A86 assembler package V4.

02 September 25, 1995

The entire package is Copyright 1986--1995 Eric Isaacson.

All rights reserved.

PLEASE read Chapter 1 for legal terms and conditions, how to

register for the package, and an overview of the assembler.

This manual is huge! A note about printing it occurs at

the bottom of the next page.

If you have never heard of "shareware", and you purchased

this disk from XYZ Software House, that advertises great
software for $5 per disk, you may now be asking the following

QUESTION: "What's going on here? Have I already bought an

assembler, or what?"

ANSWER: Well, no, not exactly. You've bought a disk that has
great software on it; you haven't bought the software yet.
A86, like the vast majority of software offered by the $5-
per-disk distribution houses, is free-distribution software
(also known as "shareware", or "user-supported software").
That means I retain the rights to A86, but I choose to let
people pass it around. I have no business relationship
with any distribution houses in the U.S.; I don't get a
penny of the $5 (or whatever) you paid them for the disk
containing A86. So I need and expect to be paid by you,
because I make my living out of making and supporting
shareware products.

A few distribution houses do a pretty poor job of revealing

the shareware nature of the software they sell. If you
thought you had purchased the software free and clear, you
might feel justified in being angry with them for having
misled you. And you might look around for houses that do a
better job of informing the public. But I hope you'll take
the time to consider everybody's role in the shareware
marketing scene; if you do, I think you'll conclude that
although you may have been misled, you haven't really been
cheated out of anything.

Shareware is great for authors like me, who have spent all
their years in their computer holes, learning to be great
programmers, and no time in business school learning
marketing and distribution techniques. We simply cast our
programs to the winds.

Shareware is also great for customers like you. You can

try out the software before paying for it. You'll know
that a successful shareware product is good, because only
satisfied customers pay for it. The existence of shareware
infuses healthy competition in the entire software market,
for both price and quality. In the case of A86, I'm
utterly convinced that you'll never find a better value for
an assembler, anywhere.
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 Finally, let's consider the distribution houses. They
provide a legitimate service, for which they charge a
reasonable price. The best houses act as librarians,
evaluating and cataloguing software. Most pay thousands of
dollars for advertising. Their cut is far less than the
distributor's cut for "commercial" software (they prosper
because their volume is bigger). Most customers for the
distribution houses are repeat customers, who are aware of,
and happy with, what they are getting. If it weren't for
your XYZ House, you might never have heard of A86, or might
never have figured out where to obtain it.

So I hope you'll be happy with shareware, and actively

desire to support it. You'll feel good about promoting a
healthy situation for everybody. And you'll encourage the
best programmers in the world to keep writing for you,
instead of for the big corporations.

Now that I've said that, let's move on to the package. The A86
package consists of the program A86.COM, a collection of source,
batch, and library files used by the demonstration contained in
Chapter 2, and a sequence of DOC files that, when printed out in
order, make the manual. Each chapter is a DOC file whose name
is A??.DOC, where ?? runs from 00 though 17. Exception: chapter
6 is split into A06A.DOC and A06B.DOC. The second file is
wider, and should be printed at 12 cpi if you can manage it.
The other files can print at 10cpi with margins -- You should
set your printer to a 1-inch left margin, as the DOC files do
not attempt to provide a left margin for you. (You can modify
the PAGE.8 program to output the appropriate codes!)

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TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION AND LEGAL TERMS

Introduction 1-1
Legal Terms and Conditions 1-1
Registration Benefits 1-3
Overview of A86 1-4
About the Author 1-6
How to Contact Me 1-7

CHAPTER 2 A86 DEMONSTRATION

Assembling a Very Short Program: PAGE.COM 2-1

Demonstration of Error-Reporting 2-1
Assembling a Longer Program with Library Files: REV.COM 2-1
Assembling a medium-sized program: TCOLS.COM 2-2

CHAPTER 3 OPERATION AND REQUIREMENTS

 Creating Programs to Assemble 3-1
Program Invocation 3-1
Assembler Switches 3-2
The A86 Environment Variable 3-7
Using Standard Input as a Command Tail 3-8
Strategies for Source File Maintenance 3-9
System Requirements for A86 3-9

CHAPTER 4 ELEMENTS OF THE A86 LANGUAGE

General Categories of A86 Elements 4-1

Operand Typing and Code Generation 4-3
Generating Opcodes from General Purpose Mnemonics 4-5

CHAPTER 5 SOME EXCLUSIVE FEATURES OF A86

The IF Statement 5-1

Multiple operands to PUSH, POP, INC, DEC 5-1
Repeat Counts to String Instructions 5-1
Conditional Return Instructions 5-2
A86 extensions to the MOV and XCHG instructions 5-2
Local Symbols 5-3
Operands to AAM and AAD Instructions 5-4
Single-Operand Forms of the TEST Instruction 5-4
Optimized LEA Instruction 5-4
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CHAPTER 6 THE 86 INSTRUCTION SET

Effective Addresses 6-1

Segmentation and Effective Addresses 6-3
Effective Use of Effective Addresses 6-4
Encoding of Effective Addresses 6-5
How to Read the Instruction Set Chart 6-7
The Instruction Set Chart 6-9

CHAPTER 7 THE FLOATING-POINT PROCESSOR

The 8087 and 287 Coprocessors 7-1

Extra Coprocessor Support 7-2
Emulating the 8087 by Software 7-2
The Floating Point Stack 7-3
Floating Point Initializations 7-3
Built-In Constant Names 7-4
Special Immediate FLD Form 7-4
Floating Point Operand Types 7-5
Operand Choices in A86 7-6
The 87 Instruction Set 7-6

CHAPTER 8 NUMBERS AND EXPRESSIONS

Numbers and Bases 8-1

The RADIX Directive 8-2
Floating-point Initializations 8-3
 Overview of Expressions 8-3
Types of Expression Operands 8-3
Descriptions of Operators and Specifiers 8-4
Operator Precedence 8-13
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CHAPTER 9 DIRECTIVES IN A86

Segments in A86 9-1

CODE ENDS and DATA ENDS Statements 9-2
The ORG Directive 9-2
The EVEN Directive 9-3
Data Allocation Using DB, DW, DD, DQ, and DT 9-3
The STRUC Directive 9-5
Forward References 9-6
Forward References in Expressions 9-7
The EQU Directive 9-7
Equates to Built-In Symbols 9-8
The NIL Prefix 9-9
Interrupt Equates 9-9
Duplicate Definitions 9-9
The = Directive 9-10
The PROC Directive 9-10
The ENDP Directive 9-11
The LABEL Directive 9-11
The INCLUDE Directive 9-12

CHAPTER 10 RELOCATION AND LINKAGE

.OBJ Production Made Easy 10-1

Overview of Relocation and Linkage 10-3
The NAME Directive 10-5
The PUBLIC Directive 10-5
The EXTRN Directive 10-6
MAIN: The Starting Location for a Program 10-7
The END Directive 10-7
The SEGMENT Directive 10-8
DATA SEGMENT, STRUC and CODE SEGMENT Directives 10-11
The ENDS Directive 10-11
Default Outer SEGMENT 10-12
The GROUP Directive 10-12
The SEG Operator 10-13
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CHAPTER 11 MACROS AND CONDITIONAL ASSEMBLY

Macro Facility 11-1

Simple Macro Syntax 11-1
Formatting in macro definitions and calls 11-2
Macro operand substitution 11-2
Quoted-string operands 11-3
Looping by operands in macros 11-4
The #L last operator and indefinite repeats 11-5
Character-loops 11-5
The "B"-before and "A"-after operators 11-6
Multiple-increments within loops 11-6
Negative R-loops 11-7
Nesting of loops in macros 11-8
 Implied closing of loops 11-8
Passing Operands by Value 11-8
Passing Operand Size 11-8
Generating the Number of an Operand 11-9
Parenthesized Operand Numbers 11-9
Exiting from the Middle of a Macro 11-10
Local Labels in Macros 11-10
Debugging Macro Expansions 11-11
Conditional Assembly 11-11
Conditional Assembly and Macros 11-13
Simulating MASM's Conditional Assembly Constructs 11-14
Declaring Variables in the Assembler Invocation 11-14
Null Invocation Variable Names 11-15
Changing Values of Invocation Variables 11-15

CHAPTER 12 COMPATIBILITY WITH OTHER ASSEMBLERS

Conversion of MASM programs to A86 12-1

Compatibility-symbols recognized by A86 12-5
Conversion of A86 Programs to Intel/IBM/MASM 12-5

CHAPTER 13 ASSOCIATED TOOLS AND OUTPUT FILES

Listings with A86 13-1

Listing Control Directives 13-2
Cross-reference Facility 13-3
A86LIB Source File Library Tool 13-5
Using A86.LIB in A86 Assemblies 13-6
Environment Variable A86LIB 13-6
Forcing a Library Search 13-7

CHAPTER 14 DESCRIPTIONS OF A86 ERROR MESSAGES

CHAPTER 15 RELEASE HISTORY OF A86

CHAPTER 16 RESERVED SYMBOLS IN THE A86 LANGUAGE

A17.DOC INDEX

CHAPTER 1 INTRODUCTION AND LEGAL TERMS

Introduction

A86 is the finest assembler available, at any cost under any

terms, for the 86-family of microprocessors (IBM-PC, compatibles,
and not-so-compatibles). In contrast to software firms who
attempt to restrict the distribution of their products via
protection-schemes, I encourage free distribution, and trust that
those who use my products will pay for them.
Please keep in mind the fundamental good spirit of
free-distribution software as you endure the following barrage of
legalities. Then evaluate the outstanding value that the A86
package offers you. I assure you that you will not be
disappointed.

Legal Terms and Conditions

This package is provided to you under the following conditions:

1. You may copy the A86Vxxx.ZIP and D86Vxxx.ZIP files, and give
them to anyone who accepts these terms. The copies you
distribute must be complete and unmodified. You do not have
to be registered to distribute this package.

2. Even if you have not yet obtained full execution rights, you
may execute the programs in this package, in order to evaluate
them. If you decide that A86 is of use to you, you must
become a registered user by sending $50 US, ($52 if you are
outside North America) to:

Eric Isaacson
416 E. University Ave.
Bloomington, IN 47401-4739

For your convenience, I now accept Visa, MasterCard and

American Express, by telephone or mail. My voice number is
1-812-339-1811. My fax number is 1-812-335-1611.

For the convenience of users in Great Britain, I have

authorized the firm Atlantic Coast Plc to collect
registrations for me. We'll try to keep the prices about the
same whether you register through me or through them; it's
your choice. Their address is The Shareware Village, Colyton,
Devon EX13 6HA, telephone 01297 552222. They'll send me a
list of registered users at the end of every month, and I'll
send an acknowledgement to each user when I get the list. So
if you haven't heard from me by the end of the month following
your registration with them, please let me know.
1-2

Your registration includes the latest diskette (3.5 inch, or

high-density 5.25 inch), that also includes the A86LIB tool
available to registered users only. You may order further
updates for $10 US, or $12 US if you are overseas. Once you
register for this package, you are registered for all future
versions -- you have permanent rights to execute A86 on one
computer. As long as I'm in business, you can get the latest
version for just the update fee.

I have a combination offer for the A86 and D86 packages.

Instead of $50 + $50 = $100, I charge $80 ($82 overseas) for
both (without the printed manual). The A86+D86 registered
disk also includes a preliminary version of A386 and D386.

I also offer a printed version of this manual, covering both

 A86 and D86. The manual is printed on sheets 8.5 inches high
and 7 inches wide, with a plastic ring-style binding so the
book can lay flat. If you order with your registration the
manual is an extra $10 to the U.S., $15 overseas. If you've
already registered and now want the manual, add another $10
($12 overseas) for the update disk that the manual is bundled
with. There is a limit of one manual per computer registered
(except you may reorder when there is a substantial revision
to the manual).

Indiana residents need to add sales tax. At the current rate

of 5%, the prices for Indiana residents are $52.50 for one
product, $63 one product with manual, $84 both products,
$94.50 both products with manual, $21 manual if already
registered.

Educational institutions and training facilities MUST be

registered in order to use A86 in courses. Contact me for
special terms.

Companies and government agencies MUST be registered in order

to use A86 for their work. Again, contact me for special
terms.

3. This package may not be sold to anyone. If the package is

distributed on a diskette, any fees collected must be
specified as materials/handling, and may not exceed $10 for
the diskette.

4. The user is completely responsible for determining the fitness

or usability of this package. I will not be liable for any
damages, of any kind, arising from any failure of any programs
in this package to perform as expected.
1-3

5. Only permanent registered users can sell or distribute any

programs that you have written or modified using this
assembler. If you do sell or distribute such programs, you
must insure that your registered name (company or individual)
will always be distributed with the program, so that I can
verify your registration. Any individual or company found to
be violating these terms will be liable for triple
registration fees for every machine they own capable of
running my assembler (plus any legal and court costs).

NOTE that the only computers that need to be registered are

those executing the program A86.COM. The programs produced by
A86 are entirely yours-- there are no "run-time royalties".

6. A86 takes advantage of situations in which more than one set

of opcodes can be generated for the same instruction. (For
example, MOV AX,BX can be generated using either an 89 or 8B
opcode, by reversing fields in the following effective address
byte. Both forms are absolutely identical in functionality
and execution speed.) A86 adopts an unusual mix of choices in
such situations. This creates a code-generation "footprint"
that occupies no space in your program file, but will enable
 me to tell, and to demonstrate in a court of law, if a
non-trivial object file has been produced by A86. The
specification for this "footprint" is sufficiently obscure and
complicated that it would be impossible to duplicate by
accident. I claim exclusive rights to the particular
"footprint" I have chosen, and prohibit anyone from
duplicating it. This has at least two specific implications:

a. Any assembler that duplicates the "footprint" is mine. If

it is not identified as mine and issued under these terms,
then those who sell or distribute the assembler will be
subject to prosecution.

b. Any program marked with the "footprint" has been produced

by my assembler. It is subject to condition 5 above.

Registration Benefits

Thank you for enduring the legalities. They are there to protect
me, and also to convince you that this is my business, from which
I make my living. I'll now return to a softer sell, to try to
make you want to register for my products.

There is a certain amount of ambiguity about when you're still

evaluating A86, and when you're really using A86 and should
register for it. Some cases are clear (e.g., you're a school
using A86 to teach a course); but many are not. In practical
reality, it's up to you to decide: you are "on your honor". Also
in practical reality, most users who ought to register haven't,
yet. For most, it's not dishonesty but merely procrastination.
So I have provided some incentives, to prod you into registering.
1-4

One incentive is the printed manual, which only registered users

can purchase. I haven't left anything out of the disk version of
the manual, but the printed version is formatted and bound much
more nicely than if you print it yourself.

Another incentive, included if you register both A86 and D86, is

a preliminary test version of A386 and D386. At this writing,
all instructions though the Pentium are implemented, except
assembly to a 32-bit protected-mode segment (in which you need
override bytes for 16-bit operands instead of 32-bit operands).

Another incentive is the tool A86LIB.COM, that lets you create

libraries of source files, to be automatically searched by A86
whenever your program has undefined symbols. This means you can
effectively add procedures of arbitrary power and complexity to
A86's language.

Also, when you register you're on my mailing list. I'll

occasionally send you notices about what I've added to A86 and
D86 since the last notice. When I bring out new products, you'll
hear about them.

Finally, there are the intangible incentives. You know you've

done the right thing. You're letting me know that you appreciate
what I've done. You're letting the world know that quality
software can succeed when distributed as shareware.

Overview of A86

A86 accepts assembly language source files, and transforms them

directly into either: (1) .COM files executable under MS-DOS,
starting at offset 0100 within a code segment; (2) .OBJ files
suitable for feeding to a linker; or (3) object files starting at
offset 0, suitable for copying to ROMs. A86 is a full featured,
professional-quality program. I designed A86 to be as closely
compatible to the standard Intel/IBM assembly language as
possible, given that I insisted upon making design and language
enhancements necessary to make A86 the best possible assembler.
Some of A86's most notable features are:

* A86 is blazingly fast-- 4 times as fast as MASM V5.1. On a

typical modern computer (486/33 with source files already
disk-cached) A86 assembles at a rate of over ten thousand lines
per second. That's per second. NOT per minute, per second.
1-5

* A86 is simple to use. You can feed it a program containing

just machine instructions, without the red tape (NAME, ASSUME,
SEGMENT PARA PUBLIC, PROC, ENDP, END, PUBLIC, EXTRN, etc. etc.)
necessary with other assemblers. The output of A86 can be a
.COM file, ready to execute immediately. You don't have to go
through a linker. Or, if you want to go through a linker, A86
will produce a correct .OBJ file even if no red tape directives
are given-- the default settings are compatible with most
high-level languages. (If you have programs written for
another assembler containing the red tape directives, you may
leave them in: A86 knows about them, and is programmed to act
upon them when assembling .OBJ files, and ignore them if
assembling .COM files.)

* In spite of its simplicity, A86 encourages modular programming,

even in its .COM mode, with separately-developed source files.
This is because A86 assembles multiple source files in its
invocation line; and because A86 assembles source files faster
than other people's linkers can link their object files. You
get all the advantages of relocation/linkage systems (building
up libraries of reliable program modules that you can piece
together), without the disadvantages (excessive,
time-and-source-code-wasting, confusing red tape).

* A86 has ample capacity for really large programming projects.

Its symbol table capacity is approximately 3000 10-letter
symbols, plus room for 15K bytes of compressed macro definition
text. (10 letters is an average symbol length; A86 recognizes
up to 127 letters in a symbol.) Plus, A86's generic local label
facility effectively doubles your symbol table capacity.

* A86 has language extension features that, once you start using,
you'll never want to do without. These include multiple
 operands to PUSH and POP; conditional returns; MOV from one
segment register to another; assembly time assertion checking;
based structures; and IF (flag) (statement).

* A86's macro processor is the best, achieving an optimal balance

between ease of use and raw power. Its looping and text
concatenation abilities let you define sophisticated macros,
whose calls look just like the machine instructions that
surround them; without the clumsy invocation syntax required by
other macro processors of A86's power.

* A86 provides clear, English error messages, given right at the

point in the source code where A86 detected the error. The
messages are actually inserted into your source file, where you
can read them and correct your code at the same time. You can
remove the messages yourself, or A86 will remove them for you
when it reassembles the file. (Fear not: your original source
is preserved in x.OLD if you want it. Or you can disable this
feature and send error messages to a .ERR file.)
1-6

* A86 provides a full complement of assembly time expression

arithmetic operators, compatible with Intel/IBM assemblers. A86
also provides 4-function floating point arithmetic in assembly
time expressions used for floating point initializations (an
A86-exclusive feature).

* A86 assembles the floating point instruction set of the

8087/287/387/IIT coprocessors, and the extended instruction set
of the 186/286/NEC series, including the NEC-unique
instructions. The 386 version, A386, assembles all the new
386/486/Pentium instructions.

* A86 has a built-in source file library feature. Any undefined

procedures or macros in your program are automatically searched
for in a special library file A86.LIB, and the associated
source files are automatically assembled. This makes access to
library routines as effortless as it is in the "C" programming
language. A sample A86.LIB file is included in the shareware
version of the package. Only registered users should have the
tool A86LIB, with which you can create your own library files.

* A86 has a powerful listing facility, allowing you complete

flexibility over the format of your listings, and including an
extremely sophisticated algorithm for automatically generating
page breaks at sensible places.

* A86 works with an associated symbolic debugger, D86, to make

the finest development environment available for the PC. See
the D86 package for the details of its features.

About the Author

I am a full-time shareware author. I have worked with Intel

microprocessors since the early days of the 8080. As an employee
of Intel, I was a part of the two-man team that implemented the
first ASM86 assembler. Having worked with all the processors of
the 86 family from the beginning, I know as much as anyone about
their machine-language architecture.

A86 and D86 themselves are extremely mature, solid programs.

They have been in existence since 1984, running first under my
own, proprietary operating system; then later under the Xenix
operating system on Altos computers, used by myself and my
clients. I have been making a decent living from my products for
some time now, and with your much-appreciated support, I will
continue to improve my products, and enhance them with new,
related offerings.
1-7

How to Contact Me

I have no plans to move from my present location at least through

the millennium. So you can write to:

Eric Isaacson Software

416 East University Ave.
Bloomington, IN 47401-4739

or call 1-812-339-1811 voice, or 1-812-335-1611 fax.

Sorry, I can't guarantee to return everybody's long distance

calls. If you'd like to be SURE I'll get back to you, please
invite me to call you back collect, or tell me to charge the cost
of the call to your credit card.

I also have a section on Compuserve: just type GO ZIPKEY to any !

prompt. (ZIPKEY is the name of my other product line, a pop-up
zipcode directory.) My Internet address is
71333.3154@compuserve.com.

If you have extraordinary difficulty contacting me, write the ASP

Ombudsman, 545 Grover Road, Muskegon, MI 49442-9427. He'll also
try to mediate any business problems people might have with
shareware authors or distributors.

PLEASE contact me if you find bugs in my programs; I'll fix them!

I accept bug reports from anyone, registered or non-registered,
no questions asked. It's very frustrating to hear about people
telling each other about bugs, and not telling me. I still await
Greg Wettstein's bug list.

CHAPTER 2 A86 DEMONSTRATION

To give you a feeling for the operation of A86, I have provided

some source files for you to assemble. You should make sure your
current directory (or a PATH directory) is the one that contains
this assembler package, and perform the following operations to
see the assembler package in action:
Assembling a Very Short Program: PAGE.COM

First, let's assemble a very short program; a program that sends

an ASCII form feed (hex 0C) to your line printer. The source for
this program is PAGE.8; type the command TYPE PAGE.8 to see how
simple this program is: note the lack of red tape directives
(NAME, ASSUME, END, PUBLIC, etc.) required by other assemblers.
Now type the command A86 PAGE.8 to assemble the file. If you are
working on a hard disk, make sure you don't blink your eyes after
typing the command; you'll miss the assembly, because A86 is
FAST.

You now have a file PAGE.COM, which is an executable program. If

you now type the command PAGE with your printer turned on, and if
your printer recognizes the form feed character, then it should
advance to the next page. You have just created a useful tool.
By altering the DB line in the source code that contains the form
feed, you can create tools to output other control sequences to
your printer.

Demonstration of Error Reporting

Now type the command ERDEMO, invoking the batch file ERDEMO.BAT.
This will invoke an assembly of a source file PAGE.BAD (copied
from PAGE.BL so you can run this demo again), into which I have
deliberately placed an erroneous statement, XCHG BL,AX. Note
that A86 tells you that it has inserted error messages into
PAGE.BAD, and saved the original source in PAGE.OLD.

Now use your favorite text editor to edit PAGE.BAD. You can use
your editor's string search function to find a tilde symbol,
which brackets all A86 error messages. Without altering the
messages, change the BL to BX, and exit your editor. Now type
the command A86 PAGE.BAD to reassemble the file. You should get
a successful assembly. Now type the command TYPE PAGE.BAD, and
note that A86 has removed the error messages for you. Wasn't
that easy?

Assembling a Longer Program with Library Files: REV.COM

Let's see A86 assemble a program with four source files. Type the
command A86 REV.8 to the console. A86 will assemble the REV.8
file you specified, see that there are undefined symbols in the
program, then assemble the files LINES.8, MSDOS.8, and USAGE.8,
listed in the library file A86.LIB, which I created using the
tool A86LIB available only to registered users.
2-2

REV is a tool that exists in the Unix operating system. It is a

"filter"; that is, it reads from standard input, transforms the
input, and outputs the transformed data to standard output. The
transformation that REV performs is to reverse all lines, so that
they come out backwards.
The usefulness of REV is in conjunction with other tools. In
particular, suppose you have a list of words that you wanted
sorted according to their last letters, not their first. You run
the list through REV, to get the words spelled backwards. Then
you run that output through SORT, to sort them that way. Finally,
you run the output of SORT through REV again, to get the words
spelled forwards again, but still sorted according to their
backwards spellings.

The normal usage of REV is, therefore, in conjunction with

redirection of standard input and output; e.g. REV <infile
>outfile. If you want to just see if REV works, type REV, the
enter key, your first name, the enter key, your last name, the
enter key, the F6 key, and the enter key. You'll get your first
and last name spelled backwards.

Assembling a medium-sized program: TCOLS.COM

Type the command MTCOLS to execute the batch file MTCOLS.BAT.

Observe that the file assembles the file TCOLS.8 into the program
TCOLS.COM. This assembly uses the +L and +X switches to produce
a listing file TCOLS.LST and a cross-reference file TCOLS.XRF.

Type the command TCOLS. The TCOLS program you just assembled
will execute, and notice that you have given it no parameters. It
thus gives you a self-documenting message. Note that towards the
end of the message is an example showing how TCOLS can be used to
print .XRF listings. You can do so now by turning your printer
on and typing an appropriate command; e.g., TCOLS <TCOLS.XRF 4 6
80 66 >PRN for 4 columns, skip 6 lines between pages, which are
80 columns by 66 lines.

If you examine the file TCOLS.LST with your favorite text editor,
you'll find a complete listing of the program, including the
expansion of the DEFAULT macro defined within the program.

CHAPTER 3 OPERATION AND REQUIREMENTS

Creating Programs to Assemble

Everything I say about A86 applies equally well to my A386

assembler, whose program name is A386. The additional features
of A386 consist of the 32-bit register set and additional
instructions and instruction forms, outlined in Chapter 6. A386
is available only on the registered A86+D86 disk.

Before you invoke A86 you must have an assembly-language source

program to assemble. A source program is an ASCII text file,
created with the text editor of your choice. The editor must
produce a file that is free of internal records known only to the
editor. Some of the fancier word processors will require you to
use a "plain text" mode to insure that the file is free of such
records.

This manual will fully explain to you the correct syntax of an

A86 program, but it is not intended to teach you about the
86-family instruction set, or about assembly-language interfacing
to your computer or your operating system.

The instruction set charts in Chapters 6 and 7 give concise,

one-line descriptions of each instruction, but they don't go into
any detail about instruction usage, or about how to make system
calls to input from keyboard or disk, output to screen, printer
or disk, etc. For that, you need a book that covers the MS-DOS
operating system and the BIOS for the IBM-PC. I am currently
using DOS Programmer's Reference by Terry Dettmann, available by
mail from Public Brand Software at 1-800-426-3475. At a more
instructional level, my users report that Peter Norton's Assembly
Language Guide to the IBM-PC has been helpful.

Program Invocation

To invoke A86, you must provide a program invocation line, either

typed to the console when the DOS command prompt appears, or
included in a batch file. The program invocation line consists
of the program name A86, followed by assembler switches
(described in the next section), and the names of the source
files you want to assemble, and of the output files you want to
produce. If the output files all have their standard extensions,
they may appear in any order: before, after, or even intermixed
with the source file names. If they don't have their standard
extensions, you must give the source file names first, followed
by the word TO, followed by the output file names. Each
non-standard name following the word TO will be assigned to the
first previously- unassigned output file in order: program,
symbols, listing, then cross-reference.

You may use the wild card delimiters * and ? if you wish, to
denote a group of source files to be assembled. A86 will sort
all matching names into alphabetical order for each wild card
specification; so the files will be assembled in the same order
even if they get jumbled up within a directory.
3-2

If you provide a name without a period or an extension, A86 will

use that as the output program file name, appending to it the
default extension as follows:

1. .OBJ if you invoked the +O switch, for linkable object file

production.

2. .BIN if there is no +O switch, but there is an ORG 0 of in

your program, without a later ORG 256.

3. .COM otherwise.

If you want your program file to have no extension, you end the
file name with a period.

You may omit any of the output file names if you wish. If you do
so, A86 will output the program source.COM (or source.OBJ or
source.BIN), where "source" is a name derived from the list of
source files, according to the rules described in the section
"Strategies for Source File Maintenance" later in this chapter.
Any of the other output files will use the name of the program
output file, combined with the standard extension for that output
file.

Assembler Switches

In addition to input and output file names, you may intersperse

assembler switch settings anywhere after the A86 program name.
They are all acted upon immediately, no matter where they are on
the command line. Some of the switches are discussed in more
detail elsewhere; I'll summarize them here:

+C causes the assembler to output symbol names with lower

case letters to its OBJ and SYM files. The case of letters
is still ignored during assembly. I output the name as it
appears in the last PUBLIC or EXTRN directive containing it;
if there is no such directive, I use the first occurrence of
the symbol to control which letters are output lower case.
(+C duplicates Microsoft MASM's /mx switch.)

+c causes the assembler to consider the case of letters

within all non-built-in symbols as significant both during
assembly and for output. Thus, for example, you can define
different symbols X and x. (+c duplicates MASM's /ml
switch.)

+D causes the default base for numeric constants to be

decimal, even if the constants have leading zeroes.

-D causes the default base to be hexadecimal if there is a

leading zero; decimal otherwise.

+E causes the error-message-augmented source file to be

written to yourname.ERR within the current directory, in all
cases. With +E, A86 will never rewrite your original source
file.
3-3

-E causes A86 to insert error messages into your source file,

whenever the file is in the current directory. If the file
is not in the current directory, A86 writes an ERR file no
matter what the E switch setting is.

+F causes A86 to generate the 287 form of floating point

instructions (no implicit FWAIT bytes are generated before
the instructions). This mode can also be specified in the
program with the .287 directive.

+f causes A86 to support emulation of the 8087. When A86

 sees a floating point instruction, it generates external
references to be resolved by the standard emulation library
(provided by Microsoft, Borland, etc.). When you LINK your
program to the emulation library, the floating point
instructions are emulated by software. NOTE you must be
assembling to a linkable OBJ file for this mode to have
effect; otherwise, +f is ignored.

-F causes emulation and default-287 to be disabled. You'll

still get 287 generation if there is a .287 directive in your
program.

+G n causes A86 to implement one or more of the

following minor options for code-generation. All these
options enhance MASM compatibility. The first three do so at
the expense of program size. The number n should be the sum
of the numbers for each of the options selected. For
example, +G10 will select the options numbered 2 and 8.

1 causes A86 to generate a longer (3-byte) instruction

form for an unconditional JMP instruction to a forward
reference local label, e.g. JMP >L1. A86 normally
assumes that since it's a local label, it will be nearby
and the short, 2-byte form will work. With this option
your code will usually be longer than necessary, but
you'll be spared having to occasionally go back and code
an explicit JMP LONG >L1.

2 causes A86 to refrain from optimizing the LEA

instruction. Without this option A86 will replace an LEA
with a shorter, equivalent MOV when it sees the chance.

4 causes A86 to generate a slightly more inefficient

internal format for memory references within an OBJ file.
The Power C compiler's MIX utility requires the
inefficient form. The makers of Power C refused to
support their customers on this by enhancing MIX, so I am
forced to offer this option.

8 causes A86 to assume that all ambiguous forward

reference operands to instructions other than jumps or
calls refer to memory variables and not offsets or
constant values. You can override this on a one-by-one
basis, with the OFFSET operator.
3-4

16 causes A86 to require that undefined names be explicitly

declared with an EXTRN when A86 is producing a linkable
.OBJ file. This switch has no effect when A86 is making
a .COM file. Without the switch, A86 will, when
assembling to an .OBJ file, quietly assume that all
undefined names are valid external references.

-G causes A86 to revert to its default for all the above

options.

+H n causes A86 to produce n extra listing lines, as

 necessary, containing excess hex bytes of object code
produced by each source line. Without this switch, A86
produces extra hex lines only if it producing extra lines
anyway to display excess source code.

+I n controls the indentation of the source line within an

A86 listing line (and thus the amount of room allocated for
the hex object display). You may have any indentation up to
127. The default value is 34, which gives room for 6 hex
object bytes. You may add or subtract 3 to that value, to
increase or decrease the hex object bytes. If you add 128 to
the value of n, the listing of the line number is suppressed
(so you can subtract 6 from the indentation to get the same
number of hex digits).

+L n causes A86 to produce a listing file of the assembly.

The n is optional, and controls what will be included in the
listing. You add numbers from the following table to produce
an n which specifies the combination of elements you want:

Use at most one of the following three values:

* Use 1 if you want A86 to eliminate all conditional

assembly control lines (#IF, #ENDIF, etc.) and all
conditionally skipped code from the listing.

* Use 2 if you want A86 to include the control lines but

not the skipped code.

* Use 3 if you want A86 to include both control lines

and skipped code.

4 causes A86 to include the lines produced by macro

expansions. Otherwise, only the macro call line is
listed.

8 causes listing control lines (TITLE, SUBTTL, PAGE) to be

listed.

16 causes the hex output object pointer to be included on

all listing lines, whether they have produced hex object
bytes or not. Otherwise, the 4-digit pointer is included
only on lines producing object bytes; on other lines, the
field is blank (but the line itself is listed!).
3-5

32 causes a symbol-table listing to be produced at the end

of the listing file.

If n is not given after +L, a value of 39 is assumed: symbol

table, hex pointer only if there are object bytes, no listing
control lines, and macro expansion lines, conditional control
lines, and skipped lines are all listed. If +L is not given
at all, then a listing file will be produced only if it is
explicitly named on the invocation line.

+O causes A86 to produce a linkable .OBJ file when the output

 file name extension is not explicitly given.

-O causes A86 to produce an executable .COM file when the

output file name extension is not explicitly given.

+P n controls which processor A86 is assembling for. You

choose one of the following values for n, for the level of
Intel processor support you wish:

* Choose 0 for the base 8086/8086 instruction set only.

Use this value if you want your programs to run on
all machines.

* Choose 1 for instructions up through the 186.

* Choose 2 for instructions up through the 286. Use

this value if you are assembling for a more recent
processor (386, 486, etc.), until A386 is available.

In addition, you may add the value 32 to n if you wish the

NEC-specific instructions to be assembled. Since the NEC
processor include the 186 instructions, you specify +P33 to
assemble for a NEC processor; use +P35 if you wish to allow
all possible instructions (which is what older versions of
A86 allowed).

The switch setting +P64 specifies assembly of the base-level

8086/8088 instruction set, but with special handling of a few
of the later instruction forms, to generate compatible code.
Specifically: all shift and rotate instructions with an
immediate shift value greater than 1 will generate that many
repetitions of the 1-value instruction form; e.g., ROL BL,3
will generate 3 copies of ROL BL,1. The mnemonics PUSHA and
POPA will be honored, generating appropriate sequences of
multiple PUSHes and POPs.

The default setting for this switch is for A86 to assemble

for the processor on which it is currently running; +P64 if
that processor is an 8086 or 8088.

+S suppresses the creation of the symbol table (.SYM) file.

This is overridden if you give an explicit symbols file name
in the invocation line.
3-6

+T n controls various options conerning the titling and

pagination of listing files. You add together your selection
of options from the following list, to produce a value of n
containing the combination you want:

The following values control the automatic incrementing of

section numbers in an A86 listing. The section number
appears to the left of the hyphen in a page number. Use at
most one of the following three values:

* Use 1 if you want A86 to refrain from incrementing

the section number whenever there is a change to the
 source file. If you choose this option, and you
don't manually increment the section number in a PAGE
directive, then page numbers will be single numbers
without any hyphenation.

* Use 2 if you want A86 to increment the section number

for each new main source file, but not for INCLUDE
files.

* Use 3 if you want A86 to increment the section number

for all source files, INCLUDE files, and returns from
INCLUDE.

4 causes A86 to automatically issue a page break at

reasonable places in the code (file breaks, and gaps of
consecutive blank lines of various lengths occurring near
the end of a page). This is described in more detail in
Chapter 13.

8 causes A86 to issue the page-ending formfeed instead of

the last linefeed, rather than in addition to it. The
advantage of this is that you can specify one more line on
a page for many printers. The disadvantage is that some
editors and file-viewing programs will not display such a
file correctly; e.g. LIST will not show the last line of a
page.

16 causes A86 to use the current source-file name as a

default TITLE if there has been no TITLE directive in the
program; as an implicit SUBTTL at the beginning of each
file if there is a TITLE explicitly given.

32 causes A86 to output a 2-line gap, followed by a line

containing the name of the source file, every time the
source file changes in the listing.

If -T (or +T0) is specified, then no titling or pagination is

done at all. If the switch is not specified at all, a
default value of 52 (auto-paging, auto-titling, and source
file line, but no auto-section and no FF instead of LF). Of
course, if there is no listing file the T switch has no
effect.
3-7

+W n controls the characteristics of wide lines in the

A86 listing file. You add together your selection of options
from the following list, to produce a value of n containing
the combination you want:

0--3 (at most one value!) determines the indentation from

the beginning of the source line for wrapped-around source
code on subsequent lines. The indentation will be 5
spaces times the number given: 0,5,10,15 spaces
respectively.

4 causes A86 to truncate lines that exceed the page width.

Exception: if A86 is outputting a line with trailing hex
 bytes anyway (via the H switch), any wrapped-around source
will be shown on those lines.

8 causes A86 to start with a listing line width of 79.

Without this option, the width is 131. The width can be
set to other values in the source code via the PAGE
directive, described in Chapter 13.

16 causes the title lines at the head of the page not to

exceed 79 bytes in width, even if the other listing lines
are longer. This option is handy if you are viewing a
listing on the screen, with a viewing program that
truncates long lines: the page headers will still be
visible during normal scrolling.

+X causes A86 to produce a cross-reference file (the same

file that was produced by the XREF utility in earlier
versions of A86).

Unless otherwise stated, the default setting for all the switches
is "minus". Multiple switches can be specified with a single
sign; e.g. +OG15L55 is the same as +O+G15+L55.

The A86 Environment Variable

To allow you to customize A86, the assembler examines the MS-DOS

environment variable named "A86" when it is invoked. If there is
such a variable, its contents are inserted before the invocation
command tail, as if you had typed them yourself.

For example, if you execute the command SET A86=+O while in DOS
(typically in the AUTOEXEC.BAT file run when the computer is
started), then the O switch will be "plus", unless overridden
with a "minus" setting in the command line.
3-8

You may also include one or more file names in the A86
environment variable. Those files will always be assembled
first, before the files you specify on the command line. This
allows you to set up a library of macro definitions, which will
always be automatically available to your programs. Thus, for
example, the DOS command SET A86=C:\A86\MACDEF.8 +O will cause
both the O switch to default ON, but will also cause the file
MACDEF.8 of subdirectory A86 of drive C to always be assembled.

Using Standard Input as a Command Tail

The following feature is a bit advanced. If you're not familiar

with the practice of redirecting standard input, you may safely
skip this section.

A86 can also be configured to take its command arguments from

standard input, in addition to the invocation command tail or the
A86 environment variable. This allows A86 to be used in those
menu-driven systems that don't generate command tails for
programs. It also allows other programs to create lists of files
to be assembled, then "pipe" the list to A86.

Here's how the feature works: when the command argument A86 is an
ampersand &, A86 will prompt for standard input. If the
ampersand is seen but there are other things following it, the
ampersand is ignored.

For example, you can place a list of file names and switch
settings into a file called FILELIST. You can then invoke the
assembler via

A86 <FILELIST &

which will cause the contents of the FILELIST file to be used as

a command line.

You may place an ampersand at the end of your A86 environment

variable. If you do so, then A86 will prompt for file names
whenever it is invoked without any command arguments (you type
A86 followed immediately by the ENTER key to the MS-DOS prompt).
This is the mode used if you have a menu system that can't
generate an invocation command tail.

Note: when you redirect standard input so that it comes from a

file, A86 will read all the lines of the file (up to a limit of
1023 bytes), and substitute spaces for the line breaks. Thus you
may give the file names on individual lines, for readability.
However, if the feature is invoked manually (no redirection), so
that you are typing in the line after the prompt, A86 will take
the first line only. You need to give all your switches and
files on that one line.
3-9

Strategies for Source File Maintenance

A86 encourages modular programming, by letting you break your

source into separate files, with complete impunity. A86 has no
concern whatsoever for file breaks-- it treats the sequence of
files as a single source code stream.

You should consider one or more of the following strategies,

which I have adopted in my source file management:

1. I name all my A86 source files with the same extension, which
is found on no other files. The particular extension I have
chosen is ".8". I did not choose the more common .ASM, because
I have a few source files designed for MSDOS's assembler. If
you don't like .8, I would suggest .A86.

2. I keep a separate subdirectory on my hard disk for each

multi-source-file A86 program I have. Then the simple command
"A86 *.8" performs the assembly for the current directory's
program.

3. I exploit the fact that A86 expands wild cards into

 alphabetical order. Whenever I want a source file to be
assembled first (e.g., when it contains variable
declarations), I append a decimal digit to the start of the
file name: 0 for the first file, 1 for the second, etc., for
however many files that need to be explicitly ordered. If a
file needs to come last, I append a "Z" to the start of the
file name.

To accommodate this strategy, I have programmed A86 to a

somewhat complicated algorithm for determining the default
output file name. I use the name of the first source file;
but I truncate the first character if it is a decimal digit.
However, you may have a general-purpose file that must come
first; so I have provided the following exception: if you have
a source file whose name begins with the digit "9", that name
(without the 9) is used. If you don't like this, you can
always explicitly give the program name you want: "A86 *.8
MYPROG".

System Requirements for A86

A86 requires MS-DOS V2.00 or later. No BIOS or lower-level calls

are made by A86, so A86 should run on any MS-DOS machine. Please
let me know if you find this not to be the case.

A86 itself is a small program, and it is fairly flexible about

the memory it uses. You can assemble with only 40K bytes of
memory beyond the program itself, which in the current version is
under 35K bytes-- a total of 75K bytes beyond the operating
system. There is no longer any limit on the size of source files
assembled under A86. The more memory you have, the more capacity
A86 has, in symbol table size and output file size. If it can,
A86 will use up to 400K bytes of memory.

CHAPTER 4 ELEMENTS OF THE A86 LANGUAGE

This chapter begins the description of the A86 language. It's a

bit more tutorial in nature than the rest of the manual. I'll
start by describing the elementary building blocks of the
language.

General Categories of A86 Elements

The statements in an A86 source file can be classified in three

general categories: instruction statements, data allocation
statements, and assembler directives. An instruction statement
uses an easily remembered name (a mnemonic) and possibly one or
more operands to specify a machine instruction to be generated.
A data allocation statement reserves, and optionally initializes,
memory space for program data. An assembler directive is a
statement that gives special instructions to the assembler.
Directives are unlike the instruction and data allocation
statements in that they do not specify the actual contents of
memory. Examples of the three types of A86 statements are given
below. These are provided to give you a general idea of what the
different kinds of statements look like.

Instruction Statements

MOV AX,BX
CALL SORT_PROCEDURE
ADD AL,7

Data Allocation Statements

A_VARIABLE DW 0
DB 'HELLO'

Assembler Directives

CODE SEGMENT
ITEM_COUNT EQU 5

The statements in an A86 source file are made up of reserved

symbols, user symbols, numbers, strings, special characters, and
comments.

Symbols are the "words" of the A86 language. All symbols are a
collection of consecutive letters, numbers, and assorted special
characters: _, @, $, and ?. Symbols cannot begin with digits:
anything that begins with a digit is a number. Symbols can begin
with any of the special characters just listed. Symbols can also
begin with a period, which is the only place within the symbol
name a period can appear.
4-2

Reserved symbols have a built-in meaning to the assembler:

instruction mnemonics (MOV, CALL), directive names (DB, STRUC),
register names, expression operators, etc. User symbols have
meanings defined by the programmer: program locations, variable
names, equated constants, etc. The user symbol name is
considered unique up to 127 characters, but it can be of any
length (up to 255 characters). Examples of user symbols are:
COUNT, L1, and A_BYTE.

Numbers in A86 may be expressed as decimal, hexadecimal, octal,

binary, or decimal "K". These must begin with a decimal digit
and, except in the case of a decimal or hexadecimal number, must
end with "x" followed by a letter identifying the base of the
number. A number without an identifying base is hexadecimal if
the first digit is 0; decimal if the first digit is 1 through 9.
Examples of A86 numbers are: 123 (decimal), 0ABC (hexadecimal),
1776xQ (octal), 10100110xB (binary), and 32K (decimal 32 times
1024).

Strings are characters enclosed in either single or double

quotes. Examples of strings are: '1st string' and "SIGN-ON
MESSAGE, V1.0". If you wish to include a quote mark within a
string, you can double it; for example, 'that''s nice' specifies
a single quote mark within the string. The single quote and
double quote are two of many special characters used in the
assembly language. Others, run together in a list, are: ! $ ? ;
: = , [ ] . + - ( ) * / >. The space and tab characters are also
special characters, used as separators in the assembly language.

A comment is a sequence of characters used for program

documentation only; it is ignored by the assembler. Comments
begin with a semicolon (;) and run to the end of the line on
which they are started. Examples of lines with comments are
shown below:

; This entire line is a comment.

MOV AX,BX ; This is a comment next to an instruction statement.

Alternatively, for compatibility with other assemblers, I provide

the COMMENT directive. The next non-blank character after
COMMENT is a delimiter to a comment that can run across many
lines; all text is ignored, until a second instance of the
delimiter is seen. For example,

COMMENT 'This comment

runs across two lines'

I don't like COMMENT, because I think it's very dangerous. If,

for example, you have two COMMENTs in your program, and you
forget to close the first one, the assembler will happily ignore
all source code between the comments. If that source code does
not happen to contain any labels referenced elsewhere, the error
may not be detected until your program blows up. For multiline
comments, I urge you to simply start each line with a semicolon.
4-3

Statements in the A86 are line oriented, which means that

statements may not be broken across line boundaries. A86 source
lines may be entered in a free form fashion; that is, without
regard to the column orientation of the symbols and special
characters.

PLEASE NOTE: Because an A86 line is free formatted, there is no

need for you to put the operands to your instructions in a
separate column. You organize things into columns when you want
to visually scan down the column; and you practically never scan
operands separate from their opcodes. Realizing this, you may
wish to separate your operands from the mnemonic with a space
instead of a tab, making the line less disjointed and hence
easier to read. You will also have room for a longer comment
after the instruction.

Operand Typing and Code Generation

A86 is a strongly typed assembly language. What this means is

that operands to instructions (registers, variables, labels,
constants) have a type attribute associated with them which tells
the assembler something about them. For example, the operand 4
has type "number", which tells the assembler that it is a
numerical constant, rather than a register or an address in the
code or data. The following discussion explains the types
associated with instruction operands and how this type
information is used to generate particular machine opcodes from
general purpose instruction mnemonics.

Registers

The 8086 has 8 general purpose word (two-byte) registers:

AX,BX,CX,DX,SI,DI,BP, and SP. The first four of those registers
are subdivided into 8 general purpose one-byte registers
AH,AL,BH,BL,CH,CL,DH, and DL. There are also 4 16-bit segment
registers CS,DS,ES, and SS, used for addressing memory; and the
implicit instruction-pointer register (referred to as IP,
although "IP" is not part of the A86 assembly language).

My A386 assembler supports the two additional segment registers

FS and GS, plus the 32-bit general registers
EAX,EBX,ECX,EDX,ESI,EDI,EBP, and ESP. The lower 16 bits of each
32-bit register is the corresponding 16-bit register (without the
E in its name).

Variables

A variable is a unit of program data with a symbolic name,

residing at a specific location in 8086 memory. A variable is
given a type at the time it is defined, which indicates the
number of bytes associated with its symbol. Variables defined
with a DB statement are given type BYTE (one byte), and those
defined with the DW statement are given type WORD (two bytes).
Examples:
4-4

BYTE_VAR DB 0 ; A byte variable.

WORD_VAR DW 0 ; A word variable.

Labels

A label is a symbol referring to a location in the program code.

It is defined as an identifier, followed by a colon (:), used to
represent the location of a particular instruction or data
structure. Such a label may be on a line by itself or it may
immediately precede an instruction statement (on the same line).
In the following example, LABEL_1 and LABEL_2 are both labels for
the MOV AL,BL instruction.

LABEL_1:
LABEL_2: MOV AL,BL

In the A86 assembly language, labels have a type identical to

that of constants. Thus, the instruction MOV BX,LABEL_2 is
accepted, and the code to move the immediate constant address of
LABEL2 into BX, is generated.

IMPORTANT: you must understand the distinction between a label

and a variable, because you may generate a different instruction
than you intended if you confuse them. For example, if you
declare XXX: DW ?, the colon following the XXX means that XXX is
a label; the instruction MOV SI,XXX moves the immediate constant
address of XXX into the SI register. On the other hand, if you
declare XXX DW ?, with no colon, then XXX is a word variable; the
same instruction MOV SI,XXX now does something different: it
loads the run-time value of the memory word XXX into the SI
register. You can override the definition of a symbol in any
usage with the immediate-value operator OFFSET or the
memory-variable opertors B,W,D,Q, or T. Thus, MOV SI,OFFSET XXX
loads the immediate value pointing to XXX no matter how XXX was
declared; MOV SI,XXX W loads the word-variable at XXX no matter
how XXX was declared.

Constants

A constant is a numerical value computed from an assembly-time

expression. For example, 123 and 3 + 2 - 1 both represent
constants. A constant differs from a variable in that it
specifies a pure number, known by the assembler before the
program is run, rather than a number fetched from memory when the
program is running.
4-5

Generating Opcodes from General Purpose Mnemonics

My A86 assembly language is modeled after Intel's ASM86 language,

which uses general purpose mnemonics to represent classes of
machine instructions rather than having a different mnemonic for
each opcode. For example, the MOV mnemonic is used for all of
the following: move byte register to byte register, load word
register from memory, load byte register with constant, move word
register to memory, move immediate value to word register, move
immediate value to memory, etc. This feature saves you from
having to distinguish "move" from "load," "move constant" from
"move memory," "move byte" from "move word," etc.

Because the same general purpose mnemonic can apply to several

different machine opcodes, A86 uses the type information
associated with an instruction's operands in determining the
particular opcode to produce. The type information associated
with instruction operands is also used to discover programmer
errors, such as attempting to move a word register to a byte
register.

The examples that follow illustrate the use of operand types in

generating machine opcodes and discovering programmer errors. In
each of the examples, the MOV instruction produces a different
8086 opcode, or an error. The symbols used in the examples are
assumed to be defined as follows: BVAR is a byte variable, WVAR
is a word variable, and LAB is a label. As you examine these MOV
instructions, notice that, in each case, the operand on the right
is considered to be the source and the operand on the left is the
destination. This is a general rule that applies to all
two-operand instruction statements.

MOV AX,BX ; (8B) Move word register to word register.

MOV AX,BL ; ERROR: Type conflict (word,byte).
MOV CX,5 ; (B9) Move constant to word register.
MOV BVAR,AL ; (A0) Move AL register to byte in memory.
MOV AL,WVAR ; ERROR: Type conflict (byte,word).
MOV LAB,5 ; ERROR: Can't use label/constant as dest. to MOV.
MOV WVAR,SI ; (89) Move word register to word in memory.
MOV BL,1024 ; ERROR: Constant is too large to fit in a byte.

CHAPTER 5 SOME EXCLUSIVE FEATURES OF A86

The IF Statement

As a "nudge" in the direction of structured programming, A86

offers the IF statement. Suppose you want to conditionally skip
around just one instruction. Ordinarily, this would require, for
example:

JNZ >L1 ; skip the following move if NZ

MOV AX,BX ; make this move only if Z
L1: ; this label exists only for the above skip

You may replace the above code with the single line:

IF Z MOV AX,BX

The above line generates exactly the same code as the previous 3
lines-- a conditional jump of the opposite condition, around the
statement given in the tail of the IF statement. The statement
can be a macro call, giving you the opportunity to skip something
more complicated.

You may use any condition that would follow the "J" in a
conditional jump instruction, except CXZ, which does not have a
reverse condition. The assembler interprets the condition by
appending a "J" to the beginning of the condition; so that the
symbols "C", "NC", "Z", "NZ", etc. are not reserved by the
assembler, and can be defined in other contexts.

Multiple operands to PUSH, POP, INC, DEC

A86 will accept any number of register operands for the

instructions PUSH, POP, INC, and DEC; it will generate the
appropriate machine instruction for each operand. For example,
the statement PUSH AX,BX is the same as the two statements PUSH
AX and PUSH BX.

A numeric operand appearing in an INC or DEC statement will cause

the previous INC(s) or DEC(s) to be propagated that number of
times. For example, the statement INC AX,4 will generate 4 INC
AX instructions. The statement DEC AL,BX,2 will generate DEC AL,
DEC BX, DEC AL, DEC BX. Sorry, numeric operands are not allowed
if any of the operands affected was a forward reference or
relocatable quantity; e.g., INC FOO,2 where FOO is undefined. In
most such cases, you'll want to code the more efficient ADD FOO,2
anyway.

Repeat Counts to String Instructions

A86 will accept a numeric operand to the string instructions

STOSB, STOSW, MOVSB, and MOVSW. This causes A86 to generate that
many copies of the given instruction. For small values (usually
2 through 4), this is more efficient than loading the number into
CX and using the REP prefix.
5-2

Conditional Return Instructions

Programmers accustomed to the conditional return instructions of

the 8080/Z80 will appreciate the following feature: A86 allows
the operand to a conditional jump instruction to be one of the
three RET instructions RET, RETF, or IRET. The assembler will
find a nearby return instruction of the indicated flavor, and use
that as the target for the conditional jump. For example, JZ RET
is the replacement for the 8080's RZ return-if-zero instruction.
In other 8086 assembly languages, you have to find the nearby
instruction yourself, attach a label to it, and use that label.
Note that it does not suffice to attach a label to a single RET
instruction and use that label throughout the program: the range
of conditional jumps is only 128 bytes in either direction.

What happens if A86 does not find a nearby return instruction? In

that case, A86 issues an error, "02 Jump > 128", for the next
matching return instruction in the program. If there is no
subsequent return instruction, the return mnemonic will appear as
an undefined symbol at the end of the program. In either case,
you correct the problem by inserting a free-standing return
instruction at some nearby point in the program, where it will
not affect the existing code (typically following an
unconditional JMP instruction). If there is no good place to
insert a return instruction, you can always replace the "Jcond
RET" with an "IF cond RET".

A86 extensions to the MOV and XCHG instructions

There are a number of MOV and XCHG instructions available in A86

that are not a part of the machine instruction set.

First, moves between segment registers, and of immediate

constants into segment registers are allowed. For example, if
you code MOV ES,DS , the assembler will generate a PUSH DS
followed by a POP ES; which will effect the move that you
intended. If you code MOV DS,0 , the assembler will generate
PUSH AX; MOV AX,0; MOV DS,AX; POP AX. This is mainly a
convenience for D86 users to load segment registers manually.

Second, MOV allows 3 operands. A statement MOV x,y,z is

equivalent to the two statements MOV y,z followed by MOV x,y.
Sorry, but segment overrides are not allowed in conjunction with
3-operand MOVs. The override preceding the MOV is ambiguous in
its meaning; and overrides within operands cannot be handled
correctly by A86. You'll have to code two MOV instructions if
you want either or both to have a segment override.

Third, A86 accepts a MOV of a word-sized memory operand into

another word-sized memory operand. A86 handles this the same way
it handles a MOV of segment registers: it generates a PUSH of the
source followed by a POP of the destination.
5-3

Finally, A86 allows the XCHG of a segment register (except CS)

with any other word-sized quantity, as well as the XCHG of two
word-sized memory quantities. If there is no machine instruction
available for XCHG a,b, then A86 generates PUSH a followed by MOV
a,b followed by POP b.

Local Symbols

If you examine most assembly language program symbol tables, you

will find that the symbols can be partitioned into two levels of
significance. About half the symbols are the names of
procedures and variables having global significance. If the
names of these symbols are chosen intelligently and carefully,
the program's readability improves drastically. (They usually
aren't chosen well, most often because the assembler restricts
symbols to 6 letters, or because the programmer's habits are
influenced by such assemblers.)

The other half of the symbols in a program have a much lower,

local significance. They are only place markers used to
implement small loops and local branching (e.g., "skip the next 2
instructions if the Z-flag is set"). Assigning full-blown names
to these symbols reduces the readability of your program in two
ways: First, it is harder to recognize local jumps for what they
are-- they are usually the assembly language equivalent of high
level language constructs like IF statements and WHILE loops.

Second, it is harder to follow the global, significant symbols

because they are buried in a sea of the place marker symbols in
the symbol table.

A86 solves this problem with local symbols. If a symbol in your

program consists of a single letter followed by one or more
decimal digits (L3, X123, Y37, etc.), then the symbol is a local
symbol. Local symbols do not appear in the A86 +X
cross-reference listing. They can also be redefined to something
completely different later in the program. Local symbols can be
of any type: labels, memory variables, etc.

Because local symbols can be redefined, you must take care to

specify which one you are referring to in your program. If your
reference is a forward reference (the label occurs further down
in the program from the reference), then the reference must be
preceded by a ">". For example,

L2:
 MOVSB
INC BX
LOOP L2 ; lack of ">" means L2 is above this statement
.
.
JNZ >L2 ; ">" indicates L2 is below this statement
.
JMP >L2 ; JMP L2 is disallowed here: cannot overlap ranges
.
L2:
5-4

I recommend that you assign all your local labels the names L0
through L9. If your program is so complex that it needs more
than 10 place holders in any one stretch of code, then that
stretch needs to be rewritten.

Operands to AAM and AAD Instructions

Those of you who have examined 86 family opcodes with an eagle

eye will have noticed a somewhat spurious "0A" opcode generated
after every AAM or AAD instruction. The opcode is there to
provide the constant divisor or multiplicand for the instruction.
Believe it or not, there wasn't enough room in the microcode of
the original 8086 to hold this constant! Although Intel has
never announced the generality of AAM and AAD, it is there: you
can substitute any other constant for 0A (decimal 10), and that
constant will be used. A86 supports this by letting you give a
constant byte-sized operand to AAM or AAD. Particularly useful
are the instructions AAM 16, which unpacks AL into nibbles AH and
AL; and AAD 16, which reverses the process, packing nibbles AH
and AL into AL.

WARNING: A couple of my users point out to me that the AAD

instruction with a general operand won't work on the NEC V20 and
V30 chips. The operand is assumed to be 10 no matter what it
really is. Since a large number of PC "speed up" kits involve
switching to NEC chips, this will be seen on many PC's. You
should not use AAD with an operand if you want your program to
run on everybody's machine. Too bad. AAM works fine, though.

Single-Operand Forms of the TEST Instruction

A86 allows the TEST instruction to have a single operand, to set

the flags according to the value of the operand. If the operand
is a register, A86 generates a TEST of the register with itself.
If the operand is a memory quantity, A86 generates a TEST of the
memory with the constant -1 (i.e., the quantity will be ANDed
with an all 1's constant). For example, instead of TEST DL,DL,
you can code simply TEST DL. Instead of TEST WVAR,0FFFF, you can
code simply TEST WVAR.

Optimized LEA Instruction

Many assembly-language programmers are in the habit of using, for
example, LEA SI,MEMLOC instead of the equivalent MOV SI,OFFSET
MEMLOC to load an immediate value that represents the pointer to
a memory location. However, the LEA instruction form generates
one more byte of object code than the MOV form. A86 recognizes
this situation and generates the more-efficient MOV instruction
when it can. This also applies to register moves: MOV AX,BX
instead of LEA AX,[BX].
5-5

I've gotten a little flak from some users about this feature.
They claim it violates my policy against "behind your back"
actions. But I feel that this feature is completely equivalent
to code optimizations in other situations: the short JMP form
instead of the equivalent near JMP; a byte operand to ADD SI,4
instead of a word operand; the one-byte XCHG AX,BX instead of the
general XCHG rw,ew form; etc, etc, etc. In situations where there
is absolute functional equivalence between forms, A86 tries to
generate the most efficient form. But for those who are not
convinced, I offer the +G2 switch, described in Chapter 3.

Some users have also gotten the mistaken impression, from reading
Intel's confusing specs, that the longer LEA is sometimes faster
than the shorter MOV. This is never the case: those users are
reading the clock counts for the memory-fetch forms of MOV, not
the register-only or immediate-value forms. If you don't believe
it, try timing 1000 consecutive LEA's in a loop that executes
50000 times, vs. a similar loop with the equivalent MOV.

CHAPTER 6 THE 86 INSTRUCTION SET

In this chapter we discuss in detail the instruction set

supported by both the A86 and A386 assemblers. To use any of the
32-bit registers, the extra segment regsiters FS and GS, or the
instructions marked with a "3", "4", or "5" in the instruction
list, you need my A386 assembler, available only if you register
both A86 and D86.

Effective Addresses

Most memory data accessing in the 86 family is accomplished via

the mechanism of the effective address. Wherever an effective
address specifier "eb", "ew" "ed", or "ev" appears in the list of
instructions, you may use a wide variety of actual operands in
that instruction. These include general registers, memory
variables, and a variety of indexed memory quantities.

GENERAL REGISTERS: Wherever an "ew" appears, you can use any of

the 16-bit registers AX,BX,CX,DX,SI,DI,SP, or BP. Wherever an
"eb" appears, you can use any of the 8-bit registers
AL,BL,CL,DL,AH,BH,CH, or DH. Whenever an "ed" occurs, you can
(on A386) use any of the 32-bit registers
EAX,EBX,ECX,EDX,ESI,EDI,EBP, or ESP. For A86, "ev" is the same
as "ew"; for A386, it means you can use either a 16-bit or a
32-bit register. For example, the "ADD ev,rv" form subsumes the
16-bit register-to-register adds; for example, ADD AX,BX; ADD
SI,BP; ADD SP,AX. On A386, this form also includes 32-bit
register-to-register adds; e.g., ADD EBX,ESI. At the machine
level the 16-vs.-32 distinction is made by an "operand override"
opcode byte, 66H, that A386 places before the instruction to
signal a switch from 16-bits to 32-bits.

MEMORY VARIABLES: Wherever an "eb", "ew", "ed", or "ev" appears,

you can use a memory variable of the indicated size: byte, word,
doubleword, or either-word-or-doubleword. Variables are
typically declared in the DATA segment, using a DD declaration
for a doubleword variable, a DW declaration for a word variable,
or a DB declaration for a byte variable. For example, you can
declare variables:

DATA_PTR DW ?
ESC_CHAR DB ?

Later, you can load or store these variables:

MOV ESC_CHAR,BL ; store the byte variable ESC_CHAR

MOV DATA_PTR,081 ; initialize DATA_PTR
MOV SI,DATA_PTR ; load DATA_PTR into SI for use
LODSW ; fetch the word pointed to by DATA_PTR

Alternatively, you can address specific unnamed memory locations

by enclosing the location value in square brackets; for example,

MOV AL,[02000] ; load contents of location 02000 into AL

6-2

Note that A86 discerned from context (loading into AL) that a
BYTE at 02000 was intended. Sometimes this is impossible, and
you must specify byte or word:

INC B[02000] ; increment the byte at location 02000

MOV W[02000],0 ; set the WORD at location 02000 to zero

INDEXED MEMORY: The 86 supports the use of certain registers as

base pointers and index registers into memory. BX and BP are the
base registers; SI and DI are the index registers. You may
combine at most one base register, at most one index register,
and a constant number into a run time pointer that determines the
location of the effective address memory to be used in the
instruction. These can be given explicitly, by enclosing the
index registers in brackets:

MOV AX,[BX]
MOV CX,W[SI+17]
MOV AX,[BX+SI+5]
MOV AX,[BX][SI]5 ; another way to write the same instr.
Or, indexing can be accomplished by declaring variables in a
based structure (see the STRUC directive in Chapter 9):

STRUC [BP] ; NOTE: based structures are unique to A86!

BP_SAVE DW ? ; BP_SAVE is a word at [BP]
RET_ADDR DW ? ; RET_ADDR is a word at [BP+2]
PARM1 DW ? ; PARM1 is a word at [BP+4]
PARM2 DW ? ; PARM2 is a word at [BP+6]
ENDS ; end of structure
INC PARM1 ; equivalent to INC W[BP+4]

Finally, indexing can be done by mixing explicit components with

declared ones:

TABLE DB 4,2,1,3,5
MOV AL,TABLE[BX] ; load byte number BX of TABLE

The 386 processor also supports indexing using any of the eight
32-bit general registers. This type of indexing is of limited
use for memory referencing from real-mode programs (most programs
running under DOS), since offsets greater than 64K are disallowed
in real mode (you will get a General Protection Fault if you try
it). 32-bit indexing is, however, useful in conjunction with the
LEA instruction, giving an extremely powerful register arithmetic
instruction. For example, LEA ECX,[EAX+2*EBX+17000] performs two
additions and a multiplication, all in a single machine
instruction. Since no memory accessed is actually attempted,
this kind of LEA usage is allowed in real-mode DOS programs.
6-3

In 32-bit indexing, you may use one or two of any of the 32-bit
general registers. You may also scale one of the indexing
registers, by multiplying it by 2, 4, or 8. You may also add or
subtract a constant of any size up to a doubleword capacity to
the indexed quantity. If you use the same register twice and
scale one of the instances of that register, you get, in effect,
an odd-number scaling (3, 5, or 9) of that register; e.g., A386
will allow LEA EAX,[9*EBX] as an abbreviation for LEA
EAX,[8*EBX+EBX].

Due to coding restrictions, the ESP register can be used only

once within an indexed quantity, and cannot be scaled.

Some more examples of 32-bit indexing are:

XCHG DX,[EAX]
MOV AL,[EAX+EBX]
ADD EBX,[ESI+8*ECX+3391811]
LEA ECX,[4*EBX-7]

Segmentation and Effective Addresses

The 86 family has four segment registers, CS, DS, ES, and SS,
used to address memory. The 386 and later processors add two
more segment registers FS and GS. Each segment register points
to 64K bytes of memory within the 1-megabyte memory space of the
86. (The start of the 64K is calculated by multiplying the
segment register value by 16; i.e., by shifting the value left by
one hex digit.) If your program's code, data and stack areas can
all fit in the same 64K bytes, you can leave all the segment
registers set to the same value. In that case, you won't have to
think about segment registers: no matter which one is used to
address memory, you'll still get the same 64K. If your program
needs more than 64K, you must point one or more segment registers
to other parts of the memory space. In this case, you must take
care that your memory references use the segment registers you
intended.

Each effective address memory access has a default segment

register, to be used if you do not explicitly specify which
segment register you wish. For most effective addresses, the
default segment register is DS. The exceptions are those
effective addresses that use the BP register for indexing. All
BP-indexed memory references have a default of SS. (This is
because BP is intended to be used for addressing local variables,
stored on the stack.)
6-4

If you wish your memory access to use a different segment

register, you provide a segment override byte before the
instruction containing the effective address operand. In the A86
language, you code the override by giving the name of the segment
register you wish before the instruction mnemonic. For example,
suppose you want to load the AL register with the memory byte
pointed to by BX. If you code MOV AL,[BX], the DS register will
be used to determine which 64K segment BX is pointing to. If you
want the byte to come from the CS-segment instead, you code CS
MOV AL,[BX]. Be aware that the segment override byte has effect
only upon the single instruction that follows it. If you have a
sequence of instructions requiring overrides, you must give an
override byte before every instruction in the sequence. (In that
case, you may wish to consider changing the value of the default
segment register for the duration of the sequence.)

NOTE: This method for providing segment overrides is unique to

the A86 assembler! The assemblers provided by Intel and IBM
(MS-DOS) attempt to figure out segment allocation for you, and
plug in segment override bytes "behind your back". In order to
do this, those assemblers require you to inform them which
variables and structures are pointed to by which segment
registers. That is what the ASSUME directive in those assemblers
is all about. I wrote Intel's first 86 assembler, ASM86, so I
have been watching the situation since day one. Over the years,
I have concluded that the ASSUME mechanism creates far, far more
confusion that it solves. So I scrapped it; and the result is an
assembler with far less red tape. But if your program needs more
than 64K, you do have to manage those segment registers yourself;
so take care!

Effective Use of Effective Addresses

Remember that all of the common instructions of the 86 family
allow effective addresses as operands. (The only major functions
that don't are the AL/AX specific ones: multiply, divide, and
input/output). This means that you don't have to funnel many
numbers through AL or AX just to do something with them. You can
perform all the common arithmetic, PUSH/POP, and MOVes from any
general register to any general register; from any memory
location (indexed if you like) to any register; and (this is most
often overlooked) from any register TO memory. The only thing
you can't do in general is memory-to-memory. Among the more
common operations that inexperienced 86 programmers overlook are:

* setting memory variables to immediate values

* testing memory variables, and comparing them to constants

* preserving memory variables by PUSHing and POPping them

* incrementing and decrementing memory variables

* adding into memory variables

6-5

Encoding of Effective Addresses

This section outlines the number of program opcode bytes

generated by effective-address specifications. This will let you
make judgments when trying to keep your program as small as
possible. The precise opcodes generated are explained in the
text files EFF86.DOC in the A86 package, and EFF386.DOC in the
A386 package.

Every instruction with an 16-bit effective address has an encoded

byte, known as the effective address byte, following the
instruction's main opcode. (For obscure reasons, Intel calls
this byte the ModRM byte.) If the effective address is a memory
variable, or an indexed memory location with a non-zero constant
offset, then the effective address byte is immediately followed
by the offset amount. Amounts in the range -128 to +127 are
given by a single signed byte. Amounts outside that range are
represented by a 2-byte offset.

In the instruction chart given later in this chapter,

effective-address specification opcodes are denoted by a slash /
followed either by the letter "r" or an octal digit. The meaning
of the r-or-digit is explained in the EFF*.DOC files. For
example, the instruction DIV CX falls under the DIV eb form in
the instruction chart. The instruction occupies two bytes: the
main opcode byte 0F6H, followed by a single effective address
byte with no constant offsets involved. Similarly, the
instruction DIV B[BX] occupies two bytes. For DIV B[BX+7] you
must add an offset byte for the 7, making a total of three bytes.
For DIV B[BX+1000] you must add a 2-byte offset for the 1000,
making a total of 4 bytes. For DIV B[02000] (more typically
coded with a symbolic name such as DIV MY_VAR_NAME), the
instruction is also 4 bytes: the main opcode byte, the effective
address byte, and the offset of the memory variable.
An anomalous case is the operand [BP]. The effective-address
byte encoding for this particular operand was usurped by the
simple-variable case. When A86 sees [BP], it must specify an
8-bit offset whose value is zero. Thus, the instruction DIV
B[BP] occupies three bytes, not two. This anomaly does not apply
to [BP+SI] or [BP+DI].

In A386, 32-bit indexing is signalled by a special address

override opcode byte (67H) preceding the instruction. Following
the override byte is the instruction's main opcode, followed by
the effective-address specification. For a simple memory
variable, the specification consists of a single
effective-address byte followed by the 4-byte offset of the
variable. For indexing involving a single, non-scaled index
register other than ESP, the specification consists of a single
byte followed by the constant offset component. For indexing
involving two registers, scaling, or the ESP register, there are
two bytes followed by the constant offset component. The
constant offset component occpies no space if the the offset is
zero, one byte if between -128 to +127, and 4 bytes otherwise.
There is no provision for a 16-bit-word-sized offset if you are
using 32-bit indexing.
6-6

Note the distinction between the address override byte (67H) and
the operand override byte (66H). A86 must supply an address
override when the instruction involves a memory operand whose
address has 32 bits. A86 must supply an operand override when
the data being manipulated has 32 bits. In general, when a
32-bit register name appears inside the square brackets, that's
an address override; when it appears outside the square brackets,
that's an operand override. Examples:

MOV DX,[BX] ; needs neither override in a 16-bit segment

MOV DX,[EBX] ; needs an address override
MOV EDX,[BX] ; needs an operand override
MOV EDX,[EBX] ; needs both overrides

Also note that the generation of these override bytes is handled

automatically by A86 when it scans the operands to an
instruction. The only exceptions to this are the no-operand
string operations: REP MOVSW, LODSD, SCASB, etc. For these
instructions, the operand size is signalled by the last letter
(B, W, or D) of the mnemonic; however, the addressing mode is not
signalled by the mnemonic. If you are in 16-bit mode, as all
simple DOS programs are, you need to precede a string instruction
with an explicit A4 prefix if you wish to use 32-bit addressing
([ESI] and/or [EDI] with count ECX). If you are assembling to a
32-bit protected-mode segment (when that is implemented) you will
need to use an explicit A2 prefix if you wish to use 16-bit
addressing ([SI] and/or [DI] with count CX).

Here are some examples of instruction size involving 32-bit

indexing in a real-mode segment: DIV B[EBX] requires an address
override byte, the single instruction opcode byte 0F6H, and an
effective address byte: total 3 bytes. DIV B[EBX+7] adds the
offset byte 07, making the total 4 bytes. DIV B[EBX+1000] forces
the offset to be 4 bytes, making the total 7 bytes. DIV
B[EBX+EDI*2] does not require an offset, but the extra index
register expands the effective address specifier to two bytes,
making the total 4 bytes. Similarly, DIV B[ESP] requires two
effective address bytes (total 4 instruction bytes), because the
ESP register is a special case. Finally, DIV
ES:D[EBX+EDI*2+1000] requires three overrides (segment override
ES, operand override for the D, and address override for 32-bit
indexing), the main opcode byte, two effective address opcode
bytes, and a 4-byte offset: total 10 bytes.

The [BP] extra-byte anomaly applies, in 32-bit mode, to [EBP] as

well. In fact, the anomaly also applies when another indexing
register (scaled or not) is added to [EBP]. A386 must generate
an offset byte whose value is 0 when it sees any no-offset forms
involving [EBP].
6-7

The 386 and later processors, when running in protected mode,

allow segments whose default word-size is 32 bits instead of
16-bits. In such segments, the usage of the operand and address
override bytes is reversed: 32-bit operands do not require the
operand-override byte, and 16-bit operands do. (8-bit operands
never require an operand-override byte.) 32-bit memory addresses
do not require an address-override byte; 16-bit addresses do.
This mode will be recognized by A386 whenever the USE32 directive
is used; however, at the time of this writing, this feature is
not yet implemented. All DOS programs, which run in real mode,
have a default of 16 bits.

How to Read the Instruction Set Chart

The following chart summarizes the machine instructions you can

program with A86. In order to use the chart, you need to learn
the meanings of the specifiers (each given by 2 lower case
letters) that follow most of the instruction mnemonics. Each
specifier indicates the type of operand (register byte, immediate
word, etc.) that follows the mnemonic to produce the given
opcodes. The "v" type, for A86, is the same as "w" -- it denotes
a 16-bit word. On A386, "v" denotes either a word or doubleword,
depending on the presence of an operand override prefix byte.

"c" means the operand is a code label, pointing to a part of the

program to be jumped to or called. A86 will also accept a
constant offset in this place (or a constant segment-offset
pair in the case of "cp"). "cb" is a label within about 128
bytes (in either direction) of the current location. "cv" is
a label within the same code segment as this program; "cp" is
a pair of constants separated by a colon-- the segment value
to the left of the colon, and the offset to the right. The
offset is always a word in A86; it can be either a word or a
doubleword in A386. Note that in both the cb and cv cases,
the object code generated is the offset from the location
following the current instruction, not the absolute location
of the label operand. In some assemblers (most notably for
 the Z-80 processor) you have to code this offset explicitly
by putting "$-" before every relative jump operand in your
source code. You do NOT need to, and should not do so with
A86.

"e" means the operand is an Effective Address. The concept of

an Effective Address is central to the 86 machine
architecture, and thus to 86 assembly language programming.
It is described in detail at the start of this chapter. We
summarize here by saying that an Effective Address is either
a general purpose register, a memory variable, or an indexed
memory quantity. For example, the instruction "ADD rb,eb"
includes the instructions: ADD AL,BL, and ADD CH,BYTEVAR, and
ADD DL,B[BX+17].
6-8

"i" means the operand is an immediate constant, provided as part

of the instruction itself. "ib" is a byte-sized constant;
"iw" is a constant occupying a full 16-bit word. The operand
can also be a label, defined with a colon. In that case, the
immediate constant which is the location of the label is
used. Examples: "MOV rw,iw" includes the instructions: MOV
AX,17, or MOV SI,VAR_ARRAY, where "VAR_ARRAY:" appears
somewhere in the program, defined with a colon. NOTE that if
VAR_ARRAY were defined without a colon, e.g., "VAR_ARRAY DW
1,2,3", then "MOV SI,VAR_ARRAY" would be a "MOV rw,ew" NOT a
"MOV rw,iw". The MOV would move the contents of memory at
VAR_ARRAY (in this case 1) into SI, instead of the location
of the memory. To load the location, you can code "MOV
SI,OFFSET VAR_ARRAY".

"m" means a memory variable or an indexed memory quantity; i.e.,

any Effective Address EXCEPT a register.

"r" means the operand is a general purpose register. The 8 "rb"

registers are AL,BL,CL,DL,AH,BH,CH,DH; the 8 "rw" registers
are AX,BX,CX,DX,SI,DI,BP,SP.

"rv/m" is used in the Bit Test instructions to denote either

a word-or-doubleword register, or an array of bits in memory
that can any length.

NOTE: The following chart gives all instructions for all

processors through the Pentium. You must take care to use only
the instructions appropriate for the target processor of your
program (the P switch will enforce this for you: see Chapter 3).
If an instruction form does not run on all processors, there is a
letter or digit just before the description field. "N" means the
instruction runs only on NEC processors (which are rare nowdays).
A digit x means the instruction runs on the x86 or later: 1 for
186, 2 for 286, 3 for 386, 4 for 486, 5 for Pentium.
Instructions with 3 or greater are recognized only by my A386
assembler, received only by those who register both A86 and D86.

6-9
Opcodes Instruction Description

67 or nil A2 (prefix) 3 Use 16-bit address (indexing) in next

instruction
67 or nil A4 (prefix) 3 Use 32-bit address (indexing) in next
instruction
37 AAA ASCII adjust AL (carry into AH) after
addition
D5 0A AAD ASCII adjust before division (AX = 10*AH +
AL)
D4 0A AAM ASCII adjust after multiply (AL/10: AH=Quo
AL=Rem)
3F AAS ASCII adjust AL (borrow from AH) after
subtraction

14 ib ADC AL,ib Add with carry immediate byte into AL

15 iv ADC eAX,iv Add with carry immediate vword into eAX
80 /2 ib ADC eb,ib Add with carry immediate byte into EA byte
10 /r ADC eb,rb Add with carry byte register into EA byte
83 /2 ib ADC ev,ib Add with carry immediate byte into EA vword
81 /2 iv ADC ev,iv Add with carry immediate vword into EA vword
11 /r ADC ev,rv Add with carry vword register into EA vword
12 /r ADC rb,eb Add with carry EA byte into byte register
13 /r ADC rv,ev Add with carry EA vword into vword register

04 ib ADD AL,ib Add immediate byte into AL

05 iv ADD eAX,iv Add immediate vword into eAX
80 /0 ib ADD eb,ib Add immediate byte into EA byte
00 /r ADD eb,rb Add byte register into EA byte
83 /0 ib ADD ev,ib Add immediate byte into EA vword
81 /0 iv ADD ev,iv Add immediate vword into EA vword
01 /r ADD ev,rv Add vword register into EA vword
02 /r ADD rb,eb Add EA byte into byte register
03 /r ADD rv,ev Add EA vword into vword register
0F 20 ADD4S N Add CL nibbles BCD, DS:SI into ES:DI (CL
even,NZ)

24 ib AND AL,ib Logical-AND immediate byte into AL

25 iv AND eAX,iv Logical-AND immediate vword into eAX
80 /4 ib AND eb,ib Logical-AND immediate byte into EA byte
20 /r AND eb,rb Logical-AND byte register into EA byte
83 /4 ib AND ev,ib Logical-AND immediate byte into EA vword
81 /4 iv AND ev,iv Logical-AND immediate vword into EA vword
21 /r AND ev,rv Logical-AND vword register into EA vword
22 /r AND rb,eb Logical-AND EA byte into byte register
23 /r AND rv,ev Logical-AND EA vword into vword register
63 /r ARPL ew,rw 2 Adjust RPL of EA word not smaller than RPL
of rw

62 /r BOUND rv,m2v 2 INT 5 if rw not between 2 vwords at [m]

inclusive
0F BC BSF rv,ev 3 Set rv to lowest position of NZ bit in ev
0F BD BSR rv,ev 3 Set rv to highest position of NZ bit in ev
0F C8+r BSWAP rd 4 Swap bytes 1,4 and 2,3 of dword register

0F BA/4 ib BT rv/m,ib 3 Set Carry flag to bit # ib of array at rv/m

0F A3/r BT rv/m,rv 3 Set Carry flag to bit # rv of array at rv/m
0F BA/7 ib BTC rv/m,ib 3 Set CF to, then compl bit ib of array at rv/m
0F BB/r BTC rv/m,rv 3 Set CF to, then compl bit rv of array at rv/m
0F BA/6 ib BTR rv/m, 3 Set CF to, then reset bit ib of array at rv/m
0F B3/r BTR rv/m,rv 3 Set CF to, then reset bit rv of array at rv/m
0F BA/5 ib BTS rv/m,ib 3 Set CF to, then set bit ib of array at rv/m
0F AB/r BTS rv/m,rv 3 Set CF to, then set bit rv of array at rv/m
6-10

9A cp CALL cp Call far segment, immediate 4- or 6-byte

address
E8 cv CALL cv Call near, offset relative to next
instruction
FF /3 CALL ep Call far segment, address at EA memory
location
FF /2 CALL ev Call near, offset absolute at EA vword
0F FF ib CALL80 ib N Call 8080-emulation code at INT number ib

98 CBW Convert byte into word (AH = top bit of AL)

99 CDQ 3 Convert dword to qword (EDX = top bit of EAX)
F8 CLC Clear carry flag
FC CLD Clear direction flag so SI and DI will
increment
FA CLI Clear interrupt enable flag; interrupts
disabled

0F 12/0 CLRBIT eb,CL N Clear bit CL of eb

0F 13/0 CLRBIT ew,CL N Clear bit CL of ew
0F 1A/0 ib CLRBIT eb,ib N Clear bit ib of eb
0F 1B/0 ib CLRBIT ew,ib N Clear bit ib of ew
0F 06 CLTS 2 Clear task switched flag
F5 CMC Complement carry flag

3C ib CMP AL,ib Subtract immediate byte from AL for flags

only
3D iv CMP eAX,iv Subtract immediate vword from eAX for flags
only
80 /7 ib CMP eb,ib Subtract immediate byte from EA byte for
flags only
38 /r CMP eb,rb Subtract byte register from EA byte for
flags only
83 /7 ib CMP ev,ib Subtract immediate byte from EA vword for
flags only
81 /7 iv CMP ev,iv Subtract immediate vword from EA vword,
flags only
39 /r CMP ev,rv Subtract vword register from EA vword for
flags only
3A /r CMP rb,eb Subtract EA byte from byte register for
flags only
3B /r CMP rv,ev Subtract EA vword from vword register for
flags only

0F 26 CMP4S N Compare CL nibbles BCD, DS:SI - ES:DI (CL

even,NZ)
A6 CMPS mb,mb Compare bytes [SI] - ES:[DI], advance SI,DI
A7 CMPS mv,mv Compare vwords [SI] - ES:[DI], advance SI,DI
A6 CMPSB Compare bytes DS:[SI] - ES:[DI], advance
SI,DI
A7 CMPSD Compare dwords DS:[SI] - ES:[DI], advance
SI,DI
A7 CMPSW Compare words DS:[SI] - ES:[DI], advance
SI,DI

0F C7 /1 CMPX8 mq 5 If EDXEAX=mq then mq:=ECXEBX, else EAXEDX:=mq

0F B0 /r CMPXCHG eb,rb 4 If AL=eb then set eb to rb, else set AL to eb
0F B1 /r CMPXCHG ev,rv 4 If eAX=ev then set ev to rv, else set eAX to
ev
0F A2 CPUID 5 If EAX=1 set EDXEAX to CPU identification
values
99 CWD Convert word to doubleword (DX = top bit of
AX)
98 CWDE 3 Sign-extend word AX to doubleword EAX
2E CS (prefix) Use CS segment for the following memory
reference

27 DAA Decimal adjust AL after addition

2F DAS Decimal adjust AL after subtraction
FE /1 DEC eb Decrement EA byte by 1
FF /1 DEC ev Decrement EA vword by 1
48+rv DEC rv Decrement vword register by 1

F6 /6 DIV eb Unsigned divide AX by EA byte (AL=Quo AH=Rem)

F7 /6 DIV ev Unsigned divide eDXeAX by EA vword (eAX=Quo
eDX=Rem)
3E DS (prefix) Use DS segment for the following memory
reference
6-11

C8 iw 00 ENTER iw,0 1 Make stack frame, iw bytes local storage, 0

levels
C8 iw 01 ENTER iw,1 1 Make stack frame, iw bytes local storage, 1
level
C8 iw ib ENTER iw,ib 1 Make stack frame, iw bytes local storage, ib
levels
26 ES (prefix) Use ES segment for the following memory
reference
F(any) Floating point set is in Chapter 7
F4 HLT Halt

F6 /7 IDIV eb Signed divide AX by EA byte (AL=Quo AH=Rem)

F7 /7 IDIV ev Signed divide eDXeAX by EA vword (eAX=Quo
eDX=Rem)
F6 /5 IMUL eb Signed multiply (AX = AL * EA byte)
F7 /5 IMUL ev Signed multiply (eDXeAX = eAX * EA vword)
0F AF /r IMUL rv,ev 3 Signed multiply ev into rv
6B /r ib IMUL rv,ib 1 Signed multiply imm byte into vword register
69 /r iv IMUL rv,iv 1 Signed multiply imm vword into vword register
69 /r iv IMUL rv,ev,iv 1 Signed multiply (rv = EA vword * imm vword)
6B /r ib IMUL rv,ev,ib 1 Signed multiply (rv = EA vword * imm byte)

E4 ib IN AL,ib Input byte from immediate port into AL

EC IN AL,DX Input byte from port DX into AL
E5 ib IN eAX,ib Input vword from immediate port into eAX
ED IN eAX,DX Input vword from port DX into eAX

FE /0 INC eb Increment EA byte by 1

FF /0 INC ev Increment EA vword by 1
40+rv INC rv Increment vword register by 1

6C INS eb,DX 1 Input byte from port DX into [DI], advance DI

6D INS ev,DX 1 Input vword from port DX into [DI], advance
DI
6C INSB 1 Input byte from port DX into ES:[DI],
advance DI
6D INSD 3 Input dword from port DX into ES:[DI],
advance DI
6D INSW 1 Input vword from port DX into ES:[DI],
advance DI

CC INT 3 Interrupt 3 (trap to debugger) (far call,

with flags
CD ib INT ib Interrupt numbered by immediate byte
pushed first)
CE INTO Interrupt 4 if overflow flag is 1
0F 08 INVD 4 Invalidate the Data Cache without writing
0F 01 /7 INVLPG m 4 Invalidate the TLB Entry that points to m
CF IRET Interrupt return (far return and pop flags)
CF IRETD 3 Interrupt return (pop EIP, ECS, Eflags)

77 cb JA cb Jump short if above (CF=0 and ZF=0)

above=UNSIGNED
73 cb JAE cb Jump short if above or equal (CF=0)
72 cb JB cb Jump short if below (CF=1)
below=UNSIGNED
76 cb JBE cb Jump short if below or equal (CF=1 or ZF=1)
72 cb JC cb Jump short if carry (CF=1)

E3 cb JCXZ cb Jump short if CX register is zero

74 cb JE cb Jump short if equal (ZF=1)
E3 cb JECXZ cb 3 Jump short if ECX register is zero
7F cb JG cb Jump short if greater (ZF=0 and SF=OF)
greater=SIGNED
7D cb JGE cb Jump short if greater or equal (SF=OF)
7C cb JL cb Jump short if less (SF>OF)
less=SIGNED
7E cb JLE cb Jump short if less or equal (ZF=1 or SF>OF)
6-12

EB cb JMP cb Jump short (signed byte relative to next

instruction)
EA cp JMP cp Jump far (4- or 6-byte immediate address)
E9 cv JMP cv Jump near (vword offset relative to next
instruction)
0F 8n cv Jcond LONG cv 3 Jump, if condition, to offset >127 away
FF /4 JMP ev Jump near to EA vword (absolute offset)
FF /5 JMP md Jump far (4-byte address in memory
doubleword)

76 cb JNA cb Jump short if not above (CF=1 or ZF=1)

72 cb JNAE cb Jump short if not above or equal (CF=1)
73 cb JNB cb Jump short if not below (CF=0)
77 cb JNBE cb Jump short if not below or equal (CF=0 and
ZF=0)
73 cb JNC cb Jump short if not carry (CF=0)

75 cb JNE cb Jump short if not equal (ZF=0)

7E cb JNG cb Jump short if not greater (ZF=1 or SF>OF)
7C cb JNGE cb Jump short if not greater or equal (SF>OF)
7D cb JNL cb Jump short if not less (SF=OF)
7F cb JNLE cb Jump short if not less or equal (ZF=0 and
SF=OF)

71 cb JNO cb Jump short if not overflow (OF=0)

7B cb JNP cb Jump short if not parity (PF=0)
79 cb JNS cb Jump short if not sign (SF=0)
75 cb JNZ cb Jump short if not zero (ZF=0)
70 cb JO cb Jump short if overflow (OF=1)

7A cb JP cb Jump short if parity (PF=1)

7A cb JPE cb Jump short if parity even (PF=1)
7B cb JPO cb Jump short if parity odd (PF=0)
78 cb JS cb Jump short if sign (SF=1)
74 cb JZ cb Jump short if zero (ZF=1)

9F LAHF Load: AH = flags SF ZF xx AF xx PF xx CF

0F 02 /r LAR rv,ew 2 Load: high(rw) = Access Rights byte,
selector ew
C5 /r LDS rv,ep Load EA pointer into DS and vword register
8D /r LEA rv,m Calculate EA offset given by m, place in rv
C9 LEAVE 1 Set SP to BP, then POP BP (reverses previous
ENTER)
C4 /r LES rv,ep Load EA pointer into ES and vword register
0F B4 /r LFS rv,ep 3 Load EA pointer into FS and vword register

0F 01 /2 LGDT m 2 Load 6 bytes at m into Global Descriptor

Table reg
0F B5 /r LGS rv,ep 3 Load EA pointer into GS and vword register
0F 01 /3 LIDT m 2 Load 6 bytes into Interrupt Descriptor Table
reg
0F 00 /2 LLDT ew 2 Load selector ew into Local Descriptor Table
reg
0F 01 /6 LMSW ew 2 Load EA word into Machine Status Word
F0 LOCK (prefix) Assert BUSLOCK signal for the next
instruction

0F 33/r LODBITS rb,rb N Load AX with DS:SI,bit rb (incr. SI,rb),

rb+1 bits
0F 3B/0 ib LODBITS rb,ib N Load AX with DS:SI,bit rb (incr. SI,rb),
ib+1 bits
AC LODS mb Load byte [SI] into AL, advance SI
AD LODS mv Load vword [SI] into eAX, advance SI
AC LODSB Load byte [SI] into AL, advance SI
AD LODSD Load dword [SI] into EAX, advance SI
AD LODSW Load word [SI] into AX, advance SI
6-13

E2 cb LOOP cb noflags DEC CX; jump short if CX>0

E1 cb LOOPE cb noflags DEC CX; jump short if CX>0 and equal
(ZF=1)
E0 cb LOOPNE cb noflags DEC CX; jump short if CX>0 and not
equal
E0 cb LOOPNZ cb noflags DEC CX; jump short if CX>0 and ZF=0
E1 cb LOOPZ cb noflags DEC CX; jump short if CX>0 and zero
(ZF=1)

0F 03 /r LSL rv,ev 2 Load: rv = Segment Limit, selector ew

0F B2 /r LSS rv,ep 3 Load EA pointer into SS and vword register
0F 00 /3 LTR ew 2 Load EA word into Task Register

A0 iv MOV AL,xb Move byte variable (offset iv) into AL

A1 iv MOV eAX,xv Move vword variable (offset iv) into eAX
0F 22 /4 MOV CR4,rd 5 Move rd into control register 4
0F 22 /n MOV CRn,rd 3 Move rd into control register n (=0,2, or 3)

0F 23 /n MOV DRn,rd 3 Move rd into debug register n (=0,1,2,3)

0F 23 /n MOV DRn,rd 3 Move rd into debug register n (=6,7)
0F 26 /n MOV TRn,rd 3 Move rd into test register TRn (=6,7)

C6 /0 ib MOV eb,ib Move immediate byte into EA byte

88 /r MOV eb,rb Move byte register into EA byte
C7 /0 iv MOV ev,iv Move immediate vword into EA vword
89 /r MOV ev,rv Move vword register into EA vword

8C /r MOV ew,segreg Move segment register into EA word

B0+rb ib MOV rb,ib Move immediate byte into byte register
8A /r MOV rb,eb Move EA byte into byte register
0F 20 /4 MOV rd,CR4 5 Move control register 4 into rd
0F 20 /n MOV rd,CRn 3 Move control register n (=0,2, or 3) into rd

0F 21 /n MOV rd,DRn 3 Move debug register n (=0,1,2,3) into rd

0F 21 /n MOV rd,DRn 3 Move debug register n (=6,7) into rd
0F 24 /n MOV rd,TRn 3 Move test register TRn (=6,7) into rd
B8+rw iv MOV rv,iv Move immediate vword into vword register
8B /r MOV rv,ev Move EA vword into vword register

8E /r MOV segreg,mw Move EA word into segment register (except

CS)
A2 iv MOV xb,AL Move AL into byte variable (offset iv)
A3 iv MOV xv,eAX Move eAX into vword register (offset iv)
A4 MOVS mb,mb Move byte [SI] to ES:[DI], advance SI,DI
A5 MOVS mv,mv Move vword [SI] to ES:[DI], advance SI,DI
A4 MOVSB Move byte DS:[SI] to ES:[DI], advance SI,DI
A5 MOVSD 3 Move dword DS:[SI] to ES:[DI], advance SI,DI
A5 MOVSW Move word DS:[SI] to ES:[DI], advance SI,DI

0F BF /r MOVSX rd,ew 3 Move word to dword, with sign-extend

0F BE /r MOVSX rv,eb 3 Move byte to vword, with sign-extend
0F B7 /r MOVZX rd,ew 3 Move word to dword, with zero-extend
0F B6 /r MOVZX rv,eb 3 Move byte to vword, with zero-extend
8C /r MOVZX rw,seg 3 Move segment register into EA word
6-14

F6 /4 MUL eb Unsigned multiply (AX = AL * EA byte)

F7 /4 MUL ev Unsigned multiply (eDXeAX = eAX * EA vword)
F6 /3 NEG eb Two's complement negate EA byte
F7 /3 NEG ev Two's complement negate EA vword
NIL (prefix) Special "do-nothing" opcode assembles no code
90 NOP No Operation

F6 /2 NOT eb Reverse each bit of EA byte

F7 /2 NOT ev Reverse each bit of EA word
0F 16/0 NOTBIT eb,CL N Complement bit CL of eb
0F 17/0 NOTBIT ew,CL N Complement bit CL of ew
0F 1E/0 ib NOTBIT eb,ib N Complement bit ib of eb
0F 1F/0 ib NOTBIT ew,ib N Complement bit ib of ew

66 or nil O2 (prefix) 3 Use 16-bit data operand in the next

instruction
66 or nil O4 (prefix) 3 Use 32-bit data operand in the next
instruction
0C ib OR AL,ib Logical-OR immediate byte into AL
0D iv OR eAX,iv Logical-OR immediate word into eAX
80 /1 ib OR eb,ib Logical-OR immediate byte into EA byte
08 /r OR eb,rb Logical-OR byte register into EA byte
83 /1 ib OR ev,ib Logical-OR immediate byte into EA word
81 /1 iv OR ev,iv Logical-OR immediate word into EA word
09 /r OR ev,rv Logical-OR word register into EA word
0A /r OR rb,eb Logical-OR EA byte into byte register
0B /r OR rv,ev Logical-OR EA word into word register

E6 ib OUT ib,AL Output byte AL to immediate port number ib

E7 ib OUT ib,eAX Output word eAX to immediate port number ib
EE OUT DX,AL Output byte AL to port number DX
EF OUT DX,eAX Output word eAX to port number DX
6E OUTS DX,eb 1 Output byte [SI] to port number DX, advance
SI
6F OUTS DX,ev 1 Output word [SI] to port number DX, advance
SI
6E OUTSB 1 Output byte DS:[SI] to port number DX,
advance SI
6F OUTSD 3 Output dword DS:[SI] to port number DX,
advance SI
6F OUTSW 1 Output word DS:[SI] to port number DX,
advance SI

1F POP DS Set DS to top of stack, increment SP by 2

07 POP ES Set ES to top of stack, increment SP by 2
0F A1 POP FS 3 Set FS to top of stack, increment SP by 2
0F A9 POP GS 3 Set GS to top of stack, increment SP by 2
8F /0 POP mv Set memory word to top of stack, increment
SP by 2
58+rw POP rv Set word register to top of stack, increment
SP by 2
17 POP SS Set SS to top of stack, increment SP by 2

61 POPA 1 Pop DI,SI,BP,SP,BX,DX,CX,AX (SP value is

ignored)
61 POPAD 3 Pop EDI,ESI,EBP,x,EBX,EDX,ECX,EAX (ESP ign.)
9D POPF Set flags register to top of stack,
increment SP by 2
9D POPFD 3 Set eflags reg to top of stack, incr SP by 2
 6-15

0E PUSH CS Set [SP-2] to CS, then decrement SP by 2

1E PUSH DS Set [SP-2] to DS, then decrement SP by 2
06 PUSH ES Set [SP-2] to ES, then decrement SP by 2
0F A0 PUSH FS 3 Set [SP-2] to FS, then decrement SP by 2
0F A8 PUSH GS 3 Set [SP-2] to GS, then decrement SP by 2
6A ib PUSH ib 1 Push sign-extended immediate byte
68 iv PUSH iv 1 Set [SP-2] to immediate word, then decrement
SP by 2
FF /6 PUSH mv Set [SP-2] to memory word, then decrement SP
by 2
50+rw PUSH rv Set [SP-2] to word register, then decrement
SP by 2
16 PUSH SS Set [SP-2] to SS, then decrement SP by 2

60 PUSHA 1 Push AX,CX,DX,BX,original SP,BP,SI,DI

60 PUSHAD 3 Push EAX,ECX,EDX,EBX,original ESP,EBP,ESI,EDI
9C PUSHF Set [SP-2] to flags register, then decrement
SP by 2
9C PUSHFD 3 Set [SP-4] to eflags reg, then decr SP by 4

D0 /2 RCL eb,1 Rotate 9-bit quantity (CF, EA byte) left once

D2 /2 RCL eb,CL Rotate 9-bit quantity (CF, EA byte) left CL
times
C0 /2 ib RCL eb,ib 1 Rotate 9-bit quantity (CF, EA byte) left ib
times
D1 /2 RCL ev,1 Rotate v+1-bit quantity (CF, EA word) left
once
D3 /2 RCL ev,CL Rotate v+1-bit quantity (CF, EA word) left
CL times
C1 /2 ib RCL ev,ib 1 Rotate v+1-bit quantity (CF, EA word) left
ib times

D0 /3 RCR eb,1 Rotate 9-bit quantity (CF, EA byte) right

once
D2 /3 RCR eb,CL Rotate 9-bit quantity (CF, EA byte) right CL
times
C0 /3 ib RCR eb,ib 1 Rotate 9-bit quantity (CF, EA byte) right ib
times
D1 /3 RCR ev,1 Rotate v+1-bit quantity (CF, EA word) right
once
D3 /3 RCR ev,CL Rotate v+1-bit quantity (CF, EA word) right
CL times
C1 /3 ib RCR ev,ib 1 Rotate v+1-bit quantity (CF, EA word) right
ib times
0F 32 RDMSR 5 Read Model Specific Reg #ECX to EDXEAX
0F 31 RDTSC 5 Read Time Stamp Counter to EDXEAX

F3 REP (prefix) Repeat following MOVS,LODS,STOS,INS, or OUTS

CX times
65 REPC (prefix) N Repeat following CMPS or SCAS CX times or
until CF=0
F3 REPE (prefix) Repeat following CMPS or SCAS CX times or
until ZF=0
64 REPNC (prfix) N Repeat following CMPS or SCAS CX times or
until CF=1
F2 REPNE (prfix) Repeat following CMPS or SCAS CX times or
until ZF=1
F2 REPNZ (prfix) Repeat following CMPS or SCAS CX times or
until ZF=1
F3 REPZ (prefix) Repeat following CMPS or SCAS CX times or
until ZF=0

CB RETF Return to far caller (pop offset, then seg)

C3 RET Return to near caller (pop offset only)
CA iw RETF iw RET (far), pop offset, seg, iw bytes
C2 iw RET iw RET (near), pop offset, iw bytes pushed
before Call

D0 /0 ROL eb,1 Rotate 8-bit EA byte left once

D2 /0 ROL eb,CL Rotate 8-bit EA byte left CL times
C0 /0 ib ROL eb,ib 1 Rotate 8-bit EA byte left ib times
D1 /0 ROL ev,1 Rotate 16- or 32-bit EA vword left once
D3 /0 ROL ev,CL Rotate 16- or 32-bit EA vword left CL times
C1 /0 ib ROL ev,ib 1 Rotate 16 or 32-bit EA vword left ib times
0F 28/0 ROL4 eb N Rotate nibbles: Heb=Leb HAL,Leb=LAL
LAL=Heb
6-16

D0 /1 ROR eb,1 Rotate 8-bit EA byte right once

D2 /1 ROR eb,CL Rotate 8-bit EA byte right CL times
C0 /1 ib ROR eb,ib 1 Rotate 8-bit EA byte right ib times
D1 /1 ROR ev,1 Rotate 16- or 32-bit EA vword right once
D3 /1 ROR ev,CL Rotate 16- or 32-bit EA vword right CL times
C1 /1 ib ROR ev,ib 1 Rotate 16- or 32-bit EA vword right ib times
0F 2A/0 ROR4 eb N Rotate nibbles: Leb=Heb Heb=LAL AL=eb
0F AA RSM 5 Resume from System Management mode

9E SAHF Store AH into flags SF ZF xx AF xx PF xx CF

D0 /4 SAL eb,1 Multiply EA byte by 2, once
D2 /4 SAL eb,CL Multiply EA byte by 2, CL times
C0 /4 ib SAL eb,ib 1 Multiply EA byte by 2, ib times
D1 /4 SAL ev,1 Multiply EA vword by 2, once
D3 /4 SAL ev,CL Multiply EA vword by 2, CL times
C1 /4 ib SAL ev,ib 1 Multiply EA vword by 2, ib times

D0 /7 SAR eb,1 Signed divide EA byte by 2, once

D2 /7 SAR eb,CL Signed divide EA byte by 2, CL times
C0 /7 ib SAR eb,ib 1 Signed divide EA byte by 2, ib times
D1 /7 SAR ev,1 Signed divide EA vword by 2, once
D3 /7 SAR ev,CL Signed divide EA vword by 2, CL times
C1 /7 ib SAR ev,ib 1 Signed divide EA vword by 2, ib times

1C ib SBB AL,ib Subtract with borrow immediate byte from AL

1D iv SBB eAX,iv Subtract with borrow immediate word from eAX
80 /3 ib SBB eb,ib Subtract with borrow immediate byte from EA
byte
18 /r SBB eb,rb Subtract with borrow byte register from EA
byte
83 /3 ib SBB ev,ib Subtract with borrow immediate byte from EA
word
81 /3 iv SBB ev,iv Subtract with borrow immediate word from EA
word
19 /r SBB ev,rv Subtract with borrow word register from EA
word
1A /r SBB rb,eb Subtract with borrow EA byte from byte
register
1B /r SBB rv,ev Subtract with borrow EA word from word
register

AE SCAS mb Compare bytes AL - ES:[DI], advance DI

AF SCAS mv Compare vwords eAX - ES:[DI], advance DI
AE SCASB Compare bytes AL - ES:[DI], advance DI
AF SCASD Compare dwords EAX - ES:[DI], advance DI
AF SCASW Compare words AX - ES:[DI], advance DI

0F 14/0 SETBIT eb,CL N Set bit CL of eb

0F 15/0 SETBIT ew,CL N Set bit CL of ew
0F 1C/0 ib SETBIT eb,ib N Set bit ib of eb
0F 1D/0 ib SETBIT ew,ib N Set bit ib of ew
0F 9n /r SETcond eb 3 Set eb byte to 1 if condition, 0 if not
0F 01 /0 SGDT m 2 Store 6-byte Global Descriptor Table
register to M

D0 /4 SHL eb,1 Multiply EA byte by 2, once

D2 /4 SHL eb,CL Multiply EA byte by 2, CL times
C0 /4 ib SHL eb,ib 1 Multiply EA byte by 2, ib times
D1 /4 SHL ev,1 Multiply EA word by 2, once
D3 /4 SHL ev,CL Multiply EA word by 2, CL times
C1 /4 ib SHL ev,ib 1 Multiply EA word by 2, ib times

0F A5/r SHLD ev,rv,CL 3 Set ev to high of ((ev,rv) SHL CL)

0F A4/r ib SHLD ev,rv,ib 3 Set ev to high of ((ev,rv) SHL ib)
6-17

D0 /5 SHR eb,1 Unsigned divide EA byte by 2, once

D2 /5 SHR eb,CL Unsigned divide EA byte by 2, CL times
C0 /5 ib SHR eb,ib 1 Unsigned divide EA byte by 2, ib times
D1 /5 SHR ev,1 Unsigned divide EA word by 2, once
D3 /5 SHR ev,CL Unsigned divide EA word by 2, CL times
C1 /5 ib SHR ev,ib 1 Unsigned divide EA word by 2, ib times
0F AD/r SHRD ev,rv,CL 3 Set ev to low of ((rv,ev) SHR CL)
0F AC/r ib SHRD ev,rv,ib 3 Set ev to low of ((rv,ev) SHR ib)

0F 01 /1 SIDT m 2 Store 6-byte Interrupt Descriptor Table

register to M
0F 00 /0 SLDT ew 2 Store Local Descriptor Table register to EA
word
0F 01 /4 SMSW ew 2 Store Machine Status Word to EA word
36 SS Use SS segment for the following memory
reference
F9 STC Set carry flag
FD STD Set direction flag so SI and DI will
decrement
FB STI Set interrupt enable flag, interrupts enabled

0F 31/r STOBITS rb,rb N Store AX to ES:DI,bit rb (incr. DI,rb), rb+1

bits
0F 39/0 ib STOBITS rb,ib N Store AX to ES:DI,bit rb (incr. DI,rb), ib+1
bits
AA STOS mb Store AL to byte [DI], advance DI
AB STOS mv Store eAX to word [DI], advance DI
AA STOSB Store AL to byte ES:[DI], advance DI
AB STOSD Store EAX to dword ES:[DI], advance DI
AB STOSW Store AX to word ES:[DI], advance DI
0F 00 /1 STR ew 2 Store Task Register to EA word

2C ib SUB AL,ib Subtract immediate byte from AL

2D iv SUB eAX,iv Subtract immediate word from eAX
80 /5 ib SUB eb,ib Subtract immediate byte from EA byte
28 /r SUB eb,rb Subtract byte register from EA byte
83 /5 ib SUB ev,ib Subtract immediate byte from EA word
81 /5 iv SUB ev,iv Subtract immediate word from EA word
29 /r SUB ev,rv Subtract word register from EA word
2A /r SUB rb,eb Subtract EA byte from byte register
2B /r SUB rv,ev Subtract EA word from word register
0F 22 SUB4S N Sub CL nibbles BCD, DS:SI - ES:DI (CL
even,NZ)

A8 ib TEST AL,ib AND immediate byte into AL for flags only

A9 iv TEST eAX,iv AND immediate word into eAX for flags only
F6 /0 ib TEST eb,ib AND immediate byte into EA byte for flags
only
84 /r TEST eb,rb AND byte register into EA byte for flags only
F7 /0 iv TEST ev,iv AND immediate word into EA word for flags
only
85 /r TEST ev,rv AND word register into EA word for flags only
84 /r TEST rb,eb AND EA byte into byte register for flags only
85 /r TEST rv,ev AND EA word into word register for flags only

0F 10/0 TESTBIT eb,CL N Test bit CL of eb, set Z flag

0F 11/0 TESTBIT ev,CL N Test bit CL of ew, set Z flag
0F 18/0 ib TESTBIT eb,ib N Test bit ib of eb, set Z flag
0F 19/0 ib TESTBIT ew,ib N Test bit ib of ew, set Z flag
6-18

0F 00 /4 VERR ew 2 Set ZF=1 if segment can be read, selector ew

0F 00 /5 VERW ew 2 Set ZF=1 if segment can be written to,
selector ew
9B WAIT Wait until BUSY pin is inactive (HIGH)
0F 09 WBINVD 4 Write Back and Invalidate the Data Cache
0F 30 WRMSR 5 Write EDXEAX to Model Specific Reg #ECX
0F C0 /r XADD eb,rb 4 Exchange eb with rb then add into new eb
0F C1 /r XADD ev,rv 4 Exchange ev with rv then add into new ev

9r XCHG eAX,rv Exchange word register with eAX

86 /r XCHG eb,rb Exchange byte register with EA byte
87 /r XCHG ev,rv Exchange word register with EA word
86 /r XCHG rb,eb Exchange EA byte with byte register
9r XCHG rv,eAX Exchange with word register
87 /r XCHG rv,ev Exchange EA word with word register

D7 XLAT mb Set AL to memory byte [BX + unsigned AL]

D7 XLATB Set AL to memory byte DS:[BX + unsigned AL]
34 ib XOR AL,ib Exclusive-OR immediate byte into AL
35 iv XOR eAX,iv Exclusive-OR immediate word into eAX
80 /6 ib XOR eb,ib Exclusive-OR immediate byte into EA byte
30 /r XOR eb,rb Exclusive-OR byte register into EA byte
83 /6 ib XOR ev,ib Exclusive-OR immediate byte into EA word
81 /6 iv XOR ev,iv Exclusive-OR immediate word into EA word
31 /r XOR ev,rv Exclusive-OR word register into EA word
32 /r XOR rb,eb Exclusive-OR EA byte into byte register
33 /r XOR rv,ev Exclusive-OR EA word into word register

"N" next to the instruction description means that instruction works

only
on NEC chips. A digit x means that instruction works only on the x86
or later processor. See the note just before the chart.

CHAPTER 7 THE FLOATING-POINT PROCESSOR

In this chapter, we'll refer to the various Central Processing

Units (CPUs) as the "86". Thus "86" refers to either the 8088,
8086, 80186, 80286, etc. We'll refer to the various coprocessors
as the "87". Thus "87" refers to either the 8087, the 287, the
387, or the special IIT-2C87 processor.

The 8087 and 287 Coprocessors

All IBM-PC's, and most clones, contain a socket for a floating

point coprocessor. If you shell out between $70 and $200, and
plug the appropriate chip into that socket, then a host of
floating point instructions is added to the assembly language
instruction set. The 486 DX series has the floating-point
processor built into the main CPU chip.

The original IBM-PC, and the XT, accept the original floating
point chip, the 8087. Later processors accept corresponding
chips: the 287 for the 286, the 387 for the 386, etc. From a
programming standpoint, the 8087 and 287 are nearly identical:
the 287 adds the instructions FSETPM and FSTSW AX, and ignores
the instructions FENI and FDISI. There is, however, a rather
nasty design flaw in the 8087, that was corrected in the 287.

To understand the flaw, you must understand how the 86 and 87

work as coprocessors. Whenever the 86 sees a floating point
instruction, it communicates the instruction, and any associated
memory operands, to the 87. Then the 86 goes on to its next
instruction, operating in parallel with the 87. That's OK, so
long as the following instructions don't do one of the following:

1. Execute another floating point instruction; or

2. Try to read the results of the still-executing floating

point instruction.

If they do, then you must provide an instruction called WAIT (or
synonymously FWAIT), which halts the 86 until the 87 is finished.
For almost all floating point instructions, it should not be
necessary to provide an explicit FWAIT; the 86 ought to know that
it should wait. For the 8087, it IS necessary to give an
explicit FWAIT before each floating point instruction: that is
the flaw.

Because of the flaw, all assemblers supporting the 8087 will

silently insert an FWAIT code (hex 9B) before all 87
instructions, except those few (the FN instructions other than
FNOP) not requiring the FWAIT. A86 will insert the opcode as
well, when it is assembling for the original 8087.
7-2

The are three ways to tell A86 whether it is assembling for an

8087 or a 287-or-later processor. First, A86 will use a default
for the processor on which it is currently assembling: no .287
for an 8086, 8088, 186, or NEC; .287 for a 286 or later. Second,
this can be overridden by the switch +F (the F must be
capitalized), to signal that the 287 is the target processor, or
-F to specify the 8087. Third, an 8087 setting can be further
overridden in the source code, with the directive ".287",
compatible with Microsoft's assembler.

When A86 is assembling for the 287 or later, it ceases outputting

FWAIT directives that are unnecessary for the 287, ignores the
instructions FENI, FDISI, FNENI, and FNDISI, and honors the
instructions FSETPM and FSTSW AX.

WARNING: The most common mistake 87 programmers make is to try to

read the results of an 87 operation in 86 memory, before the
results are ready. At least on my computer, the system often
crashes when you do this! If your program runs correctly when
single stepped, but crashes when set loose, then chances are you
need an extra explicit FWAIT somewhere.

Extra Coprocessor Support

A86 now supports two additional coprocessors available for

PC-compatibles: the 80387, available for 386-based machines, and
the IIT-2C87, a 287-plug-compatible chip that adds a couple of
unique instructions. The IIT-2C87 has two extra banks of on-chip
8-number stacks, that can be switched in with the FBANK
instruction, and a matrix multiply instrction that uses all three
banks as input. (For details contact Specialty Software
Development Corp., 110 Wild Basin Road, Austin TX 78746.) Both
chips incorporate the correction to the 8087's FWAIT design flaw,
so you can assemble with the .287 directive. The extra
instructions for these chips are marked by "387 only:" and "IIT
only:" in the chart at the end of this chapter.

Emulating the 8087 by Software

There is a software package provided with many compilers

(Borland's Turbo C and most Microsoft compilers, for example)
that emulates the 8087 instruction set. The emulator is very
cleverly implemented so that the programmer need not know whether
a floating point chip will be available, or whether emulation
will be necessary. This is done by having the linker replace all
floating point machine instructions with INT calls to certain
interrupts, dedicated to emulation. The interrupt handlers
interpret the operands to the instructions, and emulate the 8087.
7-3

You can tell A86 that the emulator might be used, by providing a
+f switch in the invocation line, or in the A86 environment
variable (make sure the f is lower case). Since your program
will be linked to the emulator, you must be producing an OBJ
file, not a COM file, for emulation support to take effect.
Whenever a floating point instruction is assembled, A86 will
generate an external reference at the opcode for the instruction.
Then, if the emulation package is linked with your program, the
opcodes will be replaced by the INT calls. If a special
non-emulation module is linked, the opcodes will be left alone,
and the floating point instructions will be executed directly.

For the later processors (286 and beyond), emulation can be

provided that executes when the floating-point instructions
themselves are seen, so the +f games are not necessary.

The Floating Point Stack

The 87 has its own register set, of 8 floating point numbers

occupying 10 bytes each, plus 14 bytes of status and control
information. Many of the 87's instructions cause the numbers to
act like a stack, much like a Hewlett-Packard calculator. For
this reason, the numbers are called the floating point stack.

The standard name for the top element of the floating point stack
is either ST or ST(0); the others are named ST(1) through ST(7).
Thus, for example, the instruction to add stack element number 3
into the top stack element is usually coded FADD ST,ST(3).

I find this notation painfully verbose. Especially bad are the

parentheses, which are hard to type, and which add visual clutter
to the program. To alleviate this problem while retaining
language compatibility, I name my stack elements simply 0 through
7. I recognize ST as a synonym for 0. I allow expression
elements to be concatenated; concatenation is the same as
addition. Thus, when A86 sees ST(3), it computes 0+3 = 3. So
you can code the old way, FADD ST,ST(3), or you can code the
concise way, FADD 0,3 or simply FADD 3.

Floating Point Initializations

In general, you use the 87 by loading numbers from 86 memory to

the 87 stack (using FLD instructions), calculating on the 87
stack, and storing the results back to 86 memory (using FST and
FSTP instructions). There are seven constant numbers built into
the 87 instruction set: zero, one, Pi, and four logarithmic
conversion constants. These can be loaded using the FLD0, FLD1,
FLDPI, FLDL2T, FLDL2E, FLDLG2, and FLDLN2 instructions. All
other constants must be declared in, then loaded from, 86 memory.
Integer constant words and doublewords can be loaded via FILD.
Non-integer constant doubleword, quadwords, and ten-byte numbers
can be loaded via FLD.
7-4

A86 allows you to declare constants loaded via FLD as floating

point numbers, using scientific notation if you like. As an
exclusive feature, A86 allows you to use any of the 4 arithmetic
functions +, -, *, / in expressions involving floating point
numbers. A86 will even do type conversion if one of the two
operands is given as an integer; though for clarity I recommend
that you always give floating point constants with their decimal
point.

Built-In Constant Names

A86 offers another exclusive feature: the built-in symbols

PI ratio of circumference to diameter of a circle

L2T log base 2 of 10

L2E log base 2 of the calculus constant e = 2.71828...

LG2 log base 10 of 2

LN2 natural log (base e) of 2

You can use these symbols in expressions, to declare useful

constants. For example, you can declare the degrees-to-radians
conversion constant:

DEG_TO_RAD DT PI/180.

Special Immediate FLD Form

Yet another exclusive A86 feature is the instruction form FLD

constant. This form is intended primarily to facilitate "fooling
around" with the 87 when using D86; but it is also useful for
quick-and-dirty programs. For example, the instruction FLD 12.3
generates the following sequence of code bytes (without
explicitly using the local labels given):

CS FLD T[M1]
JMP >M2
M1 DT 12.3
M2:

Obviously, this form is not terrifically efficient: you can

always save the JMP by placing the constant outside of the
instruction stream; and the CS override might not be needed. But
the form is very, very convenient!
NOTE that the preceding 2 sections imply that you can get
careless and code, for example, FLD PI when you intended FLDPI.
Though the two are functionally equivalent, the first form takes
a whopping 17 bytes; and second, only 2 bytes. Be careful!
7-5

Floating Point Operand Types

The list of floating point instructions contains a variety of

operand types. Here is a brief explanation of those types:

0 stands for the top element of the floating point stack.

A synonym for 0 is ST or ST(0).

i stands for element number i of the floating point stack.

i can range from 0 through 7. A synonym for i is ST(i).

mem10r is a 10-byte memory quantity (typically declared with a

DT directive) containing a full precision floating point
number. Intel recommends that you NOT store your numbers
in full precision; that you use the following double
precision format instead. Full precision numbers are
intended for storage of intermediate results (on the
stack); they exist to insure maximum accuracy for
calculations on double precision numbers, which is the
official external format of 87 numbers.

mem8r is an 8-byte memory quantity (typically declared with a

DQ directive) containing a double precision floating
point number. This is the best format for floating
point numbers on the 87. The 87 takes the same amount
of time on double precision calculations as it does on
single precision. The only extra time is the memory
access of 4 more bytes; negligible in comparison to the
calculation time.

mem4r is a 4-byte quantity (typically defined with a DD

directive) containing a single precision floating point
number.

mem10d is a 10-byte quantity (also defined via DT) containing a

special Binary Coded Decimal format recognized by the
FBLD and FBSTP instructions. This format is useful for
input and output of floating point numbers.

mem4i is a 4-byte quantity representing a signed integer in

two's-complement notation.

mem2i is a 2-byte quantity representing a signed integer in

two's-complement notation.

mem14 and mem94 are 14- and 94-byte buffers containing the 87
machine state.
7-6

Operand Choices in A86

In the "standard" assembly language, the choice of operands for
floating point instructions seems inconsistent to me. For
example, to subtract stack i from 0, you must provide two
operands; to do the equivalent comparison, you must provide only
one operand. A86 smooths out these inconsistencies by allowing
more choices for operands: FADD i is equivalent to FADD 0,i. FCOM
0,i is equivalent to FCOM i. The same holds for the other main
arithmetic instructions. FXCH 0,i and FXCH i,0 are allowed. So
if you wish to retain compatibility with other assemblers, you
should use their more restrictive instruction list, not the
following one.

The 87 Instruction Set

Following is the 87 instruction set. The "w" in the opcode field

is the FWAIT opcode, hex 9B, which is suppressed if .287 is
selected. Again, "0", "1", and "i" stand for the associated
floating point stack registers, not constant numbers! Constant
numbers in the descriptions are given with decimal points: 0.0,
1.0, 2.0, 10.0.

Opcode Instruction Description

w D9 F0 F2XM1 0 := (2.0 ** 0) - 1.0

w DB F1 F4X4 IIT only: 4 by 4 matrix multiply
w D9 E1 FABS 0 := |0|
w DE C1 FADD 1 := 1 + 0, pop
w D8 C0+i FADD i 0 := i + 0
w DC C0+i FADD i,0 i := i + 0
w D8 C0+i FADD 0,i 0 := i + 0
w D8 /0 FADD mem4r 0 := 0 + mem4r
w DC /0 FADD mem8r 0 := 0 + mem8r
w DE C0+i FADDP i,0 i := i + 0, pop
w DB E8 FBANK 0 IIT only: set bank pointer to default
w DB EB FBANK 1 IIT only: set bank pointer to bank 1
w DB EA FBANK 2 IIT only: set bank pointer to bank 2
w DF /4 FBLD mem10d push, 0 := mem10d
w DF /6 FBSTP mem10d mem10d := 0, pop

w D9 E0 FCHS 0 := -0
9B DB E2 FCLEX clear exceptions
w D8 D1 FCOM compare 0 - 1
w D8 D0+i FCOM 0,i compare 0 - i
w D8 D0+i FCOM i compare 0 - i
w D8 /2 FCOM mem4r compare 0 - mem4r
w DC /2 FCOM mem8r compare 0 - mem8r
7-7

w D8 D9 FCOMP compare 0 - 1, pop

w D8 D8+i FCOMP 0,i compare 0 - i, pop
w D8 D8+i FCOMP i compare 0 - i, pop
w D8 /3 FCOMP mem4r compare 0 - mem4r, pop
w DC /3 FCOMP mem8r compare 0 - mem8r, pop
w DE D9 FCOMPP compare 0 - 1, pop both
w D9 FF FCOS 387 only: 0 := cosine(0)

w D9 F6 FDECSTP decrement stack pointer

w DB E1 FDISI disable interrupts (.287 ignore)

w DE F9 FDIV 1 := 1 / 0, pop
w D8 F0+i FDIV i 0 := 0 / i
w DC F8+i FDIV i,0 i := i / 0
w D8 F0+i FDIV 0,i 0 := 0 / i
w D8 /6 FDIV mem4r 0 := 0 / mem4r
w DC /6 FDIV mem8r 0 := 0 / mem8r

w DE F8+i FDIVP i,0 i := i / 0, pop

w DE F1 FDIVR 1 := 0 / 1, pop
w D8 F8+i FDIVR i 0 := i / 0
w DC F0+i FDIVR i,0 i := 0 / i
w D8 F8+i FDIVR 0,i 0 := i / 0
w D8 /7 FDIVR mem4r 0 := mem4r / 0
w DC /7 FDIVR mem8r 0 := mem8r / 0
w DE F0+i FDIVRP i,0 i := 0 / i, pop

w DB E0 FENI enable interrupts (.287 ignore)

w DD C0+i FFREE i empty i

w DE /0 FIADD mem2i 0 := 0 + mem4i

w DA /0 FIADD mem4i 0 := 0 + mem2i
w DE /2 FICOM mem2i compare 0 - mem2i
w DA /2 FICOM mem4i compare 0 - mem4i
w DE /3 FICOMP mem2i compare 0 - mem2i, pop
w DA /3 FICOMP mem4i compare 0 - mem4i, pop

w DE /6 FIDIV mem2i 0 := 0 / mem2i

w DA /6 FIDIV mem4i 0 := 0 / mem4i
w DE /7 FIDIVR mem2i 0 := mem2i / 0
w DA /7 FIDIVR mem4i 0 := mem4i / 0
w DF /0 FILD mem2i push, 0 := mem2i
w DB /0 FILD mem4i push, 0 := mem4i
w DF /5 FILD mem8i push, 0 := mem8i

w DE /1 FIMUL mem2i 0 := 0 * mem2i

w DA /1 FIMUL mem4i 0 := 0 * mem4i

w D9 F7 FINCSTP increment stack pointer

9B DB E3 FINIT initialize 87
w DF /2 FIST mem2i mem2i := 0
w DB /2 FIST mem4i mem4i := 0
w DF /3 FISTP mem2i mem2i := 0, pop
w DB /3 FISTP mem4i mem4i := 0, pop
w DF /7 FISTP mem8i mem8i := 0, pop
7-8

w DE /4 FISUB mem2i 0 := 0 - mem2i

w DA /4 FISUB mem4i 0 := 0 - mem4i
w DE /5 FISUBR mem2i 0 := mem2i - 0
w DA /5 FISUBR mem4i 0 := mem4i - 0

w D9 C0+i FLD i push, 0 := old i

w DB /5 FLD mem10r push, 0 := mem10r
w D9 /0 FLD mem4r push, 0 := mem4r
w DD /0 FLD mem8r push, 0 := mem8r
w D9 E8 FLD1 push, 0 := 1.0
w D9 /5 FLDCW mem2i control word := mem2i

w D9 /4 FLDENV mem14 environment := mem14

w D9 EA FLDL2E push, 0 := log base 2.0 of e
w D9 E9 FLDL2T push, 0 := log base 2.0 of 10.0
w D9 EC FLDLG2 push, 0 := log base 10.0 of 2.0
w D9 ED FLDLN2 push, 0 := log base e of 2.0
w D9 EB FLDPI push, 0 := Pi
w D9 EE FLDZ push, 0 := +0.0

w DE C9 FMUL 1 := 1 * 0, pop
w D8 C8+i FMUL i 0 := 0 * i
w DC C8+i FMUL i,0 i := i * 0
w D8 C8+i FMUL 0,i 0 := 0 * i
w D8 /1 FMUL mem4r 0 := 0 * mem4r
w DC /1 FMUL mem8r 0 := 0 * mem8r
w DE C8+i FMULP i,0 i := i * 0, pop

DB E2 FNCLEX nowait clear exceptions

DB E1 FNDISI disable interrupts (.287 ignore)
DB E0 FNENI enable interrupts (.287 ignore)
DB E3 FNINIT nowait initialize 87
w D9 D0 FNOP no operation

DD /6 FNSAVE mem94 mem94 := 87 state

D9 /7 FNSTCW mem2i mem2i := control word
D9 /6 FNSTENV mem14 mem14 := environment
DF E0 FNSTSW AX AX := status word
DD /7 FNSTSW mem2i mem2i := status word
w D9 F3 FPATAN 0 := arctan(1/0), pop

w D9 F8 FPREM 0 := REPEAT(0 - 1)
w D9 F5 FPREM1 387 only: 0 := REPEAT(0 - 1) IEEE compat.
w D9 F2 FPTAN push, 1/0 := tan(old 0)

w D9 FC FRNDINT 0 := round(0)
w DD /4 FRSTOR mem94 87 state := mem94
w DD /6 FSAVE mem94 mem94 := 87 state
w D9 FD FSCALE 0 := 0 * 2.0 ** 1
9B DB E4 FSETPM set protection mode
w D9 FE FSIN 387 only: 0 := sine(0)
w D9 FB FSINCOS 387 only: push, 1 := sine, 0 := cos(old 0)
w D9 FA FSQRT 0 := square root of 0
7-9

w DD D0+i FST i i := 0
w D9 /2 FST mem4r mem4r := 0
w DD /2 FST mem8r mem8r := 0
w D9 /7 FSTCW mem2i mem2i := control word
w D9 /6 FSTENV mem14 mem14 := environment
w DD D8+i FSTP i i := 0, pop
w DB /7 FSTP mem10r mem10r := 0, pop
w D9 /3 FSTP mem4r mem4r := 0, pop
w DD /3 FSTP mem8r mem8r := 0, pop
w DF E0 FSTSW AX AX := status word
w DD /7 FSTSW mem2i mem2i := status word

w DE E9 FSUB 1 := 1 - 0, pop
w D8 E0+i FSUB i 0 := 0 - i
w DC E8+i FSUB i,0 i := i - 0
w D8 E0+i FSUB 0,i 0 := 0 - i
w D8 /4 FSUB mem4r 0 := 0 - mem4r
w DC /4 FSUB mem8r 0 := 0 - mem8r
w DE E8+i FSUBP i,0 i := i - 0, pop
w DE E1 FSUBR 1 := 0 - 1, pop
w D8 E8+i FSUBR i 0 := i - 0
w DC E0+i FSUBR i,0 i := 0 - i
w D8 E8+i FSUBR 0,i 0 := i - 0
w D8 /5 FSUBR mem4r 0 := mem4r - 0
w DC /5 FSUBR mem8r 0 := mem8r - 0
w DE E0+i FSUBRP i,0 i := 0 - i, pop

w D9 E4 FTST compare 0 - 0.0

w DD E0+i FUCOM i 387 only: unordered compare 0 - i
w DD E1 FUCOM 387 only: unordered compare 0 - 1
w DD E8+i FUCOMP i 387 only: unordered compare 0 - i, pop
w DD E9 FUCOMP 387 only: unordered compare 0 - 1, pop
w DA E9 FUCOMPP 387 only: unordered compare 0 - 1, pop
both

9B FWAIT wait for 87 ready

w D9 E5 FXAM C3 -- C0 := type of 0
w D9 C9 FXCH exchange 0 and 1
w D9 C8+i FXCH 0,i exchange 0 and i
w D9 C8+i FXCH i exchange 0 and i
w D9 C8+i FXCH i,0 exchange 0 and i
w D9 F4 FXTRACT push, 1 := expo, 0 := sig
w D9 F1 FYL2X 0 := 1 * log base 2.0 of 0, pop
w D9 F9 FYL2XP1 0 := 1 * log base 2.0 of (0+1.0), pop

CHAPTER 8 NUMBERS AND EXPRESSIONS

Numbers and Bases

A86 supports a variety of formats for numbers. In non-computer

life, we write numbers in a decimal format. There are ten
digits, 0 through 9, that we use to describe numbers; and each
digit position is ten times as significant as the position to its
right. The number ten is called the "base" of the decimal
format. Computer programmers often find it convenient to use
other bases to specify numbers used in their programs. The most
commonly-used bases are two (binary format), sixteen (hexadecimal
format), and eight (octal format).

The hexadecimal format requires sixteen digits. The extra six

digits beyond 0 through 9 are denoted by the first six letters of
the alphabet: A for ten, B for eleven, C for twelve, D for
thirteen, E for fourteen, and F for fifteen.

In A86, a number must always begin with a digit from 0 through 9,

even if the base is hexadecimal. This is so that A86 can
distinguish between a number and a symbol that happens to have
digits in its name. If a hexadecimal number would begin with a
letter, you precede the letter with a zero. For example, hex A0,
which is the same as decimal 160, would be written 0A0.

Because it is necessary for you to append leading zeroes to many

hex numbers, and because you never have to do so for decimal
numbers, I decided to make hexadecimal the default base for
numbers with leading zeroes. Decimal is still the default base
for numbers beginning with 1 through 9.

Large numbers can be given as the operands to DD, DQ, or DT

directives. For readability, you may freely intersperse
underscore characters anywhere with your numbers.

The default base can be overridden, with a letter or letters at

the end of the number: B or xB for binary, O or Q for octal, H
for hexadecimal, and D or xD for decimal. Examples:

077Q octal, value is 8*7 + 7 = 63 in decimal notation

123O octal if the "O" is a letter: 64 + 2*8 + 3 = 83 decimal
1230 decimal 1230: shows why you should use "Q" for octal!!
01234567H large constant
0001_0000_0000_0000_0003R real number specified in hexadecimal
100D superfluous D indicates decimal base
0100D hex number 100D, which is 4096 + 13 = 5009 in decimal
0100xD decimal 100, since xD overrides the default hex format
0110B hex 110B, which is 4096 + 256 + 11 = 4363 in decimal
0110xB binary 4+2 = 6 in decimal notation
110B also binary 4+2 = 6, since "B" is not a decimal digit
8-2

The last five examples above illustrate why an "x" is sometimes

necessary before the base-override letter "B" or "D". If that
letter can be interpreted as a hex digit, it is; the "x" forces
an override interpretation for the "B" or "D". By the way, the
usage of lower case for x and upper case for the following
override letter is simply a recommendation; A86 treats upper-and
lower-case letters equivalently.

A86 also accepts a "base" of K. The number preceding the K is

interpreted as a decimal number which is multplied by 1024.
Thus, 2K is 2048, 16K is 16384, etc.

The RADIX Directive

The above-mentioned set of defaults (hex if leading zero, decimal

otherwise) can be overridden with the RADIX (or, for
compatibility, .RADIX) directive. The RADIX directive consists
of the word RADIX followed by a number from 2 to 16. The default
base for the number is ALWAYS decimal, regardless of any (or no)
previous RADIX commands. The number gives the default base for
ALL subsequent numbers, up to (but not including) the next RADIX
command. If there is no number following RADIX, then A86 returns
to its initial mixed default of hex for leading zeroes, decimal
for other leading digits.

As an alternative to the RADIX directive, I provide the D switch,

which causes A86 to start with decimal defaults. You can put +D
into the A86 command invocation, or into the A86 environment
variable. The first RADIX command in the program will override
the D switch setting.

Following are examples of radix usage. The numbers in the

comments are all in decimal notation.

DB 10,010 ; produces 10,16 if RADIX was not seen yet

; and +D switch was not specified
RADIX 10
DB 10,010 ; produces 10,10
RADIX 16
DB 10,010 ; produces 16,16
RADIX 3 ; for Martian programmers in Heinlein novels
DB 10,100 ; produces 3,9
RADIX
DB 10,010 ; produces 10,16
8-3

Floating Point Initializations

A86 allows floating point numbers as the operands to DD, DQ, and
DT directives. The numbers are encoded according to the IEEE
standard, followed by the 8087 and 287 coprocessors. The format
for floating point constants is as follows: First, there is a
decimal number containing a decimal point. There must be a
decimal point, or else the number is interpreted as an integer.
There must also be at least one decimal digit, either to the left
or right of the decimal point, or else the decimal point is
interpreted as an addition (structure element) operator.
Optionally, there may follow immediately after the decimal number
the letter E followed by a decimal number. The E stands for
"exponent", and means "times 10 raised to the power of". You may
provide a + or - between the E and its number. Examples:

0.1 constant one-tenth

.1 the same
300. floating point three hundred
30.E1 30 * 10**1; i.e., three hundred
30.E+1 the same
30.E-1 30 * 10**-1; i.e., three
30E1 not floating point: hex integer 030E1
1.234E20 scientific notation: 1.234 times 10 to the 20th
1.234E-20 a tiny number: 1.234 divided by 10 to the 20th

Overview of Expressions
Most of the operands that you code into your instructions and
data initializations will be simple register names, variable
names, or constants. However, you will regularly wish to code
operands that are the results of arithmetic calculations,
performed either by the machine when the program is running (for
indexing), or by the assembler (to determine the value to
assemble into the program). A86 has a full set of operators that
you can use to create expressions to cover these cases. They are
given in the "Descriptions of Operators and Specifiers" section
later in this chapter.

Types of Expression Operands

Numbers and Label Addresses

A number or constant (16-bit number) can be used in most

expressions. A label (defined with a colon) is also treated as
a constant and so can be used in expressions.

Variables

A variable stands for a byte- or word-memory location. You may

add or subtract constants from variables; when you do so, the
constant is added to the address of the variable. You typically
do this when the variable is the name of a memory array.
8-4

Index Expressions

An index expression consists of a combination of a base register

[BX] or [BP], and/or an index register [SI] or [DI], with an
optional constant added or subtracted. You will usually want to
precede the bracketed expression with B, W, or D; to specify the
kind of memory unit (byte, word, or doubleword) you are referring
to. The expression stands for the memory unit whose address is
the run-time value(s) of the base and/or index registers added to
the constant. See the Effective Address section and the
beginning of this chapter for more details on indexed memory.

Descriptions of Operators and Specifiers

HIGH/LOW

Syntax: HIGH operand

LOW operand

These operators are called the "byte isolation" operators. The

operand must evaluate to a 16-bit number. HIGH returns the
high order byte of the number; LOW the low order byte.

For example,

MOV AL,HIGH(01234) ; AL = 012

TENHEX EQU LOW(0FF10) ; TENHEX = 010
These operators can be applied to each other. The following
identities apply:

LOW LOW Q = LOW Q

LOW HIGH Q = HIGH Q
HIGH LOW Q = 0
HIGH HIGH Q = 0

BY

Syntax: operand BY operand

This operator is a "byte combination" operator. It returns the

word whose high byte is the left operand, and whose low byte is
the right operand. For example, the expression 3 BY 5 is the
same as hexadecimal 0305. The BY operator is exclusive to A86. I
added it to cover the following situation: Suppose you are
initializing your registers to immediate values. Suppose you
want to initialize AH to the ASCII value 'A', and AL to decimal
10. You could code this as two instructions MOV AH,'A' and MOV
AL,10; but you realize that a single load into the AX register
would save both program space and execution time. Without the BY
operator, you would have to code MOV AX,0410A, which disguises
the types of the individual byte operands you were thinking
about. With BY, you can code it properly: MOV AX,'A' BY 10.

Addition (combination)
8-5

Syntax: operand + operand

operand . operand
operand PTR operand
operand operand

As shown in the above syntax, addition can be accomplished in

four ways: with a plus sign, with a dot operator, with a PTR
operator, and simply by juxtaposing two operands next to each
other. The dot and PTR operators are provided for compatibility.
The dot is used in structure field notation; PTR is used in
expressions such as BYTE PTR 0. (See Chapter 12 for
recommendations concerning PTR.)

If either operand is a constant, the answer is an expression with

the typing of the other operand, with the offsets added. For
example, if BVAR is a byte variable, then BVAR + 100 is the byte
variable 100 bytes beyond BVAR.

Other examples:

DB 100+17 ; simple addition

CTRL EQU -040
MOV AL,CTRL'D' ; a nice notation for control-D!
MOV DX,[BP].SMEM ; --where SMEM was in an unindexed structure
DQ 10.0 + 7.0 ; floating point addition

Subtraction
Syntax: operand - operand

The subtraction operator may have operands that are:

a. both absolute numbers

b. variable names that have the same type

The result is an absolute number; the difference between the two

operands.

Subtraction is also allowed between floating point numbers; the

answer is the floating point difference.

Multiplication and Division

Syntax: operand * operand (multiplication)

operand / operand (division)
operand MOD operand (modulo)

You may only use these operators with absolute or floating point
numbers, and the result is always the same type. Either operand
may be a numeric expression, as long as the expression evaluates
to an absolute or floating point number. Examples:

CMP AL,2 * 4 ; compare AL to 8

MOV BX,0123/16 ; BX = 012
DT 1.0 / 7.0
8-6

Shifting Operators

Syntax: operand SHR count (shift right)

operand SHL count (shift left)
BIT count (bit number)

The shift operators will perform a "bit-wise" shift of the

operand. The operand will be shifted "count" bits either to the
right or the left. Bits shifted into the operand will be set to
0.

The expression "BIT count" is equivalent to "1 SHL count"; i.e.,

BIT returns the mask of the single bit whose number is "count".
The operands must be numeric expressions that evaluate to
absolute numbers. Examples:

MOV BX, 0FACBH SHR 4 ; BX = 0FACH

OR AL,BIT 6 ; AL = AL OR 040; 040 is the mask for bit 6

Logical Operators

Syntax: operand OR operand

operand XOR operand
operand AND operand
NOT operand

The logical operators may only be used with absolute numbers.

They always return an absolute number.

Logical operators operate on individual bits. Each bit of the

answer depends only on the corresponding bit in the operand(s).

The functions performed are as follows:

1. OR: An answer bit is 1 if either or both of the operand bits

is 1. An answer bit is 0 only if both operand bits are 0.

2. XOR: This is "exclusive OR." An answer bit is 1 if the

operand bits are different; an answer bit is 0 if the operand
bits are the same.

3. AND: An answer bit is 1 only if both operand bits are 1. An

answer bit is 0 if either or both operand bits are 0.

4. NOT: An answer bit is the opposite of the operand bit. It

is 1 if the operand bit is 0; 0 if the operand bit is 1.

Examples:

11110000xB OR 00110011xB = 11110011xB

11110000xB XOR 00110011xB = 11000011xB
11110000xB AND 00110011xB = 00110000xB
NOT 00110011xB = 11001100xB

Boolean Negation Operator

Syntax: ! operand
8-7

The exclamation-point operator, rather than reversing each

individual bit of the operand, considers the entire operand as a
boolean variable to be negated. If the operand is non-zero (any
of the bits are 1), the answer is 0. If the operand is zero, the
answer is 0FFFF.

Because ! is intended to be used in conditional assembly

expressions (described in Chapter 11), there is also a special
action when ! is applied to an undefined name: the answer is the
defined value 0FFFF, meaning it is TRUE that the symbol is
undefined. Similarly, when ! is applied to some defined quantity
other than an absolute constant, the answer is 0, meaning it is
FALSE that the operand is undefined.

Relational Operators

Syntax: operand EQ operand (equal)

operand NE operand (not equal)
operand LT operand (less than)
operand LE operand (less or equal)
operand GT operand (greater than)
operand GE operand (greater or equal)

The relational operators may have operands that are either both
absolute numbers, or both variable names that have the same type.
The result of a relational operation is always an absolute
number. They return an 8-or 16-bit result of all 1's for TRUE
and all 0's for FALSE. Examples:

MOV AL, 3 EQ 0 ; AL = 0 (false)

MOV AX, 2 LE 15 ; AX = 0FFFFH (true)

String Comparison Operators

Syntax: string EQ string (equal)

string NE string (not equal)
string = string (equal ignoring case)

In order to subsume the string comparison facilities offered by

MASM's special conditional-assembly directives IFIDN and IFDIF,
A86 allows the relational operators EQ and NE to accept string
arguments. For this syntax to be accepted by A86, both strings
must be bounded using the same delimiter (either single quotes
for both strings, or double quotes for both strings). For a
match (EQ returns TRUE or NE returns FALSE), the strings must be
the same length, and every character must match exactly.
8-8

An additional A86-exclusive feature is the = operator, which

returns TRUE if the characters of the strings differ only in the
bit masked by the value 020. Thus you may use = to compare a
macro parameter to a string containing nothing but letters. The
comparison will be TRUE whether the macro parameter is upper-case
or lower-case. No checking is made to detect non-letters, so if
you use = on strings containing non-letters, you may get some
false TRUE results. Also, = is accepted when it is applied to
non-strings as well-- the corresponding values are interpreted as
two-byte strings, with the 020 bits masked away before
comparison.

B,W,D,F,Q,T Memory Variable Specifiers

Syntax: B operand D operand Q operand

operand B operand D operand Q
W operand F operand T operand
operand W operand F operand T

B, W, D, F, Q, and T convert the operand into a byte, word,

doubleword, far, quadword, and ten-byte variable, respectively.
The operand can be a constant, or a variable of the other type.
Examples:

ARRAY_PTR:
DB 100 DUP (?)
WVAR DW ?
MOV AL,ARRAY_PTR B ; load first byte of ARRAY_PTR array into AL
MOV AL,WVAR B ; load the low byte of WVAR into AL
MOV AX,W[01000] ; load AX with the memory word at loc. 01000
LDS BX,D[01000] ; load DS:BX with the doubleword at loc. 01000
JMP F OUTSIDE_LOC ; jump to undeclared far location OUTSIDE_LOC
FLD T[BX] ; load ten-byte number at [BX] to 87 stack
For compatibility, A86 accepts the more verbose synonyms BYTE,
WORD, DWORD, FAR, QWORD, and TBYTE for B,W,D,F,Q,T, respectively.

SHORT and LONG Operators

Syntax: SHORT label

LONG label

The SHORT operator is used to specify that the label referenced

by a JMP instruction is within 127 bytes of the end of the
instruction. The LONG operator specifies the opposite: that the
label is not within 127 bytes. The appropriate operator can (and
sometimes must) be used if the label is forward referenced in the
instruction.

When a non-local label is forward referenced, the assembler

assumes that it will require two bytes to represent the relative
offset of the label (so the instruction including the opcode byte
will be three bytes). By correctly using the SHORT operator, you
can save a byte of code when you use a forward reference. If the
label is not within the specified range, an error will occur. The
following example illustrates the use of the SHORT operator.
8-9

JMP FWDLAB ; three byte instruction

JMP SHORT FWDLAB ; two byte instruction
JMP >L1 ; two byte instruction assumed for a local label

Because the assembler assumes that a forward reference local

label is SHORT, you may sometimes be forced to override this
assumption if the label is in fact not within 127 bytes of the
JMP. This is why LONG is provided:

JMP LONG >L9 ; three byte instruction

If you are bothered by this possibility, you can specify the +G

switch, which causes A86 to pessimistically generate the three
byte JMP for all forward references, unless specifically told not
to, with SHORT.

NOTE that for A86, LONG will have effect only on the operand to
an unconditional JMP instruction; not to conditional jumps. That
is because conditional jumps farther than 127 bytes are available
only on the 386. If you run into this problem, then chances are
your code is getting out of control--time to rearrange, or to
break off some of the intervening code into separate procedures.
If you insist upon leaving the code intact, you can replace the
conditional jump with an "IF cond JMP".

OFFSET Operator

Syntax: OFFSET var-name

OFFSET is used to convert a variable into the constant pointer to

the variable. For example, if you have declared XX DW ?, and
you want to load SI with the pointer to the variable XX, you can
code: MOV SI,OFFSET XX. The simpler instruction MOV SI,XX moves
the variable contents of XX into SI, not the constant pointer to
XX.

NEAR Operator

Syntax: NEAR operand

NEAR converts the operand to have the type of a code label, as if

it were defined by appearing at the beginning of a program line
with a colon after it. NEAR is provided mainly for
compatibility.

Square Brackets Operator

Syntax: [operand ]

Square brackets around an operand give the operand a memory

variable type. Square brackets are generally used to enclose the
names of base and index registers: BX, BP, SI, and DI. When the
size of the memory variable can be deduced from the context of
the expression, square brackets are also used to turn numeric
constants into memory variables. Examples:
8-10

MOV B[BX+50],047 ; move imm value 047 into mem byte at BX+50
MOV AL,[050] ; move byte at memory location 050 into AL
MOV AL,050 ; move immediate value 050 into AL

Colon Operator

Syntax: constant : operand

segreg : operand
seg_or_group_name : operand

The colon operator is used to attach a segment register value to

an operand. The segment register value appears to the left of
the colon; the rest of the operand appears to the right of the
colon.

There are three forms to the colon operator. The first form has
a constant as the segment register value. This form is used to
create an operand to a far (inter-segment) JMP or CALL
instruction. An example of this is the instruction JMP 0FFFF:0,
which jumps to the cold-boot reset location of the 86 processor.

The second form has a segment register name to the left of the
colon. This is the segment override form, provided for
compatibility. A86 will generate a segment override byte when it
sees this form, unless the operand to the right of the colon
already has a default segment register that is the same as the
given override.

I prefer the more explicit method of overrides, exclusive to A86:

simply place the segment register name before the instruction
mnemonic. For example, I prefer ES MOV AL,[BX] to MOV
AL,ES:[BX].
The third form has a segment or group name before the colon.
This form is handled in .OBJ mode when there is a group name
before the colon, and an external symbol after. In that case,
the group override is necessary for the linker to produce correct
code. In other cases, the override is not necessary and is
ignored by A86.

ST Operator

ST is ignored whenever it occurs in an expression. It is

provided for compatibility with Intel and IBM assemblers. For
example, you can code FLD ST(0),ST(1), which will be taken by A86
as FLD 0,1.

REF and DEF Operators

Syntax: REF symbol

DEF symbol

The REF operator returns a value of TRUE if the operand is a

symbol that has been referenced, false if it hasn't. Appearance
as an operand to another REF or DEF does not count as a
reference.
8-11

The DEF operator returns a value of TRUE if the operand has been
defined previously in the assembly, false if it hasn't.

REF and DEF are most often used within parameters to an IF

conditional-assembly construct.

TYPE Operator

Syntax: TYPE operand

The TYPE operator returns 1 if the operand is a byte variable; 2

if the operand is a word variable; 4 if the operand is a
doubleword variable; 8 if the operand is a quadword variable; 10
if the operand is a ten-byte variable; 0 if the operand is a
constant, and the number of bytes allocated by the structure if
the operand is a structure name (see STRUC in the next chapter).

A common usage of the TYPE operator is to represent the number of

bytes of a named structure. For example, if you have declared a
structure named LINE (as described in the next chapter) that
defines 82 bytes of storage, then two ways you might refer to the
value symbolically are as follows:

MOV CX,TYPE LINE ; loads the size of LINE into CX

DB TYPE LINE DUP ? ; allocates an area of memory for a LINE

THIS and $ Specifiers

THIS returns the value of the current location counter. It is

provided for compatibility. The dollar sign $ is the more
standard and familiar specifier for this purpose; it is
equivalent to THIS NEAR. THIS is typically used with the BYTE
and WORD specifiers to create alternate-typed symbols at the same
memory location:

BVAR EQU THIS BYTE

WVAR DW ?

I don't recommend the use of THIS. If you wish to retain Intel

compatibility, you can use the less verbose LABEL directive:

BVAR LABEL BYTE

WVAR DW ?

If you are not concerned with compatibility to lesser assemblers,

A86 offers a variety of less verbose forms. The most concise is
DB without an operand:

BVAR DB
WVAR DW ?

If this is too cryptic for you, there is always BVAR EQU B[$].
8-12

Operator Precedence

Consider the expression 1 + 2 * 3. When A86 sees this

expression, it could perform the multiplication first, giving an
answer of 1+6 = 7; or it could do the addition first, giving an
answer of 3*3 = 9. In fact, A86 does the multiplication first,
because A86 assigns a higher precedence to multiplication than it
does addition.

The following list specifies the order of precedence A86 assigns

to expression operators. All expressions are evaluated from left
to right following the precedence rules. You may override this
order of evaluation and precedence through the use of parentheses
( ). In the example above, you could override the precedence by
parenthesizing the addition: (1+2) * 3.

Some symbols that we have referred to as operators, are treated

by the assembler as operands having built-in values. These
include $, and ST. In a similar vein, a segment override term (a
segment register name followed by a colon) is recorded when it is
scanned, but not acted upon until the entire containing
expression is scanned and evaluated. The size operators B, W, D,
F, Q, and T are also recorded and applied after scanning and
evaluation.

If two operators are adjacent, the rightmost operator must have

precedence; otherwise, parentheses must be used. For example,
the expression BIT ! 1 is illegal because the leftmost operator
BIT has the higher precedence of the two adjacent operators BIT
and "!". You can code BIT (! 1).

--Highest Precedence--

1. Parenthesized expressions
2. Period
3. OFFSET, SEG, TYPE, REF, DEF, and PTR
4. HIGH, LOW, and BIT
5. Multiplication and division: *, /, MOD, SHR, SHL
6. Addition and subtraction: +,-
a. unary
b. binary
7. Relational: EQ, NE, LT, LE, GT, GE =
8. Logical NOT and !
9. Logical AND
10. Logical OR and XOR
11. Colon for long pointer, SHORT, LONG, and BY
12. DUP

--Lowest Precedence--

CHAPTER 9 DIRECTIVES IN A86

Segments in A86

The following discussion applies when A86 is assembling a .COM

See the next chapter for the discussion of segmentation for .OBJ
files.

A86 views the 86 computer's memory space as having two parts: The
first part is the program, whose contents are the object bytes
generated by A86 during its assembly of the source. A86 calls
this area the CODE SEGMENT. The second part is the data area,
whose contents are generated by the program after it starts
running. A86 calls this area the DATA SEGMENT.

Please note well that the only difference between the CODE and
DATA segments is whether the contents are generated by the
program or the assembler. The names CODE and DATA suggest that
program code is placed in the CODE segment, and data structures
go in the DATA segment. This is mostly true, but there are
exceptions. For example, there are many data structures whose
contents are determined by the assembler: pointer tables, arrays
of pre-defined constants, etc. These tables are assembled in the
CODE segment.

In general, you will want to begin your program with the

directive DATA SEGMENT, followed by all your program variables
and uninitialized data structures, using the directives DB, DW,
and STRUC. If you do not give an ORG directive, A86 will begin
the allocation immediately following the end of the .COM program.
You can end the DATA SEGMENT allocation lines with the DATA ENDS
directive, followed by the program code itself. A short program
illustrating this suggested usage follows:

DATA SEGMENT
ANSWER_BYTE DB ?
CALL_COUNT DW ?
CODE SEGMENT
JMP MAIN

TRAN_TABLE:
DB 16,3,56,23,0,9,12,7

MAIN:
MOV BX,TRAN_TABLE
XLATB
MOV ANSWER_BYTE,AL
INC CALL_COUNT
RET

A86 allows you to intersperse CODE SEGMENTs and DATA SEGMENTs

throughout your program; but in general it is best to put all
your DATA SEGMENT declarations at the top of your program, to
avoid problems with forward referencing.
9-2

CODE ENDS and DATA ENDS Statements

For compatibility with Intel/IBM assemblers, A86 provides the

CODE ENDS and DATA ENDS statements. The CODE ENDS statement is
ignored; we assume that you have not nested a CODE segment inside
a DATA segment. The DATA ENDS statement is equivalent to a CODE
SEGMENT statement.

The ORG Directive

Syntax: ORG address

ORG moves the output pointer (the location counter at which

assembly is currently taking place within the current segment) to
the value of the operand. In the CODE segment, the operand
should be an absolute constant, or an expression evaluating to an
absolute, non-forward-referenced constant. In the DATA segment,
the operand may be a forward reference or an expression
containing one or more forward references. All symbols in the
segment will be resolved when the forward references to the ORG
operand are all resolved.

There is a special side effect to ORG when it is used in the CODE

segment. If you begin your code segment with ORG 0, then A86
knows that you are not assembling a .COM program; but are instead
assembling a code segment to be used in some other context
(examples: programming a ROM, or assembling a procedure for older
versions of Turbo Pascal). The output file will start at 0, not
0100 as in a .COM file; and the default extension for the output
file will be .BIN, not .COM. However, if you later issue an ORG
0100 directive, the default will revert back to .COM.

Other than in the above example, you should not in general issue
an ORG within the CODE segment that would lower the value of the
output pointer. This is because you thereby put yourself in
danger of losing part of your assembled program. If you
re-assemble over space you have already assembled, you will
clobber the previously-assembled code. Also, be aware that the
size of the output program file is determined by the value of the
code segment output pointer when the program stops. If you ORG
to a lower value at the end of your program, the output program
file will be truncated to the lower-value address.

Again, almost no program producing a .COM file will need any ORG
directive in the code segment. There is an implied ORG 0100 at
the start of the program. You just start coding instructions,
and the assembler will put them in the right place.
9-3

The EVEN Directive

Syntax: EVEN constant

The EVEN directive coerces the current output pointer to a value

which is an exact multiple of the operand. If no operand is
given, a value of 2 is assumed. In a DATA SEGMENT or STRUC, it
does so by adding to the current output pointer if necessary. In
a code segment, it outputs an appropriate number of NOP
instruction bytes. EVEN is most often used in data segments,
before a sequence of DW directives. Machines beyond the original
8088 fetch words more quickly when they are aligned onto even
addresses; so the EVEN directive insures that your program will
have the faster access to those DW's that follow it. Also useful
are EVEN 4 for doubleword alignment, and EVEN 16 for paragraph
alignment. Be aware, though, that if you use the EVEN directive
in .OBJ mode, the containing SEGMENT directive should have an
alignment type at least as great as your EVEN operand, to achieve
the desired alignment at its final memory location.

Data Allocation Using DB, DW, DD, DQ, and DT

The 86 computer family supports the three fundamental data types

BYTE, WORD, and DWORD. A byte is eight bits, a word is 16 bits
(2 bytes), and a doubleword is 32 bits (4 bytes). In addition,
the 87 floating point processor manipulates 8-byte quantities,
which we call Q-words, and 10-byte quantities, which we call
T-bytes. The A86 data allocation statement is used to specify
the bytes, words, doublewords, Q-words, and T-bytes which your
program will use as data. The syntax for the data allocation
statement is as follows:

(optional var-name) DB (list of values)

(optional var-name) DW (list of values)
(optional var-name) DD (list of values)
(optional var-name) DQ (list of values)
(optional var-name) DT (list of values)

The variable name, if present, causes that name to be entered

into the symbol table as a memory variable with type BYTE (for
DB), WORD (for DW), DWORD (for DD), QWORD (for DQ), or TBYTE (for
DT). The variable name should NOT have a colon after it, unless
you wish the name to be a label (instructions referring to it
will interpret the label as the constant pointer to the memory
location, not its contents).

The DB statement is used to reserve bytes of storage; DW is used

to reserve words. The list of values to the right of the DB or
DW serves two purposes. It specifies how many bytes or words are
allocated by the statement, as well as what their initial values
should be. The list of values may contain a single value or more
than one, separated by commas. The list can even be missing;
meaning that we wish to define a byte or word variable at the
same location as the next variable.
9-4

If the data initialization is in the DATA segment, the values

given are ignored, except as place markers to reserve the
appropriate number of units of storage. The use of "?", which in
.COM mode is a synonym for zero, is recommended in this context
to emphasize the lack of actual memory initialization. When A86
is assembling .OBJ files, the ?-initialization will cause a break
in the segment (unless ? is embedded in a nested DUP containing
non-? terms, in which case it is a synonym for zero).

A special value which can be used in data initializations is the

DUP construct, which allows the allocation and/or initialization
of blocks of data. The expression n DUP x is equivalent to a
list with x repeated n times. "x" can be either a single value,
a list of values, or another DUP construct nested inside the
first one. The nested DUP construct needs to be surrounded by
parentheses. All other assemblers, and earlier versions of A86,
require parentheses around all right operands to DUP, even simple
ones; but this requirement has been removed for simple operands
in the current A86.

Here are some examples of data initialization statements, with

and without DUP constructs:

CODE SEGMENT
DW 5 ; allocate one word, init. to 5
DB 0,3,0 ; allocate three bytes, init. to 0,3,0
DB 5 DUP 0 ; equivalent to DB 0,0,0,0,0
DW 2 DUP (0,4 DUP 7) ; equivalent to DW 0,7,7,7,7,0,7,7,7,7

DATA SEGMENT
XX DW ? ; define a word variable XX
YYLOW DB ; no init value: YYLOW is low byte of word var
YY
YY DW ?
X_ARRAY DB 100 DUP ? ; X_ARRAY is a 100-byte array
D_REAL DQ ? ; double precision floating variable
EX_REAL DT ? ; extended precision floating variable

A character string value may be used to initialize consecutive

bytes in a DB statement. Each character will be represented by
its ASCII code. The characters are stored in the order that they
appear in the string, with the first character assigned to the
lowest-addressed byte. In the DB statement that follows, five
bytes are initialized with the ASCII representation of the
characters in the string 'HELLO':

DB 'HELLO'
9-5

Note that except for string comparisons described in the previous

chapter, the DB directive is the only place in your program that
strings of length greater than 2 may occur. In all other
contexts (including DW), a string is treated as the constant
number representing the ASCII value of the string; for example,
CMP AL,'@' is the instruction comparing the AL register with the
ASCII value of the at-sign. Note further that 2-character string
constants, like all constants in the 8086, have their bytes
reversed. Thus, while DB 'AB' will produce hex 41 followed by
hex 42, the similar looking DW 'AB' reverses the bytes: hex 42
followed by hex 41.

For compatibility, A86 now accepts double quotes, as well as

single quotes, for strings in DB directives.

The DD directive is used to initialize 32-bit doubleword pointers

to locations in arbitrary segments of the 86's memory space.
Values for such pointers are given by two numbers separated by a
colon. The segment register value appears to the left of the
colon; and the offset appears to the right of the colon. In
keeping with the reversed-bytes nature of memory storage in the
86 family, the offset comes first in memory. For example, the
statement

DD 01234:05678

appearing in a CODE segment will cause the hex bytes 78 56 34 12

to be generated, which is a long pointer to segment 01234, offset
05678.

DD, DQ, and DT can also be used to initialize large integers and
floating point numbers. Examples:

DD 500000 ; half million, too big for most 86 instructions

DD 3.5 ; single precision floating point number
DQ 3.5 ; the same number in a double precision format
DT 3.5 ; the same number in an extended precision format

The STRUC Directive

The STRUC directive is used to define a template of data to be

addressed by one of the 8086's base and/or index registers. The
syntax of STRUC is as follows:

(optional strucname) STRUC (optional effective address)

The optional structure name given at the beginning of the line

can appear in subsequent expressions in the program, with the
operator TYPE applied to it, to yield the number of bytes in the
structure template.
9-6

The STRUC directive causes the assembler to enter a mode similar

to DATA SEGMENT: assembly within the structure declares symbols
(the elements of the structure), using a location counter that
starts out at the address following STRUC. If no address is
given, assembly starts at location 0. An option not available to
the DATA SEGMENT is that the address can include one base
register [BX] or [BP] and/or one index register [SI] or [DI]. The
registers are part of the implicit declaration of all structure
elements, with the offset value increasing by the number of bytes
allocated in each structure line. For example:

LINE STRUC [BP] ; the template starts at [BP]

DB 80 DUP (?) ; these 80 bytes advance us to [BP+80]
LSIZE DB ? ; this 1 byte advances us to [BP+81]
LPROT DB ?
ENDS

The STRUC just given defines the variables LSIZE, equivalent to

B[BP+80], and LPROT, equivalent to B[BP+81]. You can now issue
instructions such as MOV AL,LSIZE; which automatically generates
the correct indexing for you.

The mode entered by STRUC is terminated by the ENDS directive,

which returns the assembler to whatever segment (CODE or DATA) it
was in before the STRUC, with the location counter restored to
its value within that segment before the STRUC was declared.

Forward References

A86 allows names for a variety of program elements to be forward

referenced. This means that you may use a symbol in one
statement and define it later with another statement. For
example:

JNZ TARGET
.
.
TARGET:
ADD AX,10

In this example, a conditional jump is made to TARGET, a label

farther down in the code. When JNZ TARGET is seen, TARGET is
undefined, so this is a forward reference.
9-7

Earlier versions of A86 were much more restricted in the kinds of

forward references allowed. Almost all of the restrictions have
now been eased, for convenience as well as compatibility with
other assemblers. In particular, you may now make forward
references to variable names. You just need to see to it that
A86 has enough information about the type of the operand to
generate the correct instruction. For example, MOV FOO,AL will
cause A86 to correctly deduce that FOO is a byte variable. You
can even code a subsequent MOV FOO,1 and A86 will remember that
FOO was assumed to be a byte variable. But if you code MOV FOO,1
first, A86 won't know whether to issue a byte or a word MOV
instruction; and will thus issue an error message. You then
specify the type by MOV FOO B,1.

In general, A86's compatibility with other assemblers has

improved dramatically for forward references. You'll need only
sprinkle a very few B's and W's into your references. And you'll
be rewarded: in many cases the word form is longer than the byte
form, so that other assemblers wind up inserting a wasted NOP in
your program. You'll wind up with tighter code by using A86!

Forward References in Expressions

A86 now allows you to include any number of forward-reference

symbols in expressions of arbitrary complexity. If the
expression is legal when the forward references are resolved,
then it will be accepted by the assembler.

A86 will also accept the reserved symbol END as a

forward-reference quantity, either by itself as an operand, or
within an expression. END will be resolved when assembly is
complete, as a label pointing to the end of the program.

For example, suppose you wish to advance the ES segment register

to point immediately beyond your program. You can code:

MOV AX,CS ; fetch the program's segment value

ADD AX,(END+15)/16 ; add in the number of paragraphs
MOV ES,AX ; ES is now loaded as desired

The EQU Directive

Syntax: symbol-name EQU expression

symbol-name EQU built-in-symbol
symbol-name EQU INT n

The expression field may specify an operand of any type that

could appear as an operand to an instruction.
9-8

As a simple example, suppose you are writing a program that

manipulates a table containing 100 names and that you want to
refer to the maximum number of names throughout the source file.
You can, of course, use the number 100 to refer to this maximum
each time, as in MOV CX,100, but this approach suffers from two
weaknesses. First of all, 100 can mean a lot of things; in the
absence of comments, it is not obvious that a particular use of
100 refers to the maximum number of names. Secondly, if you
extend the table to allow 200 names, you will have to locate each
100 and change it to a 200. Suppose, instead, that you define a
symbol to represent the maximum number of names with the
following statement:

MAX_NAMES EQU 100

Now when you use the symbol MAX_NAMES instead of the number 100
(for example, MOV CX,MAX_NAMES), it will be obvious that you are
referring to the maximum number of names in the table. Also, if
you decide to extend the table, you need only change the 100 in
the EQU directive to a 200 and every reference to MAX_NAMES will
reflect the change.

You could also take advantage of A86's strong typing, by changing

MAX_NAMES to a variable:

MAX_NAMES DB ?

or even an indexed quantity:

MAX_NAMES EQU [BX+1]

Because the A86 language is strongly typed, the instruction for

loading MAX_NAMES into the CX register remains exactly the same
in all cases: simply MOV CX,MAX_NAMES.

Equates to Built-In Symbols

A86 allows you to define synonyms for any of the assembler

reserved symbols, by EQUating an alternate name of your choosing,
to that symbol. For example, suppose you were coding a source
module that is to be incorporated into several different
programs. In some programs, a certain variable will exist in the
code segment. In others, it will exist in the stack segment. You
want to address the variable in the common source module, but you
don't know which segment override to use. The solution is to
declare a synonym, QS, for the segment register. QS will be
defined by each program: the code-segment program will have a QS
EQU CS at the top of it; the stack-segment program will have QS
EQU SS. The source module can use QS as an override, just as if
it were CS or SS. The code would be, for example, QS MOV
AL,VARNAME.
9-9

The NIL Prefix

A86 provides a mnemonic, NIL, that generates no code. NIL can be

used as a prefix to another instruction (which will have no
effect on that instruction), or it can appear by itself on a
line. NIL is provided to extend the example in the previous
section, to cover the possibility of no overrides. If your
source module goes into a program that fits into 64K, so that all
the segment registers have the same value, then code QS EQU NIL
at the top of that program.

Interrupt Equates
A86 allows you to equate your own name to an INT instruction with
a specific interrupt number. For example, if you place TRAP EQU
INT 3 at the top of your program, you can use the name TRAP as a
synonym for INT 3 (the debugger trap on the 8086).

Duplicate Definitions

A86 contains the unique feature of duplicate definitions. We

have already discussed local symbols, which can be redefined to
different values without restriction. Local symbols are the only
symbols that can be redefined. However, any symbol can be
defined more than once, as long as the symbol is defined to be
the same value and type in each definition.

This feature has two uses. First, it eases modular program

development. For example, if two independently-developed source
files both use the symbol ESC to stand for the ASCII code for
ESCAPE, they can both contain the declaration ESC EQU 01B, with
no problems if they are combined into the same program.

The second use for this feature is assertion checking. Your

deliberate redeclaration of a symbol name is an assertion that
the value of the symbol has not changed; and you want the
assembler to issue you an error message if it has changed.
Example: suppose you have declared a table of options in your
DATA segment; and you have another table of initial values for
those options in your CODE segment. If you come back months
later and add an option to your tables, you want to be reminded
to update both tables in the same way. You should declare your
tables as follows:

DATA SEGMENT
OPTIONS:
.
.
OPT_COUNT EQU $-OPTIONS ; OPT_COUNT is the size of the table

CODE SEGMENT
OPT_INITS:
.
.
OPT_COUNT EQU $-OPT_INITS ; second OPT_COUNT had better be the same!
9-10

The = Directive

Syntax: symbol-name = expression

symbol-name = built-in-symbol
symbol-name = INT n

The equals sign directive is provided for compatibility. It is

identical to the EQU directive, with one exception: if the first
time a symbol appears in a program is in an = directive, that
symbol will be taken as a local symbol. It can be redefined to
other values, just like the generic local symbols (letter
followed by digits) that A86 supports. (If you try to redefine an
EQU symbol to a different value, you get an error message.) The =
facility is most often used to define "assembler variables", that
change value as the assembly progresses.

The PROC Directive

Syntax: name PROC NEAR

name PROC FAR
name PROC

PROC is a directive provided for compatibility with Intel/IBM

assemblers. I don't like PROC; and I recommend that you do not
use it, even if you are programming for those assemblers.

The idea behind PROC is to give the assembler a mechanism whereby

it can decide for you what kind of RET instruction you should be
providing. If you specify NEAR in your PROC directive, then the
assembler will generate a near (same segment) return when it sees
RET. If you specify FAR in your PROC directive, the assembler
will generate a far RETF return (which will cause both IP and CS
to be popped from the stack). If you simply leave well enough
alone, and never code a PROC in your program, then RET will mean
near return throughout your program.

The reason I don't like PROC is because it is yet another attempt

by the assembler to do things "behind your back". This goes
against the reason why you are programming in assembly language
in the first place, which is to have complete control over the
code generated by your source program. It leads to nothing but
trouble and confusion.

Another problem with PROC is its verbosity. It replaces a simple

colon, given right after the label it defines. This creates a
visual clutter in the program, that makes the program harder to
read.

A86 provides an explicit RETF mnemonic so that you don't need to

use PROC to distinguish between near and far return instructions.
You can use RET for a near return and RETF for a far return.
9-11

The ENDP Directive

Syntax: [name] ENDP

The only action A86 takes when it sees an ENDP directive is to

return the assembler to its (sane) default state, in which RET is
a near return.

NOTE that this means that A86 does not support nested PROCs, in
which anything but the innermost PROC has the FAR attribute. I'm
sorry if I am blunt, but anybody who would subject their program
to that level of syntactic clutter has rocks in their head.
The LABEL Directive

Syntax: name LABEL NEAR

name LABEL FAR
name LABEL BYTE
name LABEL WORD
name LABEL DWORD
name LABEL QWORD
name LABEL TBYTE

LABEL is another directive provided for compatibility with

Intel/IBM assemblers. A86 provides less verbose ways of
specifying all the above LABEL forms, except for LABEL FAR.

LABEL defines "name" to have the type given, and a value equal to
the current output pointer. Thus, LABEL NEAR is synonymous with
a simple colon following the name; and LABEL BYTE, LABEL WORD,
LABEL DWORD, etc., are synonymous with DB, DW, DD, etc., with no
operands.

LABEL FAR does have a unique functionality, not found in other

assemblers. It identifies "name" as a procedure that can be
called from outside this program's code segment. Such procedures
should have RETFs instead of RETs. Furthermore, I have provided
the following feature, unique to A86: if you CALL the procedure
from within your program, A86 will generate a PUSH CS instruction
followed by a NEAR call to the procedure. Other assemblers will
generate a FAR call, having the same functional effect; but the
FAR call consumes more program space, and takes more time to
execute.

WARNING: you cannot use the above CALL feature as a forward

reference; the LABEL FAR definition must precede any CALLs to it.
This is unavoidable, since the assembler must assume that a CALL
to an undefined symbol takes 3 program bytes. All assemblers
will issue an error in this situation.
9-12

The INCLUDE Directive

A86 now allows the inclusion of alternate source files within the
middle of a "parent" source file, via the INCLUDE directive.
When you give the name INCLUDE followed by the name of a file,
A86 will insert the contents of the named file into the assembly
source stream, as if it were substituted for the INCLUDE line.
There is no limit to the size of an INCLUDE file, and INCLUDEs
may be nested (the file included may itself contain INCLUDE
directives) to any level within reason. Parentheses are optional
around the file name; if you don't give them, there must be at
least one blank between the INCLUDE and the file name.

If there is no file name whatever following the INCLUDE, A86 will

perform an A86LIB library search (see Chapter 13), and INCLUDE
all library files necessary to resolve all undefined symbols at
the point of the INCLUDE. This provides an "in-file" equivalent
to the pound-sign given on the invocation line.
CHAPTER 10 RELOCATION AND LINKAGE

A86 allows you to produce either .COM files, which can be run
immediately as standalone programs, or .OBJ files, to be fed to
the MS-DOS LINK program. In this chapter I'll discuss .OBJ mode
of A86.

.OBJ Production Made Easy

I'll start by giving you the minimum amount of information you

need to know to produce .OBJ files. If you are writing short
interface routines, and do not want to concern yourself with the
esoterica of .OBJ files (segments, groups, publics, etc.), you
can survive quite nicely by reading only this section.

There are two ways you can cause A86 to produce a .OBJ file as
its object output. One way is to explicitly give .OBJ as the
output file name: for example, you can assemble the source file
FOO.8 by giving the command "A86 FOO.8 FOO.OBJ". The other way
is to specify the switch +O (letter O not digit 0). This is
illustrated by the invocation "A86 +O FOO.8", which will have the
same effect as the first invocation.

My design philosophy for .OBJ production is to accommodate two

types of user. The first type of user is writing new code, to
link to other (usually high level language) modules. That person
should be able to write the module with a minimum of red tape,
and have A86 do the right thing. The second type of user has
existing modules written for Intel/IBM assemblers, and wants to
port them to A86. A86 should recognize and act upon all the
relocation directives (SEGMENT, GROUP, PUBLIC, EXTRN, NAME, END)
given. The assembly should work even if several files, assembled
separately under the Intel/IBM assembler, are fed to a single A86
assembly. You'll see if you read on through this entire chapter
that the multiple-files requirement causes A86 to interpret some
of the relocation directives a little differently (while
achieving compatible results).

Let's suppose you're writing new code: for example, an interface

routine to the "C" language, that multiplies a 16-bit number by
10. "C" pushes the input number onto the stack, before calling
your routine. Your code needs to get the number, multiply it by
10, and return the answer in the AX register. You can code it:

_MUL10: ; "C" expects all public names to start with "_"

PUSH BP ; "C" expects BP to be preserved
MOV BP,SP ; we use BP to address the stack
MOV AX,[BP+4] ; fetch the number N, beyond BP and the ret addr
ADD AX,AX ; 2N
MOV BX,AX ; 2N is saved in BX
ADD AX,AX ; 4N
ADD AX,AX ; 8N
 ADD AX,BX ; 8N + 2N = 10N
POP BP ; BP is restored
RET ; go back to caller
10-2

These 11 lines can be your entire source file! If you name the
file MUL10.8, A86 will create an object file MUL10.OBJ, that
conforms to the standard SMALL model of computation for high
level languages. If you use RETF instead of RET (thus, by the
way, getting the operand from BP+6 instead of BP+4), the object
module will conform to the standard LARGE model of computation.
All the red tape information required by the high level language
is provided implicitly by A86. I'll go through this information
in detail later, but you should need to read about it only if
you're curious.

What happens if you need to access symbols outside the module

you're assembling? If the type of the symbol is correctly
guessed from the instruction that refers to it, then you can
simply refer to it, and leave it undefined within the module. For
example, if A86 sees the instruction CALL PRINT with PRINT
undefined, it will assume that PRINT is a NEAR procedure. If
PRINT is never defined within the module, A86 will act as if you
declared PRINT via the directive EXTRN PRINT:NEAR. The address
of PRINT will be plugged into your instruction by LINK when it
combines A86's .OBJ file with the high level language's .OBJ
files, to make the final program.

In general, the undefined operand to any CALL or JMP instruction

is assumed to be NEAR. The second (source) operand to a MOV or
arithmetic instruction is assumed to be ABS (i.e., an immediate
constant). An undefined first (destination) operand is assumed
to be a simple memory variable, of the same size (BYTE or WORD)
as the register given in the second operand. If your external
symbol does not comply with these guidelines, you need to declare
it with an EXTRN before you use it. (You can also use EXTRN to
declare types of non-complying forward references within your
module, as you'll see later.)

If you'd like to link the MUL10 procedure to Turbo Pascal V4.0 or

later, you need to append the line CODE SEGMENT PUBLIC to the top
of the program, to name the program segment according to Turbo
Pascal's expectations. You may dispense with the leading
underscore in the name MUL10-- Turbo Pascal does not require or
expect it.

At this point, if you're a casual user, I think you've read

enough to get going! Read further only if you wish; or if you
get stuck, and need to master the esoterica.
10-3

Overview of Relocation and Linkage

When you assemble a program directly into a .COM file, the

program has just two forms: the source program, that you can
understand, and the .COM file, that the computer can "understand"
(i.e., execute). A .OBJ file is an intermediate format: neither
you nor the (executing) computer can make sense out of a .OBJ
file; only programs like LINK interpret .OBJ files. The purpose
of a .OBJ file is to allow you to assemble or compile just a part
of a program. The other parts (also in the form of .OBJ files)
can be produced at a different time; often by a different
assembler or compiler, whose source files are in a different
language. It's easy to see where the word "linkage" comes from:
the LINK program puts the pieces of a program together. The
"relocation" comes because the assembler or compiler that makes a
given program piece doesn't know how many other pieces will come
before it, or how big the other pieces will be. Each piece is
constructed as if it started at location 0 within the program;
then LINK "relocates" the piece to its true location.

Many of the relocation features of 86 assembly language are

couched in terms of LINK's point of view, so we must look at the
way LINK sees things. LINK calls a .OBJ file an "object module",
or just "module". Each module has a NAME, that can be referred
to when LINK issues diagnostic messages, such as error messages
and symbol maps. If a program symbol is used only within a
single module, it does not need to be given to LINK, except
possibly to pass along to a symbolic debugger. On the other
hand, if a program symbol is defined in one module and referenced
in other modules, then LINK needs to know the name of the symbol,
so it can resolve the references. Such a symbol is PUBLIC in the
module in which it is defined; it is "external" in the other
modules, containing references to it. Finally, exactly one
module in a program must contain the starting location for the
program; that module is called the "main module", and it must
supply the starting address (which is not necessarily at the
beginning of the module).

In the 86 family of microprocessors, the LINK system also does

much to manage the memory segments that a program will fit into,
and get its data from. The (grotesquely ornate) level of support
for segmentation was dictated by Intel, when it specified (and
IBM and the compiler makers accepted) the format that .OBJ files
will have. I attended the fateful meeting at Intel, in which the
crucial design decisions were made. I regret to say that I sat
quietly, while engineers more senior than I applied their fertile
imaginations to construct fanciful scenarios which they felt had
to be supported by LINK. Let's now review the resulting
segmentation model.
10-4

The parts of a program, as viewed by LINK, come in three

different sizes: they can be (1) pieces of a single segment, (2)
an entire single segment, or (3) a sequence of consecutive
segments in 86 memory. Size (1) should have been called
something like FRAGMENT, but is instead called SEGMENT. Size (2)
should have been called SEGMENT, but is instead called GROUP.
Size (3) should have been called "group", but is instead called
"class". Let me cling to the sensible terminology for one more
paragraph, while I describe the worst scenario Intel wanted to
support; then when I discuss individual directives, I'll
regretfully revert to the official terminology.
The scenario is as follows: suppose you have a program that
occupies about 100K bytes of memory. The program contains a core
of 20K bytes of utility routines that every part of the program
calls. You'd like every part of the program to be able to call
these routines, using the NEAR form to save memory. By gum, you
can do it! You simply(!) slice the program into three fragments:
the utility routines will go into fragment U, and the rest of the
program will be split into equal-sized 40K-byte fragments A and
B. Now you arrange the fragments in 8086 memory in the order
A,U,B. The fragments A and U form a 60K-byte block, addressed by
a segment register value G1, that points to the beginning of A.
The fragments U and B form another 60K-byte block addressed by a
segment register value G2, that points to the beginning of U. If
you set the CS register to G1 when A is executing, and G2 when B
is executing, the U fragment is accessible at all times. Since
all direct JMPs and CALLs are encoded as relative offsets, the
U-code will execute direct jumps correctly whether addressed by
G1 with a huge offset, or G2 with a small offset. Of course, if
U contains any absolute pointers referring to itself (such as an
indirect near JMP or CALL), you're in trouble.

It's now been over a decade since the fateful design meeting took
place, and I can report that the above scenario has never taken
place in the real world. And I can state with some authority
that it never will. The reason is that the only programs that
exceed 64K bytes in size are coded in high level language, not
assembly language. High-level-language compilers follow a very,
very restricted segmentation model-- no existing model comes
remotely close to supporting the scheme suggested by the
scenario. But the 86 assembly language can support it-- the
directives "G1 GROUP A,U" and "G2 GROUP B,U", followed by chunks
of code of the appropriate object size, headed by directives "A
SEGMENT", "B SEGMENT", and "U SEGMENT". The LINK program is
supposed to sort things out according to the scenario; but I
can't say (and I have my doubts) if it actually succeeds in doing
so.

The concept of "class" was added as an afterthought, to implement

the more sensible and usable features that outsiders thought
GROUPs were implementing; namely, the ability to specify that
different (and disjoint!) segments occur consecutively in memory.
This allows programs to be arranged in a consistent manner-- for
example, with all program code followed by all static data
segments followed by all dynamically allocated memory.
10-5

The NAME Directive

Syntax: NAME module_name

The NAME directive specifies that "module_name" be given to LINK

as the name of the module produced by this assembly. The symbol
"module_name" can be used elsewhere in your program without
conflict: it can even, if you like, be a built-in assembler
mnemonic (e.g. "NAME MOV" is acceptable)!

If you do not provide a NAME directive, A86 will use the name of
the output object file, without the .OBJ extension. If you
provide more than one NAME directive, A86 will use the last one
given, with no error reported.

The PUBLIC Directive

Syntax: PUBLIC sym1, sym2, sym3, ...

PUBLIC

The PUBLIC directive allows you to explicitly list the symbols

defined in this assembly, that can be used by other modules. If
you do not give any PUBLIC directives in your program, A86 will
use every relocatable label and variable name in your program,
except local labels (the redefinable labels consisting of a
letter followed by digits: L7, M1, Q234, etc.). Symbols EQUated
to constants, and symbols defined within structures and DATA
SEGMENTs, are not implicitly declared PUBLIC: you have to
explicitly include them in a PUBLIC directive.

A86 maintains an internal flag, telling it whether to figure out

for itself which symbols are PUBLIC, or to let the program
explicitly declare them. The flag starts out "implicit", and is
set to "explicit" only if A86 sees a PUBLIC directive with no
names at all, or a PUBLIC directive containing at least one name
that would have been implicitly made PUBLIC.

If you are writing new code, you'll probably want to keep the
flag "implicit". You use the PUBLIC directive only for those
symbols which have the form of local labels, but aren't (e.g., a
memory variable I1987 for 1987 income); and for absolute values
that are globally accessed -- e.g. specify "PUBLIC
OPEN_FILES_LIMIT" for a symbol defined as "OPEN_FILES_LIMIT EQU
20".

If you are porting existing code, that code will already have
PUBLIC directives in it, and A86 will go to "explicit" mode,
duplicating the functionality of other assemblers.

The PUBLIC directive with no names is used to force "explicit"

mode, thus causing (if there are no further PUBLICs with names)
the .OBJ file to declare no symbols PUBLIC.
10-6

There is another side effect to the PUBLIC directive: if a symbol

is declared PUBLIC in a module, it had better be defined in that
module. If it isn't then A86 includes it in the .UND listing of
undefined symbols in the module, and suppresses output of the
object file.

The EXTRN Directive

Syntax: EXTRN sym1:type, sym2:type, ...

where "type" is one of: BYTE WORD DWORD QWORD TBYTE FAR
or synonymously: B W D Q T F
or: NEAR ABS PROC

The EXTRN directive allows you to attach a type to a symbol that

may not yet be defined (and may never be defined) within your
program. This is often necessary for the assembler to generate
the correct instruction form when the symbol is used as an
operand. All the possible types except ABS and PROC are defined
elsewhere in the A86 language, but I list them again here for
convenience:

B or BYTE: byte-sized memory variable

W or WORD: word (2 byte) sized memory variable
D or DWORD: doubleword (4-byte) sized memory variable
Q or QWORD: quadword (8-byte) sized memory variable
T or TWORD: 10-byte-sized memory variable
NEAR: program label accessed within a segment
FAR: program label accessed from outside this segment
ABS: an absolute number (i.e., an immediate constant)
PROC: same as NEAR unless you provide a PROC EQU FAR

An example of EXTRN usage is as follows: suppose there is a word

memory variable IFARK in your program. The variable might be
declared at the end of the program; or it might be defined in a
module completely outside of this program. Without an EXTRN
directive, A86 will assemble an instruction such as "MOV
AX,IFARK" as the loading of an immediate constant IFARK into the
AX register. If you place the directive "EXTRN IFARK:W" at the
top of your program, you'll get the correct instruction form for
MOV AX,IFARK-- moving a word-memory variable into the AX
register.

A86 will allow more than one EXTRN directive for a given symbol,
as long as the same type is given every time. A86 will even
allow an EXTRN directive for a symbol that has already been
defined, as long as the type declared is consistent with the
symbol's definition. These allowances exist so that you can
assemble multiple files written for another assembler, that had
been fed separately to that assembler.
10-7

Note that EXTRN is viewed quite differently by A86 than by other

assemblers. In fact, if it weren't for those other assemblers,
I'd use the mnemonic DECLARE instead of EXTRN. A86 doesn't
really use EXTRN to determine which symbols are external-- it
uses those symbols that are undefined at the end of assembly. As
I stated earlier in the chapter, an undefined symbol can be
referenced without being declared via EXTRN. Conversely, a
defined symbol can be declared (and redeclared) via EXTRN; being
defined, such a symbol will not be specified "external" in the
.OBJ file.

Because EXTRN is useful in forward reference situations, it is

now recognized even when A86 is assembling a .COM file.

For those of you who are accustomed to the more traditional use
of EXTRN, and who do not like external records to be created
"behind your back", A86 offers the "+G16" option. If you include
"+G16" in the program invocation, A86 will require that all
undefined symbols be explicitly declared via an EXTRN. Any
undefined, undeclared symbols will be included in the .UND
listing of undefined symbols, and object-file output will be
suppressed.

MAIN: The Starting Location for a Program

I've already stated that exactly one module in a program is the

"main" module, containing the starting address of the entire
program. In A86 when assembling .OBJ files, the starting address
is given by the label MAIN. You simply provide the label "MAIN:"
where you want the program to start. The module containing MAIN
is the main module. Note that if you have the +c
case-sensitivity switch enabled, MAIN must be in all-caps.

The END Directive

Syntax: END
END start_addr

The END directive is used by other assemblers for two purposes,

both of which are now a little silly. The first purpose is to
signal the end of assembly. This was necessary back in the days
when source files were input on media such as paper tape: you had
to tell the assembler explicitly that the content of the tape has
ended. Today the operating system can tell you when you've
reached the end of the file, so this function is an anachronism.
10-8

The second purpose of END is, nonsensically, to allow you to

specify the starting location of the program. I suppose the
person who wrote the first assembler back in the 1950's was too
short on memory to implement a separate START directive, or a
MAIN label like A86 has, and decided to let END do double duty.
I've always considered the example "END START" to have an
Alice-in-Wonderland quality; it is fuel for the
high-level-language snobs who like to attack assembly language.
Please defeat the snobs, and use "MAIN:" if you are writing new
code.

For compatibility, A86 treats "END start_addr" exactly the same

as if you had coded "MAIN EQU start_addr". Note that if you want
your program to assemble under both A86 and other assemblers, you
can specify "END MAIN"-- A86 treats MAIN EQU MAIN as a legal
redefinition of the symbol MAIN.

A86 ignores END when there is no starting-address operand, thus

allowing assembly of multiple files written for other assemblers.

The SEGMENT Directive

Syntax: seg_name SEGMENT [align] [combine] [use] ['class_name']

where "align" is one of: BYTE WORD PARA PAGE
"combine" is one of: PUBLIC STACK COMMON MEMORY
AT number
"use" is one of: USE16 USE32 FLAT

The SEGMENT directive says that assembled object code will

henceforth go to a block of code whose name is "seg_name".
"seg_name" is a symbol that represents a value that can be loaded
into a segment register. If "seg_name" is not declared in a
GROUP directive, then its value should in fact be loaded into a
segment register, in order to address the code. If "seg_name" is
declared in a GROUP directive, then the code is a part of the
segment addressed by the name of the group.

A program can consist of any number of named segments, to be

combined in numerous exotic ways to produce the final program.
You can redirect your object output from one segment to another
in your assembly, by providing a SEGMENT directive before each
piece of code. You can even return to a segment you started
earlier, by repeating a SEGMENT with the same name-- the
assembler just picks up where it left off, subject to some
possible skipping for memory alignment, that I'll describe
shortly.
10-9

The specifications following the word SEGMENT help to describe

how the code in this module's part of the segment will be
combined with code for the same segment name given in other
modules; and also how this named segment will be grouped with
other named segments. Other assemblers require the
specifications to be given in the order indicated. A86 will
accept any order, and will accept commas between the
specifications if you want to provide them. The only restriction
is that "AT number" must be followed by a comma if it is not the
last specification on the line.

The "align" specification tells if each piece of code within the

segment should be aligned so that its starting address is an even
multiple of some number. BYTE alignment means there is no
requirement; WORD alignment requires each piece to start at a
multiple of 2; PARA alignment, at a multiple of 16; PAGE
alignment, at a multiple of 256. For example, suppose you have a
segment containing memory variables. You can declare the segment
with the statement "VAR_DATA SEGMENT WORD", which ensures that
the segment is aligned to an even memory address. That way you
can insure that all 16-bit and bigger memory quantities in the
segment are aligned to even addresses, for faster access on the
16-bit machines of the 86 family.

There are special rules governing alignment for multiple pieces

of the same named segment within the same program module. Other
assemblers outlaw conflicting alignment specifications in this
situation; A86 accepts them, and uses the strictest specification
given. Furthermore, the alignment given for any specification
beyond the first will control the alignment for that piece of
code within this module's chunk. For example, if a program
contains two pieces of code headed by "VAR_DATA SEGMENT WORD",
A86 will insert a byte between the pieces if the first piece has
an odd number of bytes. This insures correct assembly for
multiple files written for another assembler.

If no "align" type is given for any of the pieces of a named

segment, an alignment of PARA is assumed.

The "combine" specification tells how the chunk of code from this
module will be combined with the chunks of the same named
segment, that come from other modules. Yes, I know, that sounds
like what "align" does; but "combine" takes a different, more
major point of view:

* PUBLIC is the kind of combination we've been talking about all

along: each piece of the segment is located off the end of the
previously linked piece, subject to possible gaps for
alignment. The size of the segment is the sum of the sizes of
the pieces, plus the sizes of the gaps.

* STACK is a combination type reserved for the system's stack

segment. To illustrate how STACK segment chunks are combined,
let's describe the only way a stack segment should ever be
used. We'll call the segment STACK; and we declare it as
follows:
10-10

STACK SEGMENT WORD STACK

DW 100 DUP (?)
TOP_OF_STACK:

The code just given declares a stack area of 200 bytes (100
words) for this module. If identical code occurs in each of
three modules which are then linked together, the resulting
STACK segment will have 600 bytes (the sizes are added), but
TOP_OF_STACK will be the same address (600) for each module
(each piece is overlayed at the top of the segment). That way,
every module can declare and access the top of the stack, which
is the only static part of the stack that any code should ever
refer to.

* COMMON is a type of memory area supported by FORTRAN. Each

module's chunk of a COMMON segment starts at location 0, and
overlaps (usually duplicates) the pieces from all the other
modules. The size of a COMMON segment is the size of the
largest chunk.

* MEMORY is an obsolete combination type. All recent

implementations of LINK treat MEMORY as a synonym for PUBLIC.

* "AT number" defines a non-combinable segment at the absolute

memory location whose segment register value is "number". This
form is useful for initializing data in fixed locations, such
as the 86 interrupt vector (IVECTOR SEGMENT AT 0 followed by
ORG 4 * INT_NUMBER), or for reading fixed memory locations,
such as the BIOS variables area (BIOS_DATA SEGMENT AT 040).
The combine type specification can be repeated in subsequent
pieces of a given segment, but if it is, it must be the same in
all pieces.

Finally, if no combine type is ever given for a named segment in

a module, that segment is non-combinable-- no other modules may
define that segment; the code given in the one module constitutes
the entire segment.

The "use" specification, recognized only by future versions of

A386, tells whether the segment is a 32-bit segment. The default
value USE16 means the segment is a 16-bit segment: 32-bit
registers, memory operands, and memory indexing will be encoded
using address and operand override bytes. The value USE32 or its
synonym FLAT specify a 32-bit segment (to be loaded and run only
in protected-mode environments such as Windows or OS/2). In
32-bit segments, the 16-bit operands and addresses require the
override opcode bytes, and the 32-bit operands and addresses
don't.

At this writing, segments of type USE32 are not yet implemented.

See the documentation files accompanying A386 on your registered
A86+D86 disk, for the latest news on what is implemented in A386.
10-11

The last specification available on a SEGMENT line is the class

name, which is identified by being enclosed in single quotes.
Unlike a segment name, which can be used as an instruction
operand and hence cannot conflict with other assembler symbols, a
class name can be assigned without regard to its usage elsewhere
in the program. It can even be a built-in A86 mnemonic. In
fact, both the SMALL and LARGE high-level-language models specify
the class name 'CODE' for code segments, and the SMALL model
specifies the class name 'DATA'.

If no class name is given for a segment, A86 specifies the null

(zero length) string as the class name.

DATA SEGMENT, STRUC and CODE SEGMENT Directives

The DATA SEGMENT and STRUC directives work in .OBJ mode exactly
as they do in .COM mode-- they define a special assembly mode, in
which declarations are made, but no object code is output.
Offsets within DATA segments and structures are absolute, as in
.COM mode. Assembly resumes as before when an ENDS or CODE
SEGMENT directive is encountered.

For MASM compatibility (especially in modules written to link to

Turbo Pascal V4.0 programs), I now recognize the reserved symbols
CODE, DATA, and STACK as ordinary relocatable segment names. The
ordinary functionality takes effect whenever a SEGMENT directive
is given with CODE, DATA or STACK as the segment name, and with
one or more relocatable parameters (e.g., PUBLIC) given after
SEGMENT.
The ENDS Directive

Syntax: [seg_name] ENDS

The ENDS directive closes out the segment currently being

assembled, and returns assembly to the segment being assembled
before the last SEGMENT directive. The "seg_name", if given,
must match the name in that last SEGMENT directive. ENDS allows
you to "nest" segments inside one another. For example, you can
declare some static data variables that are specific to a certain
section of code at the top of that section:

_DATA SEGMENT BYTE PUBLIC 'DATA'

VAR1 DB ?
VAR2 DB ?
_DATA ENDS

These four lines can be inserted inside any other segment being
assembled. They will cause the two variable allocations to be
tacked onto the segment _DATA; and assembly will then continue in
whatever segment surrounded the four lines. Observe that the
"nesting" does not occur in the final program; only the
presentation of the source code is nested.
10-12

If you are not nesting segments inside one another, then the ENDS
directive serves only to lend a clean, "block-structured"
appearance to your source code. It does not assist A86 in any
particular way; in fact, it consumes a bit more object output
memory (slightly reducing object output capacity) if you have
ENDSs, rather than just starting up new segments with SEGMENT
directives.

Default Outer SEGMENT

Other assemblers outlaw any code outside of a SEGMENT

declaration, forcing you to give a SEGMENT declaration before you
can assemble anything. A86 lets you assemble just your code; you
don't have to worry about SEGMENTs if you don't want to.

If you do provide code outside of all SEGMENT declarations, A86

performs the following steps, to find a reasonable place to put
the code:

1. If there are any segments explicitly declared whose name is or

ends with "_TEXT", then the first such segment declared is
used. It is as if the SEGMENT declaration appeared at the top
of, rather than within, the program.

2. If there is no such explicit segment, A86 creates a BYTE

PUBLIC segment of class 'CODE', and proceeds to construct a
name for the segment. If there are no RETF instructions in
the outer segment, the name chosen is "_TEXT", conforming to
the SMALL model of computation. If there is a RETF
 instruction, the name chosen is "modulename_TEXT", where
"modulename" is the name of this module. Recall that
"modulename" comes from the NAME directive if there is one;
from the name of the .OBJ file if there isn't.

The GROUP Directive

Syntax: group_name GROUP seg_name1, seg_name2, ...

The GROUP directive causes A86 to tell LINK that all the listed
segments can fit into a single 64K-byte block of memory, and
instruct LINK to make that fit. (If they won't fit, LINK will
issue an error message.) Having declared the group, you can then
use "group_name" as the segment register value that will allow
simultaneous access to all the named segments. The order of
names given in the list does not necessarily determine the order
in which the segments will finally appear within the group.

The most useful application of the GROUP directive is to allow

you to structure the pieces of a program, all of whose code and
data will fit into a single 64K segment. You organize the pieces
into SEGMENTs, and declare all the SEGMENTs to be within the same
GROUP. When the program starts, all segment registers are set to
point to the GROUP, and you never have to worry about segment
registers again in the program.
10-13

WARNING: If your segments will be GROUPed in the final program,

you should have the appropriate GROUP directive in every module
assembled. If you don't, then any memory pointers generated will
be relative to the beginning of the individual named segments,
not to the beginning of the whole group.

Because of the obscure scenario I described in the Overview

section, Intel does not prohibit more than one GROUP from
containing some of the same segments; so neither does A86. Any
pointers within a segment will be calculated from the beginning
of the last GROUP within which the segment was declared. But
again, I have my doubts as to whether LINK will handle this
correctly.

The SEG Operator

Syntax: SEG operand

The SEG operator returns the segment containing its operand-- a

value suitable for loading into one of the segment registers. If
the operand is an explicit far constant such as 01811:0100, the
value returned is the lefthand component of the constant (01811
in this example). Otherwise, the result depends on A86's output
mode:

When A86 is assembling to an OBJ file, the result is the named

relocatable segment containing the operand. SEG is most useful
when the operand is not defined in this A86 module: in that case,
the segment value will be plugged in by LINK.

When A86 is assembling to a COM file, SEG always returns the CS

register, with one exception: symbols declared within a SEGMENT
AT structure return the value of the containing segment. COM
files have no facility for explicitly specifying relocatable
segments, so for compatibility A86 assumes that all non-absolute
segment references are to the program's segment itself.

CHAPTER 11 MACROS AND CONDITIONAL ASSEMBLY

Macro Facility

A86 contains an easy-to-use, but very powerful macro facility.

The facility subsumes the capabilities of most assemblers,
including operand concatenation, repeat, indefinite repeat (often
called IRP), indefinite repeat character (IRPC), passing macro
operands by text or by value, comparing macro operands to
strings, and detecting blank macro operands. Unlike other
assemblers, A86 integrates these functions into the main macro
facility; so they can be invoked without clumsy syntax, or
strange characters in the macro-call operands.

Simple Macro Syntax

All macros must be defined before they are used. A macro

definition consists of the name of the macro, followed by the
word MACRO, followed by the text of the macro, followed by #EM,
which marks the end of the macro.

Many assembly languages require a list of dummy operand names to

follow the word MACRO. A86 does not: the operands are denoted in
the text with the fixed names #1, #2, #3, ... up to a limit of
#9, for each operand in order. If there is anything following
the word MACRO, it is considered part of the macro text.

Examples:

; CLEAR sets the register operand to zero.

CLEAR MACRO SUB #1,#1 #EM

CLEAR AX ; generates a SUB AX,AX instruction

CLEAR BX ; generates a SUB BX,BX instruction

; MOVM moves the second operand to the first operand.

; Both operands can be memory variables.

MOVM MACRO
MOV AL,#2
MOV #1,AL
#EM
VAR1 DB ?
VAR2 DB ?

MOVM VAR1,VAR2 ; generates MOV AL,VAR2 followed by MOV VAR1,AL

11-2

Formatting in Macro Definitions and Calls

The format of a macro definition is flexible. If the macro text

consists of a single instruction, the definition can be given in
a single line, as in the CLEAR macro given above. There is no
particular advantage to doing this, however: A86 prunes all
unnecessary spaces, blank lines, and comments from the macro text
before entering the text into the symbol table. I recommend the
more spread-out format of the MOVM macro, for program
readability.

All special macro operators within a macro definition begin with

a hash sign # (a hex 23 byte). The letters following the hash
sign can be given in either upper case or lower case. Hash-sign
operators are recognized even within quoted strings. If you wish
the hash sign to be treated literally, and not as the start of a
special macro operator, you must give 2 consecutive hash signs:
##. For example:

FOO MACRO
DB '##1'
DB '#1'
#em

FOO abc ; produces DB '#1' followed by DB 'abc'

The format of the macro call line is also flexible. A macro call
consists of the name of the macro, followed by the operands to be
plugged into the macro. A86 prunes leading and trailing blanks
from the operands of a macro call. The operands to a macro call
are always separated by commas. Also, as in all A86 source
lines, a semi-colon occurring outside of a quoted string is the
start of a comment, ignored by A86. If you want to include
commas, blanks, or semi-colons in your operands, you must enclose
your operand in single quotes.

Macro Operand Substitution

Some macro assemblers expect the operands to macro calls to

follow the same syntax as the operands to instructions. In those
assemblers, the operands are parsed, and reduced to numeric
values before being plugged into the macro definition text. This
is called "passing by value". As its default, A86 does not pass
by value, it passes by text. The only parsing of operands done
by the macro processor is to determine the start and the finish
of the operand text. That text is substituted, without regard
for its contents, for the "#n" that appears in the macro
definition. The text is interpreted by A86 only after a complete
line is expanded and as it is assembled.
11-3

If the first non-blank character after the macro name is a comma,

then the first operand is null: any occurrences of #1 in the
macro text will be deleted, and replaced with nothing. Likewise,
any two consecutive commas with no non-blanks between them will
result in the corresponding null operand. Also, out-of-range
operands are null; for example, #3 is a null operand if only two
operands are provided in the call.

Null operands to macros are not in themselves illegal. They will

produce errors only if the resulting macro expansion is illegal.

The method of passing by text allows operand text to be plugged

anywhere into a macro, even within symbol names. For example:

; KF_ENTRY creates an entry in the KFUNCS table, consisting of a

; pointer to a KF_ action routine. It also declares the
; corresponding CF_ symbol, which is the index within the table
; for that entry.

KF_ENTRY MACRO
CF_#1 EQU ($-KFUNCS)/2+080
DW KF_#1
#EM

KFUNCS:
KF_ENTRY UP
KF_ENTRY DOWN

; The above code is equivalent to:

;
; KFUNCS:
; DW KF_UP
; DW KF_DOWN
;
; CF_UP EQU 080
; CF_DOWN EQU 081

Quoted String Operands

As mentioned before, if you want to include blanks, commas, or

semicolons in your operands, you enclose the operand in single
quotes. In the vast majority of cases in which these special
characters need to be part of operands, the user wants them to be
quoted in the final, assembled line also. Therefore, the quotes
are passed in the operand. To override this, and strip the
quotes from the string, you precede the quoted string with a hash
sign. Examples:
11-4

DBW MACRO
DB #1
DW #2
#EM

DBW 'E', E_POINTER

DBW 'W', W_POINTER

; note that if quotes were not passed, the above lines would have
; to be DBW '''E''', E_POINTER; DBW '''W''', W_POINTER

FETCH_CHAR MACRO
LODSB
#1
CALL PROCESS_CHAR
#EM

FETCH_CHAR STOSB ; generates STOSB as second instruction

FETCH_CHAR #'INC DI' ; generates INC DI as second instruction

Looping by Operands in Macros

A86's macro facility contains two kinds of loops: you can loop
once for each operand in a range of operands; or you can loop
once for each character within an operand. The first kind of
loop, the R-loop, is discussed in this section; the second kind,
the C-loop, is discussed later.

An R-loop is a stretch of macro-definition code that is repeated

when the macro is expanded. In addition to the fixed operands #1
through #9, you can specify a variable operand, whose number
changes each time through the loop. You give the variable
operand one of the 4 names #W, #X, #Y, or #Z.

An R-loop begins with #R, followed immediately by the letter

W,X,Y, or Z naming the variable, followed by the number of the
first operand to be used, followed by the number of the last
operand to be used. After the #Rxnn is the text to be repeated.
The R-loop ends with #ER. For example:

STORE3 MACRO
MOV AX,#1
#RY24 ; "repeat for Y running from 2 through 4"
MOV #Y,AX
#ER
#EM

STORE3 VAR1,VAR2,VAR3,VAR4

; the above call produces the 4 instructions MOV AX,VAR1; MOV VAR2,AX;
; MOV VAR3,AX; MOV VAR4,AX.
11-5

The #L Last Operator and Indefinite Repeats

A86 recognizes the special operator #L, which is the last operand
in a macro call. #L can appear anywhere in macro text; but its
big power occurs in conjunction with R-loops, to yield an
indefinite-repeat facility.

A common example is as follows: you can take any macro that is

designed for one operand, and easily convert it into a macro that
accepts any number of operands. You do this by placing the
command #RX1L, "repeat for X running from 1 through L", at the
start of the macro, and the command #ER at the end just before
the #EM. Finally, you replace all instances of #1 in the macro
with #X. We see how this works with the CLEAR macro:

CLEAR MACRO #RX1L

SUB #X,#X
#ER
#EM

CLEAR AX,BX ; generates both SUB AX,AX and SUB BX,BX in one macro!

It is possible for R-loops to iterate zero times. In this case,

the loop-text is skipped completely. For example, CLEAR without
any operands would produce no expanded text.

Character Loops

We have seen the R-loop; now we discuss the other kind of loop in
macros, the character loop, or C-loop. In the C-loop, the
variable W,X,Y, or Z does not represent an entire operand; it
represents a character within an operand.

You start a C-loop with #C, followed by one of the 4 letters

W,X,Y, or Z, followed by a single operand specifier-- a digit,
the letter L, another one of W,X, Y, or Z defined in an outer
loop, or one of the more complicated specifiers defined later in
this chapter. Following the #Cxn is the text of the C-loop. The
C-loop ends with #EC. The macro will loop once for every
character in the operand. That single character will be
substituted for each instance of the indicated variable operand.
For example:

PUSHC MACRO #CW1

PUSH #WX
#EC#EM

PUSHC ABC ; generates PUSH AX | PUSH BX | PUSH CX

If the C-operand is quoted in the macro call, the quotes ARE

removed from the operand before passing characters to the loop.
It is not necessary to precede the quoted string with a hash sign
in this case. If you do, the hash sign will be passed as the
first character.
11-6

If the C-operand is a null operand (no characters in it), the

loop text is skipped completely.

The "B"-Before and "A"-After Operators

So far, we have seen that you can specify operands in your macro
in fourteen different ways: 1,2,3,4,5,6,7,8,9,W,X,Y,Z,L. We now
multiply these 14 possibilities, by introducing the "A" and "B"
operators. You can precede any of the 14 specifiers with "A" or
"B", to get the adjacent operand after or before the specified
operand. For example, BL means the operand just before the last
operand; in other words, the second-to-the-last operand. AZ
means the operand just after the Z operand. You can even repeat,
up to a limit of 4 "B"s or 3 "A"s: for example, BBL is the
third-to-last operand.

Note that any operand specifier can appear in contexts other than
by itself following a # within a macro. For example, BBL could
appear as the upper limit to an R-loop: #RZ1BBL loops with Z
running from the first operand to the third-to-last operand.

In the case of the variable operand to a C-loop, the "A" and "B"
specifiers denote the characters before or after the current
looping-character. An example of this is given in the next
section.

Multiple Increments within Loops

We have seen that you end an R-loop with a #ER, and you end a
C-loop with a #EC. We now present another way to end these
loops; a way that lets you specify a larger increment to the
macro's loop counter. You can end your loops with one of the 4
additional commands #E1, #E2, #E3, or #E4.

For R-loops terminated by #ER, the variable operand advances to

the next operand when the loop is made. If you end your R-loop
with #E2, the variable operand advances 2 operands, not just one.
For #E3, it advances 3 operands; for #E4, 4 operands. The #E1
command is the same as #ER.

The most common usage of this feature is as follows: You will

recall that we generalized the CLEAR macro with the #L-variable,
so that it would take an indefinite number of operands. Suppose
we want to do the same thing with the DBW macro. We would like
DBW to take any number of operands, and alternate DBs and DWs
indefinitely on the operands. This is made possible by creating
an R-loop terminated by #E2:

DBW MACRO #RX1L

DB #X
DW #AX
#E2
#EM

DBW 'E',E_POINTER, 'W',W_POINTER ; two pairs on same line!

11-7

The #E2 terminator means that we are looping on a pair of

operands. Note the crucial usage of the "A"-after operator to
specify the second operand of the operand pair.
A special note applies to the DBW macro above: A86 just happens
to accept a DW directive with no operands (it generates no object
code, and issues no error). This means that DBW will accept an
odd number of operands with no error, and do the expected thing
(it alternates bytes and words, ending with a byte).

You could likewise generalize a macro with 3 or 4 operands, to an

indefinite number of triples or quadruples; by ending the R-loop
with #E3 or #E4. The operands in each group would be specified
by #X, #AX, #AAX, and, for #E4, #AAAX.

For C-loops terminated by #E1 through #E4, the character pointer

is advanced the specified number of characters. You use this in
much the same way as for R-loops, to create loops on pairs,
triplets, and quadruplets of characters. For example:

PUSHC2 MACRO #CZ1

PUSH #Z#AZ
#E2
#EM

PUSHC2 AXBXSIDI ; generates PUSH AX | PUSH BX | PUSH SI | PUSH DI

Negative R-loops

We now introduce another form of R-loop, called the Q-loop-- the

negative repeat loop. This loop is the same as the R-loop,
except that the operand number decrements instead of increments;
and the loop exits when the number goes below the finish-number,
not above it. The Q-loop is specified by #Qxnn instead of #Rxnn,
and #EQ instead of #ER. You can also use the multiple-decrement
forms #E1 #E2 #E3 or #E4 to terminate an Q-loop.

Example:

MOVN MACRO #QXL2 ; "negative repeat X from L down to 2"

MOV #BX,#X
#EQ#EM

MOVN AX,BX,CX,DX ; generates the three instructions:

; MOV CX,DX
; MOV BX,CX
; MOV AX,BX

Note: the above functionality is already built into the MOV

instruction of A86. The macro shows how you would implement it
if you did not already have this facility.
11-8

Nesting of Loops in Macros

A86 allows nesting of loops within each other. Since we provide

the 4 identifiers W,X,Y,Z for the loop operands, you can nest to
a level of 4 without restriction-- just use a different letter
for each nesting level. You can nest even deeper, for example,
by having two nested R-loops that use W is its indexing letter.
The only restriction to this is that you cannot refer to the W of
the outer loop from within the inner W loop. (I challenge anyone
to come up with an application in which these limitations /
restrictions cause a genuine inconvenience!)

Implied Closing of Loops

If you have a loop or loops ending when the macro ends, and if
the iteration count for those loops is 1, you may omit the #ER,
#EC, or #EQ. A86 closes all open loops when it sees #EM, with no
error.

For example, if you omit the #ER for the loop version of the
CLEAR macro, it would make no difference-- A86 automatically
places an #ER code into the macro definition for you.

Passing Operands by Value

As already stated, A86's default mode for passing operands is by

text-- the characters of the operand are copied to the macro
expansion line as-is, without any evaluation. You may override
this with the #V operator. When A86 sees #Vn in a macro
definition, it will evaluate the expression given in the text of
operand n, and pass a string representing the decimal constant
answer, instead of the original text. The operand must evaluate
to an absolute constant value, less than 65536. For example:

JLV MACRO
J#1 LABEL#V2
#EM

JINDEX = 3
JLV NC,JINDEX+1 ; generates JNC LABEL4
JINDEX = 6
JLV Z,JINDEX+2 ; generates JZ LABEL8

Passing Operand Size

The construct #Sn is translated by A86 into the decimal string

representing the number of characters in operand n. One use of
this would be to make a conditional-assembly test of whether an
operand was passed at all, as we'll see later in this chapter.
Another use is to generate a length byte preceding a string, as
required by some high-level languages such as Turbo Pascal.
Example:
11-9

LSTRING MACRO
DB #S1,'#1'
#EM

LSTRING SAMPLE ; generates DB 6,'SAMPLE'

Generating the Number of an Operand

The construct #Nn is translated by A86 into the decimal string

represented by the position number n of the macro operand. Note
that this value does not depend on the contents of the operand
that was passed to the macro. Thus, for example, #N2 would
translate simply to 2; so this usage of #N is silly. #N achieves
usefulness when n is variable: W,X,Y,Z, or L. I give an example
of #N with a loop-control variable in the next section. Here is
an example of #NL, used to generate an array of strings, preceded
by a byte telling how many strings are in the array:

ZSTRINGS MACRO
DB #NL ; generates the number of operands passed
#RX1L
DB '#X',0
#EM

ZSTRINGS TOM,DICK,HARRY ; generates DB 3 followed by strings

Parenthesized Operand Numbers

We've seen that macro operands are usually specified in your

macro definition by a single character: either a single digit or
one of the special letters W,X,Y,Z, or L. A86 also allows you to
specify a constant operand number up to 255. You do so by giving
an expression enclosed in parentheses, rather than a single
character. The expression must evaluate at the time the macro is
defined, to a constant between 0 and 255. You can use this
feature to translate many programs that use MASM's REPT
directive. For example, if the following REPT construct occurs
within a MASM macro:

TEMP = 0
REPT 100
TEMP = TEMP + 1 ; MASM needs an explicitly-set-up counter
DB TEMP
ENDM

you may translate it into an A86 loop, as follows:

#RX1(100) ; the counter X is built into the A86 loop

DB #NX
#ER

If the REPT does not occur within a macro, you must define a
macro containing the loop, which you may then immediately call.
11-10

Note that the expression enclosed in parentheses must not itself

contain any macro operators. Thus, for example, you cannot
specify #(#NY+1) to represent the operand after Y-- you must use
#AY.
Exiting from the Middle of a Macro

For MASM compatibility, A86 offers the #EX operator, which is

equivalent to MASM's EXITM directive. #EX is typically used in a
conditional-assembly block within a loop, to terminate the loop
early. When the #EX code is seen in a macro expansion, the
expansion ceases at that point, and assembly returns to the
source file (or to the outer macro in a nested call). You
couldn't use #EM to do this, because that would signal the end of
the macro definition, not just the call.

Local Labels in Macros

Some assemblers have a LOCAL pseudo-op that is used in

conjunction with macros. Symbols declared LOCAL to a macro have
unique (and bizarre) symbol names substituted for them each time
the macro is called. This solves the problem of duplicate label
definitions when a macro is called more than once.

In A86, the problem is solved more elegantly, by having a class

of generic local labels throughout assembly, not just in macros.
Recall that symbols consisting of a single letter, followed by
one or more decimal digits, can be redefined. You can use such
labels in your macro definitions.

I have recommended that local labels outside of macros be

designated L1 through L9. Within macro definitions, I suggest
that you use labels M1 through M9. If you used an Ln-label
within a macro, you would have to make sure that you never call
the macro within the range of definition of another Ln-label with
the same name. By using Mn-labels, you avoid such potential
conflicts.

The following example of a local label within a macro is taken

from the source of the macro processor itself:

; "JHASH label" checks to see if AL is a hash sign. If it is,

; it processes the hash sign term, and jumps to label.
; Otherwise, it drops through to the following code.

JHASH MACRO
CMP AL,'##' ; is the scanned character a hash sign?
JNE >M1 ; skip if not
CALL MDEF_HASH ; process the hash sign
JMP #1 ; jump to the label provided
M1:
#EM
11-11

...
L3: ; loop here to eat empty lines, leading blanks
CALL SKIP_BLANKS ; skip over the leading blanks of a line
INC SI ; advance source ptr beyond the next non-blank
JHASH L3 ; if hash sign then process, and eat more blanks
CMP AL,0A ; were the blanks terminated by a linefeed?
JE L3 ; loop if yes, nothing on this line
L5: ; loop here after a line is seen to have contents
CMP AL,';' ; have we reached the start of a comment?
JE L1 ; jump if yes, to consume the comment
JHASH >L6 ; if hash sign then process it; get next char
...
L6:
LODSB ; fetch the next definition char from the source
CMP AL,' ' ; is it blank?
JA L5 ; loop if not, to process it
...

Debugging Macro Expansions

There are two ways that A86 lets you debug your macro expansions.
First, if A86 encounters an error within a macro expansion, it
now includes the offending expansion line within the error
message. This will often allow you to spot the problem. If you
need a complete listing of the expanded macro, the A86 listing
will now give you that. These facilities replace the old EXMAC
tool, which sometimes failed to expand excessively complicated
macros the way the assembler did.

Conditional Assembly

A86 has a conditional-assembly feature which allows you to

specify that blocks of source code will or will not be assembled,
according to the values of equated user symbols. The controlling
symbols can be declared in the program (and can thus be the
result of assembly-time expressions), or they can be declared in
the assembler invocation.

You should keep in mind the difference between conditional

assembly, invoked by #IF, and the structured-programming feature,
invoked by IF without the hash sign. #IF tests a condition at
assembly time, and can cause code to not be assembled and thus
not appear in the program. IF causes code to be assembled that
tests a condition at run time, possibly jumping over code. The
skipped code will always appear in the program.

All conditional-assembly lines are identified by a hash sign # as

the first non-blank character of a line. The hash sign is
followed by one of the four reserved symbols IF, ELSEIF, ELSE or
ENDIF.
11-12

#IF starts a conditional-assembly block. On the same line,

following the #IF, you provide either a single name, or an
arbitrary expression evaluating to an absolute constant. In this
context, a single name evaluates to TRUE if it is defined and not
equal to the absolute constant zero. A name is FALSE if it is
undefined, or if it has been equated to zero. An expression is
TRUE if nonzero, FALSE if zero.

If the #IF expression evaluates to FALSE, then the following

lines of code are skipped, up to the next matching #ELSEIF,
#ELSE, or #ENDIF. If the expression is TRUE, then the following
lines of code are assembled normally. If a subsequent matching
#ELSEIF or #ELSE is encountered, then code is skipped up to the
matching #ENDIF.

#ELSEIF provides a multiple-choice facility for #IF-blocks. You

can give any number of #ELSEIFs between an #IF and its matching
#ENDIF. Each #ELSEIF has a name or expression following it on
the same line. If the construct following the #IF is FALSE, then
the assembler looks for the first TRUE construct following an
#ELSEIF, and assembles that block of code. If there are no TRUE
#ELSEIFs, then the #ELSE-block (if there is one) is assembled.

You should use the ! instead of the NOT operator in

conditional-assembly expressions. The ! operator performs the
correct translation of names into TRUE or FALSE values, and
handles the case !undefined without reporting an error.

#ELSE marks the beginning of code to be assembled if all the

previous blocks of an #IF have been skipped over. There is no
operand after the #ELSE. There can be at most one #ELSE in an
#IF-block, and it must appear after any #ELSEIFs.

#ENDIF marks the end of an #IF-block. There is no operand after

#ENDIF.

It is legal to have nested #IF-blocks; that is, #IF-blocks that

are contained within other #IF-blocks. #ELSEIF, #ELSE, and
#ENDIF always refer to the innermost nested #IF-block.

As an example of conditional assembly, suppose that you have a

program that comes in three versions: one for Texas, one for
Oklahoma, and one for the rest of the nation. The three programs
differ in a limited number of places. Instead of keeping three
different versions of the source code, you can keep one version,
and use conditional assembly on the boolean variables TEXAS and
OKLAHOMA to control the assembler output. A sample block would
be:

#if TEXAS
DB 0,1,2,3
#elseif OKLAHOMA
DB 4,5,6,7
#else
DB 8,9,10,11
#endif
11-13

If a block of code is to be assembled only if TEXAS is false,

then you would use the exclamation-point operator:

#if !TEXAS
DB 0FF
#endif

Conditional Assembly and Macros

You may have conditional-assembly blocks either in macro
definitions or in macro expansions. The only limitation is that
if you have an #IF-block in a macro expansion, the entire block
(i.e., the matching #ENDIF) must appear in the same macro
expansion. You cannot, for example, define a macro that is a
synonym for #IF.

To have your conditional-assembly block apply to the macro

definition, you provide the block normally within the definition.
For example:

X1 EQU 0
BAZ MACRO
#if X1
DB 010
#else
DB 011
#endif
#EM
BAZ
X1 EQU 1
BAZ

In the above sequence of code, the conditional-assembly block is

acted upon when the macro BAZ is defined. The macro therefore
consists of the single line DB 011, with all the
conditional-assembly lines removed from the definition. Thus,
both expansions of BAZ produce the object-code byte of 011, even
though the local label X1 has turned non-zero for the second
invocation.

To have your conditional-assembly block appear in the macro

expansion, you must literalize the hash sign on each
conditional-assembly line by giving two hash signs:

X1 EQU 0
BAZ MACRO
##if X1
DB 010
##else
DB 011
##endif
#EM
BAZ
X1 EQU 1
BAZ
11-14

Now the entire conditional-assembly block is stored in the macro

definition, and acted upon each time the macro is expanded. Thus,
the two invocations of BAZ will produce the different object
bytes 011 and 010, since X1 has become non-zero for the second
expansion.

You will usually want your conditional-assembly blocks to be

acted upon at macro definition time, to save symbol table space.
You will thus use the first form, with the single hash signs.

Simulating MASM's Conditional Assembly Constructs

Microsoft's MASM assembler has an abundance of confusing

conditional-assembly directives, all of which are subsumed by
A86's #IF expression evaluation policies. IFDEF is covered by
A86's #IF directive in conjunction with its DEF operator. IFE
and IFNDEF are duplicated by #IF followed by the
exclamation-point (boolean negation) operator, followed by a DEF
operator. IFB and IFNB test whether a macro operand has been
passed as blank -- they can be simulated by testing the size of
the operand with the #Sn operator. Finally, IFIDN and IFDIF do
string comparisons of macro operands. This is more generally
subsumed by the string-comparison capabilities of the operators
EQ, NE, and =.

Examples of translation of each of these constructs is given in

the next chapter, on compatibility with other assemblers.

Declaring Variables in the Assembler Invocation

To facilitate the effective use of conditional assembly, A86

allows you to declare boolean (true-false) symbols in the command
line that invokes the assembler. The declarations can appear
anywhere in the list of source file names. They are
distinguished from the file names by a leading equals sign =. To
declare a symbol TRUE (value = 1), give the name after the equals
sign. DO NOT put any spaces between the equals sign and the
name! To declare a symbol FALSE (value = 0), you can give an
equals sign, an exclamation point, then the name. Again, DO NOT
embed any blanks! Example: if your source files are src1.8,
src2.8, and src3.8, then you can assemble with TEXAS true by
invoking A86 as follows:

a86 =TEXAS src1.8 src2.8 src3.8

You can assemble with TEXAS explicitly set to FALSE as follows:

a86 =!TEXAS src1.8 src2.8 src3.8

Note that if TEXAS is used only as a conditional-assembly

control, then you do not need to include the =!TEXAS in the
invocation, because an undefined TEXAS will automatically be
interpreted as false.
11-15

A user pointed out to me that it's impossible to get an

equals-sign into an environment variable. So A86 now accepts an
up-arrow (hex 5E) character in place of an equals-sign for an
invocation variable.

Null Invocation Variable Names

A86 will ignore an equals-sign by itself in the invocation line,
without error. This allows you to generate assembler-invocation
lines using parameters that could be either boolean variable
names, or null strings. For example, in the previously-mentioned
TEXAS-OKLAHOMA-nation example, the program could be invoked via a
.BAT file called "AMAKE.BAT", coded as follows:

A86 =%1 *.8

You invoke A86 by typing one of the following:

amake texas
amake oklahoma
amake

The third line will produce the assembler invocation A86 = *.8;
causing no invocation variables to be declared. Thus both TEXAS
and OKLAHOMA will be false, which is exactly what you want for
the rest-of-the-nation version of the program.

Changing Values of Invocation Variables

The usual prohibition against changing the value of a symbol that

is not a local label does not apply to invocation variables. For
example, suppose you have a conditional-control variable DEBUG,
which will generate diagnostic code for debugging when it is
true. Suppose further that you have already debugged source
files src1.8 and src3.8; but you are still working on src2.8. You
may invoke A86 as follows:

A86 src1.8 =DEBUG src2.8 =!DEBUG src3.8

The variable DEBUG will be TRUE only during assembly of src2.8,

just as you want.

CHAPTER 12 COMPATIBILITY WITH OTHER ASSEMBLERS

I gave heavy priority to compatibility when I designed A86; a

priority just a shade behind the higher priorities of
reliability, speed, convenience, and power. For those of you who
feel that "close, but incompatible" is like saying "a little bit
pregnant", I'm sorry to report that A86 will not assemble all
Intel/IBM/MASM programs, unmodified. But I do think that a
majority of programs can, with a little massaging, be made to
assemble under A86. Furthermore, the massaging can be done in
such a way as to make the programs still acceptable to that old,
behemoth assembler.

I have been adding compatibility features with almost every new

version of A86. Among the features added since A86 was first
released are: more general forward references, double quotes for
strings, "=" as a synonym for EQU, the RADIX directive, the
COMMENT directive, and the COMPAT.8 file containing macros for a
number of segmentation-model directives. If you tried feeding an
old source file to a previous A86 and were dismayed by the number
of error messages you got, try again: things might be more
manageable now.

Conversion of MASM programs to A86

Following is a list of the things you should watch out for when
converting from MASM to A86:

1. You need to determine whether the program was coded as a COM

program or as an EXE program. All COM programs coded for MASM
will contain an ORG 100H directive somewhere before the start
of the code. EXE programs will contain no such ORG, and will
often contain statements that load named segments into
registers. If the program was coded as EXE, you must either
assemble it (using the +O option) to an OBJ file to be fed to
LINK, or you must eliminate the instructions that load segment
registers-- in a COM program they often aren't necessary
anyway, since COM programs are started with all segment
registers already pointing to the same value.

A good general rule is: when it doubt, try assembling to an

OBJ file.

2. You need to determine whether the program is executing with

all segment registers pointing to the same value. Simple COM
programs that fit into 64K will typically fall into this
category. Most EXE programs, programs that use huge amounts
of memory, and programs (such as memory-resident programs)
that take over interrupts typically have different values in
segment registers.
12-2

If there are different values in the segment registers, then

there may be instructions in the program for which the old
assembler generates segment-override prefixes "behind your
back". You will need to find such references, and to generate
explicit overrides for them. If there are data tables within
the program itself, a CS-override is needed. If there are
data structures in the stack segment not accessed via a
BP-index, an SS-override is needed. If ES points to its own
segment, then an ES-override is needed for accesses (other
than STOS and MOVS destinations) to that segment. In the
interrupt handlers to memory-resident programs, the "normal"
handler is often invoked via an indirect CALL or JMP
instruction that fetches the doubleword address of the normal
handler from memory, where it was stored by the initialization
code. That CALL or JMP often requires a CS-override-- watch
out!

If you want to remain compatible with the old assembler, then

code the overrides by placing the segment-register name, with
a colon, before the memory-access operand in the instruction.
If you do not need further compatibility, you can place the
segment register name before the instruction mnemonic. For
 example:

MOV AL,CS:TABLE[SI] ; if you want compatibility do this

CS MOV AL,TABLE[SI] ; if not you can do it this way

3. A86 is distributed with a file called COMPAT.8, containing a

set of macros that implement some of the directives seen in
recent versions of MASM. If you would like A86 to recognize
these directives, make sure copies of COMPAT.8 and A86.LIB are
in either the current directory, or a directory named by the
A86LIB environment variable. When you do so, the file
COMPAT.8 will automatically be included in your assembly
whenever you use one of the MASM directives. See Chapter 13
for more details about A86LIB.

4. You should use a couple of A86's switches to maximize

compatibility with MASM. I've already mentioned the +O switch
to produce .OBJ files. You should also assemble with the +D
switch, which disables A86's unique parsing of constants with
leading zeroes as hexidecimal. The RADIX command in your
program will also do this. And you should use the +G15 switch,
that disables a few other A86 features that might have reduced
compatibility. See Chapter 3 for a detailed explanation of
these switches.

5. A86 is a bit more restrictive with respect to forward

references than MASM, but not as much as it used to be. You'll
probably need to resolve just a few ambiguous references by
appending " B" or " W" to the forward reference name.

6. A86's macro-definition and conditional-assembly language is

different from MASM's. Most macros can be translated by
replacing the named parameters of the old macros with the
dedicated names #n of the A86 macro language; and by replacing
ENDM with #EM. For example, the following MASM macro:
12-3

MOVM MACRO DEST,SRC

MOV AL,DEST
MOV SRC,AL
ENDM

would be translated by eliminating the DEST,SRC declarations

on the first line, replacing DEST with #1 and SRC with #2 in
the body of the definiation, and replacing ENDM by #EM -- the
result is the MOVM macro that I presented at the beginning of
Chapter 11.

Other constructs have straightforward translations, as

illustrated by the following examples. Note that examples
involving macro parameters have double hash signs, since the
condition will be tested when the macro is expanded, not when
it is defined.

MASM construct Equivalent A86 construct

IFE expr #IF ! expr

 IFB <PARM3> ##IF !#S3
IFNB <PARM4> ##IF #S4
IFIDN <PARM1>,<CX> ##IF "#1" EQ "CX"
IFDIF <PARM2>,<SI> ##IF "#2" NE "SI"
IFDEF symbol #IF DEF symbol
IFNDEF symbol #IF ! DEF symbol
.ERR (any undefined symbol)
.ERRcond TRUE EQU 0FFFF
TRUE EQU cond
EXITM #EX
IRP ... ENDM #RX1L ... #ER
REPT 100 ...ENDM #RX1(100) ... #ER
IRPC ... ENDM #CX ... #EC

The last three constructs, IRP, REPT, and IRPC, usually occur
within macros; but in MASM they don't have to. The A86
equivalents are valid only within macros-- if they occur in
the MASM program outside of a macro, you duplicate them by
defining an enclosing macro on the spot, and calling that
macro once, right after it is defined.
12-4

7. Later versions of MASM have expanded the syntax of the PROC

directive, to support interfacing to languages such as Pascal
and C. The USES clause lists registers to be pushed at the
beginning of the procedure, and popped just before the RET
instruction for that procedure. In A86, you need to
explicitly provide such pushes and pops. MASM also allows the
declaration of parameters as local variables within the
procedure. Such parameters are passed by the calling program
on the stack, and are addressed using the BP register. The
precise offsets from the BP register depend on the particular
language of the calling program, and on whether the calls are
NEAR or FAR. In A86, you can either define the parameter
names as offsets from BP (the based STRUC feature of A86
assists in this), or replace usages of the local names with
explicit BP-index references. If you define parameter names,
keep in mind that A86 does not have local scoping of symbols:
if names are duplicated in different PROCs, you will need to
change the names to eliminate the duplications.

8. Later versions of MASM have block-structure mechanisms to

simulate the look and feel of a high-level language. A86 does
not yet support these mechanisms: you need to replace them
with the equivalent conditional-jump and LOOP instructions.

9. A86 supports the STRUC directive, with named structure

elements, just like MASM, with one exception: A86 does not
save initial values declared in the STRUC definition, and A86
does not allow assembly of instances of structure elements.

For example, the MASM construct

PAYREC STRUC
PNAME DB 'no name given'
PKEY DW ?
ENDS
 PAYREC 3 DUP (?)
PAYREC <'Eric',1811>

causes A86 to accept the STRUC definition, and to define the

structure elements PNAME and PKEY correctly; but the PAYREC
initializations need to be recoded. If it isn't vital to
initialize the memory with the specific definition values, you
could recode the first PAYREC as:

DB ((TYPE PAYREC) * 3) DUP ?

If you must initialize values, you do so line by line:

DB 'Eric '
DW ?

If there are many such initializations, you could define a

macro INIT_PAYREC containing the DB and DW lines.
12-5

Compatibility symbols recognized by A86

A86 has been programmed to ignore a variety of lines that have

meaning to Intel/IBM/MASM assemblers; but which do nothing for
A86. These include lines beginning with a percent sign, lines
beginning with ASSUME, and lines beginning with any unrecognized
symbol that begins with a period. If you are porting your
program to A86, and you wish to retain the option of returning to
the other assembler, you may leave those lines in your program.
If you decide to stay with A86, you can remove those lines at
your leisure.

In addition, there is a class of symbols now recognized by A86 in

its .OBJ mode, but still ignored in .COM mode. This includes
NAME, END, and PUBLIC.

Named SEGMENT and ENDS directives written for other assemblers

are, of course, recognized by A86's .OBJ mode. In non-OBJ mode,
A86 treats these as CODE SEGMENT directives. A special exception
to this is the directive

segname SEGMENT AT atvalue

which is treated by A86 as if it were the following sequence:

segname EQU atvalue

STRUC

This will accomplish what is usually intended when SEGMENT AT is

used in a program intended to be a COM file.

Conversion of A86 Programs to Intel/IBM/MASM

I consider this section a bit of a blasphemy, since it's a little

silly to port programs from a superior assembler, to run on an
inferior one. However, I myself have been motivated to do so
upon occasion, when programming for a client not familiar with
A86; or whose computer doesn't run A86, and who therefore wants
the final version to assemble on Intel's assembler. Since my
assembler/debugger environment is so vastly superior to any other
environment, I develop the program using my assembler, and port
it to the client's environment at the end.

The main key to success in following the above scenarios is to

exercise supreme will power, and not use any of the wonderful
language features that exist on A86, but not on MASM. This is
often not easy; and I have devised some methods for porting my
features to other assemblers:

1. I hate giving long sequences of PUSHes and POPs on separate

lines. If the program is to be ported to a lesser assembler,
then I put the following lines into a file that only A86 will
see:
12-6

PUSH2 EQU PUSH

PUSH3 EQU PUSH
POP2 EQU POP
POP3 EQU POP

I define macros PUSH2, PUSH3, POP2, POP3 for the lesser

assembler, that PUSH or POP the appropriate number of
operands. Then, everywhere in the program where I would
ordinarily use A86's multiple PUSH/POP feature, I use one or
more of the PUSHn/POPn mnemonics instead.

2. I refrain from using the feature of A86 whereby constants with

a leading zero are default-hexadecimal. All my hex constants
end with H.

3. I will usually go ahead and use my local labels L0 through L9;

then at the last minute convert them to a long set of labels
in sequence: Z100, Z101, Z102, etc. I take care to remove all
the ">" forward reference specifiers when I make the
conversion. The "Z" is used to isolate the local labels at
the end of the lesser assembler's symbol table listing. This
improves the quality of the final program so much that it is
worth the extra effort needed to convert L0--L9's to Z100--
Zxxx's.

CHAPTER 13 ASSOCIATED TOOLS AND OUTPUT FILES

Listings with A86

A86 has a powerful listing facility, that allows you to tailor

the format of your listings to your specific needs. Because the
listing pass adds a significant percentage to the time it takes
A86 to execute, the listing is not produced by default. You must
include either a +L switch, or the name of a file with a .LST
extension on the A86 invocation line.

By default (+L but nothing else specified), an A86 listing file

consists of a sequence of pages, each 59 lines long and 79
characters wide. Each page has a header line identifying A86 and
its version number, giving the name of the program output file,
the date and time of assembly, the name of the source file
currently being listed, and a page number. Note that I am not so
obnoxious as to splash my company name over the top of every page
of your listing! If both a TITLE and a SUBTTL have been
specified, the header consists of three content lines and one
skipped line; otherwise, there are just two content lines. Each
listing line has a sequential line number, a hex offset and hex
object bytes, an indicator field with "i" for include files and
"m" for macro expansions, and the source code itself. Nested
includes have no special indication; nested macros are indicated
by increasing indentation of the macro expansion line. A86 tries
to be intelligent about the formatting of its listings: it will
break up the wraparound of a long line at a word if reasonable.
It will avoid breaking up a multi-line listing of less than 10
lines. It will break pages at sensible locations (described in
detail shortly, under the PAGE directive). It will suppress
blank lines at the top and bottom of pages (but it counts them in
the sequential line numbering so you can tell they were there).

Five A86 switches, H, I, L, T, and W, allow you to control the

existence and characteristics of titling, pagination, page-number
format, page break control, source line numbering, hex object
display, and source line display. The operation of these
switches is described in detail in Chapter 3. Here are some
examples of switch settings that will produce listings meeting
some specialized needs:

+L21+T0+W12+I137 produces a listing consisting only of the source

code, with the hex offset of each line placed to the left, and
with the line truncated at 79 columns. Such a listing file would
be ideal for viewing the source file while debugging on a
primitive remote system that cannot run D86.

+L9+T0+W4+I128 produces a list file of just source code, with all

conditional-assembly lines and skipped code removed. All
titling, pagination, line numbers, and hex codes are eliminated,
so the list file could be renamed as a source file, and
reassembled. This might be useful for archival purposes, or for
giving individualized versions of a source file to parties who
don't need any of the conditional-assembly options you've
programmed.
13-2

+L+I186+H15+W12 produces a list file that concentrates on the hex

output, increasing the width to 16 bytes per line, showing up to
15 hex runover lines, and limiting the amount of source code
shown.
Listing Control Directives

In addition to the five switches just mentioned, A86 has a number

of source-code directives that control aspects of listings.

The .NOLIST directive causes all subsequent listing to be

suppressed, until a .LIST directive is seen. Line numbering and
page numbering continues during list suppression, so the result
is as if you had text-edited the listing file to remove all the
suppressed lines.

I also offer a macro-definition control code, #H, which causes

the suppression of the listing of macro expansion lines. If #H
appears anywhere within a macro definition, all calls to that
macro will be listed as the macro call line only, showing the
generated hex object bytes on that call line. This allows you to
define macros that will be listed as if they were simple machine
instructions. This effect can be achieved for all macros with an
L switch setting that doesn't include the value 4 (see Chapter
3).

The TITLE directive specifies a title that will appear at the top
of every page of the entire assembly. The title consists of the
first 60 characters starting with the first nonblank after the
word TITLE on the line. If you give more than one TITLE
directive in a program, only the first will be recognized.

The SUBTTL directive specifies a subtitle to appear at the top of

every page until another SUBTTL directive is given (or until the
next file change if you have the +T16 switch-bit value set). If
the directive is at the very top of the listing page, or it is
shortly after an automatic page break, the subtitle will take
effect on the page in which it appears. Otherwise, it will take
effect at the next page.

The PAGE directive serves several purposes. The word PAGE by

itself will force a new page in the listing, at that point. A
plus sign following the word PAGE causes a new page plus an
incremented section number -- e.g. PAGE + on page 1-17 will cause
a new page 2-1 to begin. The word PAGE followed by one or more
constant parameters will set various A86 listing variables to the
specified parameter values. The variables are as follows:

1. The length, in lines, of a listing page. Minimum is 10;

maximum is 65535.

2. The width, in characters, of the maximum listing line.

3,4,5,6. The number of lines at the end of a page, less than

which A86 guarantees will not be "widowed" after a page break
of level 1,2,3,4, respectively.
13-3

Omitted parameters (either left off the end or via leading commas
or 2 consecutive commas) will remain unchanged.

The concept of "page break levels" is unique to A86 listings: it

is my attempt to get A86 to make intelligent decisions about
where to issue new listing pages. There are 4 page break levels,
normally triggered by gaps (consecutive blank lines) in the
source code, and by source-file changes. One- and two-line gaps
cause breaks of level 1 and 2, respectively. Three-or-more-line
gaps cause a break of level 3. A source-file change causes a
break of level 4. If a page break occurs close to the end of a
page, and a break of greater level hasn't already been marked,
A86 will mark the point for a potential new page. If a page
break of equal or greater level doesn't occur before the page is
full, A86 will issue a new page at the marked point. The
definition of "close to the end of the page" is 10,20,30, and 40
lines, respectively, for break levels 1,2,3,4. Those line counts
can be changed by parmeters 3,4,5,6 of the PAGE directive, as
already described.

If you are intimidated by all this, or if you want to control

page break levels manually, you may specify a T switch value that
does not include the "auto-paging" option value 4. With that
option disabled, page break levels will occur only at places
where you issue a PAGE directive containing a special parameter
value /1, /2, /3, or /4. The leading slash indicates that a page
break of the indicated level is desired here. Such a parameter
will typically be given by itself following PAGE; but, if you
wish, it can be interspersed anywhere among other parameter
values -- it will not be "counted" for the purposes of
determining the other parameters' positions.

Cross-reference Facility

When you specify the +X switch, A86 will create a

cross-referenced symbol table listing of your program.

The output file, having a standard extension of .XRF is an

alphabetical listing of all the non-local symbols in your
program. For each symbol, A86 gives its type, the file in which
it was defined, its value, and a list of all procedures in which
the file was used. If you print this file, you typically use the
TCOLS tool to obtain a multi-column listing from A86's
single-column output.

Note the use of procedure names to identify references -- this

makes the cross-reference listing truly readable. Other
cross-reference listings often give either line numbers, which
are meaningless unless you go find the associated line; or a file
name, which doesn't give you as much useful information.

Here is a more detailed description of the various pieces of

information provided for each symbol:
13-4

1. TYPE. Labels are indicated by a colon immediately following

the symbol name. Special symbols such as macro names are
denoted by an appropriate word such as "macro" in place of the
 value on the following line. Other symbol types are described
by one or two characters, following the symbol name.
Possibilities for the first character are:

m for a simple memory variable

+ for an index memory quantity
c for a constant
i for an interrupt equate
s for a structure

If there is a second letter, it is a size attribute: b for

byte, w for word, f for far (or doubleword).

2. FILE in which the symbol was defined. The name is stripped of

its extension, which is presumably the same for all your
source files. The name is preceded by = or period, which
denotes a definition of, not a reference to the symbol.

3. VALUE, given as 4 hex digits, on the line following the

symbol. For memory variables, this is the location of the
variable. For indexed quantities, this is the
constant-displacement part of the quantity. For structures,
it is the size of the structure. For interrupt equates, it is
the number of the interrupt.

4. REFERENCES, given on indented lines following the symbol name.

All occurrences of the symbol in your program produce a
reference. If the symbol is the first thing on a line, it is
considered a "definition" of that symbol, the reference listed
is the source file name. The name is preceded by a period if
the definition was via a colon (i.e., a label); it is preceded
by an equals sign otherwise. If the symbol is not the first
thing on the line, then it is not a definition. The reference
listing consists of the name of the last definition that A86
scanned (which, if your program is organized in a standard
way, will be the name of the procedure in which the reference
occurred.

Observe that you must use the local-label facility of A86 to

make this work. If you don't use local labels as your
"place-marker" symbols, the symbol the cross-reference gives
you will often be the name of the last "place-marker" symbol,
not the name of the last procedure.

To save space, duplicate reference entries are denoted by a

single entry, followed by "*n", where n is the decimal number
of occurrences of that entry.
13-5

A86LIB Source File Library Tool

There is a tool, A86LIB.COM, available only if you are

registered, that lets you build libraries of source files. To
use A86LIB, you must first code and debug the A86 source files
that you wish to include in your library. Then you issue the
command A86LIB followed by the names of the source files.
Wildcards are accepted; so you will typically want to gather the
source files into a single directory, and use the wildcard
specification. For example, if you use the filename extension .8
for your source files, you can issue the command A86LIB *.8 to
create the library.

The library created consists of a catalog file, always named

A86.LIB, together with the source files that you fed to A86LIB to
create the catalog.

The following observations about A86LIB are in order:

1. Unlike object-code libraries, A86.LIB contains only symbol

names and file names; it does not contain the code itself. You
MUST retain the source files used to create A86.LIB, because
A86 will read those files that it needs after consulting
A86.LIB to read their names.

2. A86LIB records all non-local symbols that start a line, and

are followed by a colon, an EQU or the word MACRO. (Recall
that local symbols are those names consisting of a single
letter followed by one or more decimal digits.) A86LIB also
records all symbols appearing on lines starting with the word
PUBLIC.

3. If a symbol appears in more than one library source file, it

will be logged for the first file A86LIB sees, and not the
subsequent ones. No error will be reported, unless and until
A86 tries to assemble both files in one assembly, and sees a
conflict.

4. A86LIB is simple-minded. A86LIB does NOT expand macros; nor

does it recognize conditional-assembly directives. This is
because the library files do not stand by themselves; the
macros and conditional-assembly variables being used might
well be defined in the main program of the programs accessing
the library files.

You may update A86.LIB by running A86LIB again; either with new
files or previously-recorded ones. If A86LIB is given a file it
had already read in a previous run, then A86LIB marks all the
symbols it had logged for the file as deleted, before rereading
the file. Those symbols that are still in the file are then
"unmarked". Thus, symbols that have been deleted from the file
disappear functionally from A86.LIB, but still occupy space
within A86.LIB. What I'm getting at is this: A86LIB will
tolerate alterations in library files quite nicely; but for
optimum storage efficiency you should delete A86.LIB and rebuild
it from scratch any time you delete anything from the library.
A86LIB is so fast that this is never very painful.
13-6

Using A86.LIB in A86 Assemblies

Once you have created a library with A86LIB, you access it simply
by calling the procedures in it from your A86 program. When A86
finishes an assembly and sees that there are undefined symbols in
your program, it will automatically look for copies of A86.LIB in
the current directory (then in other directories, as described in
the next section). If any of the undefined symbols are found in
the A86.LIB catalog, the files containing them are assembled.
You see this in the list of files output to the console by A86.

The subroutines in your library or libraries are effectively a

permanent part of the A86 language. They can be called up
effortlessly in your A86 programs. In time you can build up an
impressive arsenal of library modules, making A86 as easy to
program in as most high-level languages.

You may now have macros in your A86LIB library. Here's how it
works: when A86 sees a new symbol at the beginning of a line, in
a context where it would formerly have issued an error, A86 will
first look in the A86LIB libraries for the symbol. If it's
found, A86 will INCLUDE that library file on the spot, and then
assemble the line. NOTE that if the macro is being called within
a sequence of executable instructions, the library file must
generate no output object code.

Environment Variable A86LIB

You can set an environment variable A86LIB to specify which

drives or subdirectories contain A86.LIB files. The variable
consists of a sequence of path names separated by semicolons,
just like the PATH variable used by the operating system. For
example, if you include in your AUTOEXEC.BAT file the line

SET A86LIB=C:\bin\lib;\tools\a86lib

then A86 will look for A86.LIB in the current directory, then it
will look for C:\bin\lib\A86.LIB, then \tools\a86lib\A86.LIB. A86
will keep looking in all three catalog files, assembling the
appropriate source files from any or all of them, until there are
no more undefined symbols, or there are no more source files to
assemble.

For every symbol in an A86.LIB catalog, there is recorded the

name of the library file containing the symbol. The library file
is assumed to be in the same directory as its A86.LIB file,
unless a complete path name (starting with \ or a drive
specifier) was fed to A86.LIB when A86.LIB was created.
13-7

Forcing a Library Search

You may force A86 to assemble library files before moving on to

more of your program's source files. You do this by placing a
hash sign # (hex code 23) between file names in your invocation
line. For example, suppose your program has two modules FIRST.8
and LAST.8. FIRST.8 calls subroutines from your library; but you
need the library files assembled before LAST.8 is assembled. (You
might want this because LAST.8 allocates memory space beyond the
end of your program, which would be the end of LAST.8 if it were
truly the last module.) You accomplish this by the invocation
line:

A86 FIRST.8 # LAST.8

Note that there is never any need to force a library search at

the end of your program modules: A86 always makes a library
search there, if you have any undefined symbols.

You may now also force a library search from within a source
file, by placing a line with INCLUDE by itself with no file
names, into the source code. A86 will include any library files
necessary to resolve any forward-references at the point of the
INCLUDE.

CHAPTER 14 DESCRIPTIONS OF A86 ERROR MESSAGES

A86 signals successful assembly by returning an ERRORLEVEL of 0.

If errors are detected during assembly, A86 returns an ERRORLEVEL
of 1. If undefined symbols remain at the end of assembly, A86
returns an ERRORLEVEL of 2.

ERROR 01: Unknown Mnemonic

Most assembly-language lines start with a built-in instruction

mnemonic such as MOV or ADD. The only circumstances in which
a line can start with a non-built-in symbol are if the symbol
is a macro name or INT equate, or if the symbol is now being
defined, as indicated by a limited set of following symbols: a
colon, EQU, DB, DW, etc. This line started with a
non-built-in symbol which did not fall into any of the above
categories. You might have misspelled an instruction
mnemonic, or misspelled the following word.

ERROR 02: Jump > 128

The destination operand of a conditional jump must be a label

within 128 bytes of the end of the instruction. (Precisely,
from -128 to +127 from the next instruction, which is from
-126 to +129 from the start of the conditional jump.) This
error can also occur from an unconditional JMP to a
forward-referenced local label, since A86 assumes the short
JMP form in that case. This error is reported in three
possible places:

1. At the jump. The operand is more than 126 bytes before

the jump, or the operand is not a label (e.g. you tried an
indirect conditional jump through a variable, which isn't
allowed)

2. At a label definition. In this case, you use your editor

to search backwards for references to the label. One or
more of the earliest conditional jumps found are too far
away.
 3. At a RET, RETF, or IRET instruction. You use your editor
to search backwards for that flavor of RET used as the
operand to a conditional jump (the A86 conditional return
feature). The earliest such jumps not satisfied by a
previous RET are too far away.

You usually correct this error by rearranging your code, or

(better) by breaking intervening code off into subroutines. If
desperate, you can replace "Jcond" with "IF cond JMP".

ERROR 03: [BX+BP] And [SI+DI] Not Allowed

The 86 instruction set does not support the combinations of

indexing registers indicated in the error message. In
previous versions of A86, this error was reported in other
illegal operand combinations; I've attempted to change other
cases to error 14. If you ever find otherwise, please let me
know.
14-2

ERROR 04: Bad Character In Number

All numbers, and only numbers, start with a decimal digit.

(It's illegal to have a symbol begin with a digit; e.g.
01MYVAR .) You have coded something that starts with a
decimal digit but does not have the correct format for a
numeric constant. See Chapter 8 for detailed descriptions of
the formats of both integer and floating constants.

ERROR 05: Operands Not Allowed

When this error is reported it usually means that you have

provided something more than just the mnemonic for an
instruction that does not have any operands: e.g., PUSHF,
STOSB, STC, FLDPI, CLTS. It's also called in other contexts
when the assembler expects nothing more on the line; e.g.,
NAME with more than just a single name following, or something
following the word ENDS.

ERROR 06: Symbol Required

This is reported in numerous situations where A86 requires

some sort of symbol: either a built-in assembler mnemonic, or
a symbol you define. (It's possible that a number or some
punctuation marks are legal in the context, and that they have
already been checked for.) Instead of a symbol, a punctuation
mark or out-of-context number was seen. The contexts in which
this error can occur include:

* the start of a line (characters hex 3C or greater)

* after the following at the start of a line: a symbol you

define, #, #IF, IF, CODE, or DATA

* where operands to the following directives are expected:

NAME, PUBLIC, EXTRN, GROUP, SEGMENT
 * after ">" denoting a local-label forward reference

ERROR 07: Local Symbol Required

This is reported when something other than a generic local

label (letter followed by one or more digits) follows a ">"
mark, which denotes a local-label forward reference. If you
meant "greater than" you use the GT operator instead.

ERROR 08: Too Many Operands

This is reported for instructions and directives requiring a

limited number of operands, for which the limit is exceeded.
Since operands are separated by commas, you have too many
commas-- possibly an extra comma between the mnemonic and
first operand, or at the end of the operands.

ERROR 09: Constant Required

14-3

This is reported for instructions and directives (ENTER, RET,

RADIX, etc.) requiring operands that are an immediate constant
number; and for expression operators (*, /, SHL, OR, NOT, BY,
etc.) whose operands must be constant. In some cases a
limited number of forms other than constants are acceptable,
but the assembler has already checked for and not found those
possibilities.

ERROR 10: More Operands Required

This is reported for instructions requiring two operands, for

which you have provided no operands or only one operand. You
might have left out the comma separating the operands.

ERROR 11: Constant/Label Not Allowed

This is reported when you have given a constant number in a

place where it isn't allowed-- usually as a destination
operand to an instruction, such as the first operand to a MOV
or ADD. If you meant the operand to be the memory location
with the constant offset, you must convert the type by
enclosing the operand in brackets [ ] or appending a
size-specifier (B, W, D, Q, or T) to the number.

ERROR 12: Segment Register Not Allowed

This is reported when you have used a segment register in an

instruction where it isn't allowed. The only instructions
allowing segment registers as operands are MOV, PUSH, and POP.
You can't, for example, ADD into a segment register. If you
want to do anything with a segment register value, you have to
MOV it into a general register, perform the operation, then
MOV the result back to the segment register.

ERROR 13: Byte/Word Combination Not Allowed

This is reported in a two-byte instruction in which one

 operand is byte-sized and the other word-sized; or in an
instruction with a byte-sized destination and an immediate
source whose value is not byte-sized (high byte not 0 or 0FF).
If one of the operands is a memory variable of the wrong size,
you either change the declaration of the variable (DB to DW or
vice versa) or override the size of the variable in this
instruction only, by appending a " B" or " W" to the memory
operand.

ERROR 14: Bad Operand Combination

This is reported when you attempt to add or combine terms in

an operand expression that do not allow combination. An
example of this would be DT 3.7+BX. Only constants can be
added to floating point numbers.

This is also reported when you have two operands that are
mismatched in size, and the mismatch is something other than
Byte vs. Word. Example: MOV AL,D[0100].
14-4

ERROR 15: Bad Subtraction Operands

This is reported when you attempt to subtract terms in an

operand expression that do not allow subtraction, or if the
right-hand side to a subtraction is missing. If the
right-hand side to a subtraction is a non-forward-referenced
constant, then the left side can be almost anything.
Otherwise, the operands must match; e.g., labels from
relocatable segments must be in the same segment (in which
case the answer is an absolute constant; namely, the size of
the block of memory between the two labels).

ERROR 16: Definition Conflicts With Forward Reference

This error occurs when the assembler has previously guessed

the type of a forward-referenced symbol in order to determine
what kind of instruction to generate, and the guess turned out
to be wrong. The error is reported at the time the symbol is
defined. For example, when A86 sees MOV AX,FOO, it will
assume FOO is an immediate value. This error is reported if
FOO turns out to be a word variable: FOO DW 0. You need to
search backwards from the error message, to references of FOO,
and specify the type you intend to be used: MOV AX,FOO W. If
you really did intend to load the offset of FOO and not the
memory contents, you can code MOV AX,OFFSET FOO to make the
error message go away.

ERROR 17: Divide Overflow

This is reported when the right-hand side to a division or MOD

operation is zero, or when the result of a division by a large
(>64K) number is still large.

ERROR 18: Same Type Required

This is reported when the two operands to a relational

 operator (EQ, NE, GT, GE, LT, or LE) are of different types.
The operands to a relational operator ought to be both
absolute integer constants, or labels in the same segment.

ERROR 19: CS Destination Not Allowed

This is reported if you attempt to specify CS as the

destination (first) operand to MOV, or as an operand to POP.
The only acceptable way to load CS on the 8086 is via a far
JMP, CALL, RETF, or IRET instruction. The MOV and POP forms
don't make much sense, so they were outlawed by Intel.

ERROR 20: Left Operand Not Allowed

This is reported if you have a left-hand side to an expression

operator that expects only a single operand to its right.
Those operators are BIT, NOT, OFFSET, TYPE, LOW, HIGH, SHORT,
LONG, and INT. (The mnemonic INT is considered an operator
e.g., in MSDOS EQU INT 33.) For example, you would get this
error for the expression 1 NOT 2.
14-5

ERROR 21: Bad Single Operand

This is reported if the operand is inappropriate for an

instruction INC, DEC, PUSH, POP, NOT, NEG, MUL, IMUL, DIV, or
IDIV, that takes a single operand. You should look up the
instruction in the chart in Chapter 6, to determine the proper
operand forms allowed.

ERROR 22: Bad DUP Usage

This is reported when a DUP construct occurs out of context

(e.g. in an instruction operand instead of a data
initialization); when the total number of bytes generated
would push the output pointer beyond 64K; or when there is
improper syntax for a DUP. See Chapter 9 for the description
of correct DUP usage.

ERROR 23: Number Too Large

This is reported when a numeric constant is too large for the

assembler to store in its operand buffers-- the limit for
integers is 2**80-1 = 1208925819614629174706175 decimal. The
error is also given when the exponent part of a floating point
constant is greater than 65535 in magnitude.

ERROR 24: SEGMENT or ENDS Required

This is reported if a line beginning with one of the two A86

reserved symbols CODE or DATA does not continue with one of
the reserved symbols SEGMENT or ENDS. If you meant CODE or
DATA to be a symbol you define, you have to change the name to
something else, like _CODE or _DATA.

ERROR 25: Bad CALL/JMP Operand

 This is reported if the operand to a call or jump instruction
cannot be taken as a jump destination. This occurs if the
operand is missing, or if it has a size inappropriate for
address pointers: byte, quadword, or ten-byte. The error also
occurs if the operand is a constant number, and you are
assembling to an OBJ format. In OBJ format anything jumped to
within a segment must be specified as a label within some
segment.

ERROR 26: Memory Doubleword Required

This is reported if the second operand to an LDS, LES, or

BOUND instruction is of the wrong type. The operand should be
a doubleword memory quantity; but A86 will accept a word
memory variable or a memory variable of unspecified size.

ERROR 27: Bad IN/OUT Operand

This is reported when the operands to IN or OUT do not have

the correct form. See Chapter 6 for the limited set of forms
for these instructions. One of the operands must be AL or AX;
the other must be DX or a constant between 0 and 255.
14-6

ERROR 28: type Required

This is reported when a symbol given in an EXTRN list is not

followed one of the type names B, W, D, Q, T, F, NEAR, or ABS.
The more verbose synonyms BYTE, WORD, DWORD, QWORD, and TBYTE
are also acceptable.

ERROR 29: Bad Rotate/Shift Operand

This is reported when the count (second) operand to a rotate

or shift instruction is not appropriate: it should be either
the name CL or a constant less than 32. The instructions
requiring this are ROL, ROR, RCL, RCR, SHL, SHR, SAL, SAR, and
the NEC-specific instructions SETBIT, TESTBIT, CLRBIT, and
NOTBIT.

ERROR 30: Byte-Sized Constant Required

This is reported in contexts where only a byte-sized absolute

constant is acceptable. Those contexts are: the operand to a
BIT or INT operator in an expression; the required operand to
an INT or CALL80 instruction; the optional operand to an AAM
or AAD instruction.

ERROR 31: Instruction In Data Segment Not Allowed

There are only a limited number of directives allowed with a

STRUC or a DATA segment. This error is reported when any
instructions or disallowed directives are seen in one of these
restricted environments. You have possibly neglected to
provide an ENDS directive, returning you to normal assembly.
In a STRUC, the only directives allowed are DB, DW, DD, DQ,
DT, another STRUC, ENDS, EQU, SEGMENT, GROUP, MACRO, LABEL,
 EVEN, and ORG. The DATA segment allows the same directives,
plus PROC, ENDP, DATA, and CODE.

ERROR 32: Bad String

This is reported when you start a quoted string, and do not

provide the closing quote in the same line. You might have
left it out; or you might not have intended to code a string
at all, and accidentally inserted a single- or double-quote
mark within the string. Or you might have intended a string
containing an end-of-line, which isn't allowed. You must
instead close the string and code hex bytes 0D,0A to represent
an end-of-line.

ERROR 33: Bad Data Operand

This is reported if an inappropriate operand is seen in a data

initialization (DB, DW, DD, DQ, or DT) directive. Examples of
this are indexed quantities such as [BX], non-byte quantities
in a DB, or floating point constants in a DB or DW.

ERROR 34: Index Brackets Required

14-7

This is reported if the name of a register is given in an

addition/combination operation, but the register is not
enclosed in square brackets. The only registers that may be
added are those presented as indexing registers. For example,
don't code BX+2, code [BX+2].

ERROR 35: Bad Character

This is reported when a punctuation mark or other non-standard

character is seen where it is not expected. The characters
causing this error at the beginning of a line are digits, and
the marks / - , + * ( ) & " ! -- other illegal marks at
the start of a line cause error 6, Symbol Required. The
characters causing this error elsewhere (i.e. within operands)
are all characters except letters, digits, and the marks [ ] +
- ' " > ( ) * . / :

ERROR 36: String > 2 Not Allowed

This is reported when a string with 3 or more characters is

seen outside of the places where such a string is allowed (in
a DB directive, macro operand, or relocatable SEGMENT
directive). One- and two-character strings are treated as
simple numeric constants; but longer strings require special
handling and are allowed only in the places mentioned.

ERROR 37: Misplaced Built-In Symbol

The symbol just before this error message is an A86 built-in

symbol, that is in a place where it doesn't belong. Examples
of this are: mnemonics such as MOV occurring in operands; and
symbols that aren't mnemonics such as LT occurring at the
start of the line. If you thought you could define the symbol
 to the left of this message for your own use, you were wrong.
You need to change the symbol to something else: TEST to
_TEST, for example. If you'd like to know the built-in
meaning of the symbol, you can look it up in Chapter 16.

ERROR 38: Segment Combination Not Allowed

This is reported when you attempt to add or combine a segment

or group name with another quantity. A86 currently doesn't
allow this.

ERROR 39: Bad Index Register

This is reported when you attempt to use a register other than

SI, DI, BX, or BP for indexing. Those are the only registers
that the 86 architecture allows you to place inside brackets,
to address memory.

ERROR 40: Conflicting Multiple Definition Not Allowed

14-8

This is reported when you define a symbol in two places in

your program, and the definitions aren't the same. Most often
you have simply forgotten you already had a symbol somewhere
of the same name, and you need to change the name of one of
the two symbols you've defined. A86 allows the re-use of a
symbol if it is a generic local label (a letter followed by
one or more digits), or if is defined with = instead of EQU.
A86 also allows the redefinition of a symbol if it has exactly
the same value (e.g. ESC EQU 01B in two places in your
program). See the section "Duplicate Definitions" in Chapter
9 for a detailed discussion of this feature.

ERROR 41: ENDS Has No Segment

This error occurs when A86 is assembling to an OBJ file, and

it sees an ENDS at the outermost level of segments-- the ENDS
has not been preceded by a matching SEGMENT directive. You
need to look over your SEGMENT and ENDS directives, to get
them to match up properly.

ERROR 42: Bad IF Operand

This is reported when an IF is not followed by one of the

flag-mnemonics (e.g., E, Z, NC, AE, etc.) that follow "J" in a
conditional jump instruction. Most likely the line is a
conditional assembly line intended for another assembler. In
A86, conditional assembly lines begin with a hash sign #. So
you change IF, ELSE, ENDIF to #IF, #ELSE, #ENDIF. You may
also need to change the condition following IF: IF FOO EQU 0
becomes #IF !FOO; IFDEF FOO becomes simply #IF FOO. IF
(expression) must be replaced by the two lines C1 EQU
(expression) followed by #IF C1 . See Chapter 11 for the
details of A86's syntax for conditional assembly. See Chapter
5 for the way A86 uses IF when it doesn't have a hash sign #.

ERROR 43: Parenthesis/Bracket Mismatch

 This is reported when there is a lack of balance of
parentheses ( ) or brackets [ ] in an operand expression--
there are too many left-sides, too many right-sides, or the
brackets are interleaved illegally: ( [ ) ]. Most likely you
have left out an opening or closing parenthesis/bracket in a
complicated expression; or a spurious extra ( ) [ or ] has
crept into your code.

ERROR 44: Bad Forward Reference Combination

This is reported when you try to use forward references in

expressions that are too complicated for A86 to handle. You
can add or subtract constants from forward-referenced symbols;
but you can't subtract a forward-referenced symbol from
anything, and you can't add two forward references together.
You can typically get around restrictions in forward reference
expressions by moving the expression down to an EQU directive
after the point that the symbols are defined, and making a
forward reference to the EQUated symbol that represents the
evaluated expression.
14-9

You will often get this error if you forget to put a

semi-colon before a comment: A86 will interpret your prose as
the addition of undefined (i.e., forward-referenced) terms.

This error is also reported in some situations involving

relocatable symbols in OBJ mode -- these symbols are forward
references in the sense that they are resolved only at link
time.

ERROR 45: Is It Byte Or Word?

This is reported when you have a memory operand of unspecified

size, and A86 needs to know whether the operand is byte-sized
or word-sized, in order to generate the correct instruction
form. All you need to do is to append a B or a W to the
operand, to specify the size you want. For example, if you've
coded INC [BX], you need to decide between INC B[BX] and INC
W[BX]. If you've coded ADD FOO,4 where FOO is a forward
reference, you need to specify ADD FOO B,4 or ADD FOO W,4 .

ERROR 46: Bad #-Construct

This is reported if, within a macro definition, a # is seen

that is not followed by one of the allowed macro parameter
constructs described in Chapter 11. Even in quoted strings,
the hash sign # must be literalized via ## if it is to be
taken as-is.

If you mistakenly provide a macro-loop variable (#W, #X, #Y,

or #Z) outside of any loop defining that variable, this error
is detected when the macro is expanded, even though the error
is in the macro definition.

The error is also reported if # occurs at the beginning of a

 line, and is not followed by IF, ELSEIF, ELSE, or ENDIF; or if
a conditional assembly parameter is a built-in mnemonic e.g.
#IF MOV . See Chapter 11 for the correct usage of the hash
sign in both macros and conditional assembly.

ERROR 47: #ENDIF Required

This is reported if you have an #IF without a corresponding

#ENDIF before the end of the file (or the end of the macro
expansion if the #IF was assembled during a macro expansion).
When this message appears at the end of a file, you need to
search backwards for #IFs, to find the unclosed block.

ERROR 48: #EM Required To End Macro

This is reported if you have a MACRO without an end. In A86,

the end of a macro is given by #EM. Most likely your file was
written for another assembler, and you need to convert macro
definitions. You need to change all ENDM directives to #EM.
You also need to eliminate the named parameters from the MACRO
line, and replace occurrences of the named parameters with #1,
#2, #3, etc. The & concatenation operator can be dropped. See
Chapter 11 for a full description of A86's macro syntax.
14-10

ERROR 49: End Delimiter to COMMENT Required

This is reported when the portion of code skipped in a COMMENT

directive has run to the end of the file, without the closing
delimiter being found. You need to search backwards from the
end of the file to find the COMMENT directive, figure out
where you intend the directive to end, and duplicate the
delimiter (the first non-blank following COMMENT) at that
end-point. See Chapter 4 for a full description of the
COMMENT directive.

ERROR 50: Reg,Mem Required

This is reported when you have an improper combination of

operands for a MOV, XCHG, or general arithmetic instruction
such as ADD, SUB, CMP, XOR, etc. Most often you have
attempted to provide two memory operands: MOV VAR1,VAR2 or ADD
VAR1,VAR2. One of the operands must be a register. You can
effect the memory-to-memory operation by using a register in a
two-instruction sequence; for example, MOV AX,VAR2 followed by
ADD VAR1,AX . For convenience, A86 lets you code the sequence
with the single line ADD VAR1,AX,VAR2.

If you don't wish to clobber the contents of any registers,

and the operands are word-sized, you may PUSH the source
operand and then POP to the destination operand: PUSH VAR2
followed by POP VAR1.

ERROR 51: Segment Override Not Allowed Here

For compatibility with other assemblers, A86 allows segment

override operators CS:, DS:, ES:, or SS: within expressions in
 instruction operands. The override informs the assembler that
the named segment register is to be used for the memory
reference, so that the assembler might generate a segment
override opcode byte. This error is reported when a segment
override operator occurs out of context: in A86's special
three-operand form for MOV or arithmetic instructions; within
a DATA segment or STRUC, or in an EQU directive. You might
encounter the last case if you're porting a program written
for another assembler. If so, you might have to provide
explicit overrides wherever the EQUated symbol is used. It's
possible, though, that the override is provided only to
satisfy the other assembler's segment checking mechanism, and
no overrides are generated at all. In that case, you can just
eliminate the override operator.

ERROR 52: Byte Operand Required

This is reported when an operand to one of the NEC-specific

instructions STOBITS, LODBITS, ROL4, ROR4 is of the wrong
type. STOBITS and LODBITS require the first operand to be a
byte-sized register and the second operand to be either a
byte-sized register or an immediate constant. ROL4 and ROR4
require the only operand to be a byte-sized register.

ERROR 53: Word Register Required

14-11

This is reported when the first operand to any of the

instructions LDS, LES, LEA, BOUND, IMUL, LAR, or LSL is not a
word-sized general register (AX,BX,CX,DX,SI,DI,BP, or SP).

ERROR 54: Floating-Point Chip Required

This is reported when you attempt to assemble a program with

floating point constants or floating point expressions, and
you do not have a floating point chip (8087 or 287) in your
computer system. A86 uses the 87 to assemble constants and do
arithmetic. It's time for you to buy a chip and install it in
that empty socket!

ERROR 55: Bad Floating-Point Operand

This is reported when an operand to a floating point

instruction is not of the correct type. See Chapter 7 for the
correct forms for the instruction you're coding. Some
possibilities for the error are:

* a memory operand has unspecified size, or a size not

compatible with the instruction. Integer instructions
(FIxxx) require a W or D operand; floating arithmetic
instructions require a D or Q operand.

* you've tried to specify an 86 register instead of a memory

operand.

* you've tried A86's special FLD (constant) form in OBJ mode.

Sorry, I support this only for COM mode (mainly for D86).
 * you've specified two register numbers (0 through 7), but
neither is 0.

* you've tried one of the disallowed forms FCOM i,0 or FCOMP

i,0

ERROR 56: Constant 0--7 Required

This is reported if a constant number operand to an 87

instruction, which is supposed to represent an 87 stack number
(0 through 7), does not have the right value; i.e., it's not
an integer, or it's not in the range 0 through 7.

ERROR 57: Memory Operand Required

This is reported when an operand to a floating-point or a 286

protected-mode instruction must be a memory operand, and the
operand you've provided isn't one. See Chapters 7 (for
floating) or 6 (for protected) for the correct syntax of the
instruction you're coding.

ERROR 58: Segment Or Struc Name Not Allowed

14-12

This error occurs most often when you are attempting to

assemble as a COM program a file intended to be an EXE
program. The COM format does not allow you to refer to the
value of a named segment, or to make a FAR pointer out of a
label within the program. You should either use the +O option
to produce an OBJ file, or simply eliminate the statements
intended to set the segment registers-- COM programs are
started with all segment registers already pointing to the
same value.

This error is also reported when you provide the name of a

structure, or the name of an INT equate, in a place where a
register or memory operand is expected.

ERROR 59: Word Operand Required

This is reported when something other than a word-sized

operand is provided for one of the 286 instructions ARPL,
SLDT, LLDT, STR, LTR, VERR, VERW, SMSW, or LMSW.

ERROR 60: Circular Definition Not Allowed

This is reported when a chain of macro calls or references to

undefined symbols reaches a depth of 1024. A86 assumes that
it is in an infinite loop: for example, FOO EQU FOO; or BAZ
MACRO containing an uncontrolled call to BAZ within itself.

ERROR 61: Overlapping Local Not Allowed

Recall from Chapter 5 that when you use a local label symbol
twice, you must distinguish a reference to that symbol by
 prepending a > before the symbol's name if the reference is a
forward reference. You get this error if you have followed
such a forward reference with another reference, without the
>, before the next incaration of the symbol is defined.
There's a danger that you intended the reference to be to the
previous incarnation, which A86 doesn't allow. Example:

L1: ; first incarnation of L1

JNZ >L1 ; reference to second incarnation
JMP L1 ; ERROR-- which incarnation are we referring to?
L1: ; second incarnation of L1

If you intended the JMP to be to the second L1, you should

prepend a > to the L1, just like the JNZ. If you intended the
JMP to be to the first L1, you must change one of the two
label names so that their ranges don't overlap.

ERROR 62: Instruction Not Allowed On Your CPU

14-13

This is reported when you attempt to assemble an instruction

not supported for the processor (8086/8088, 186, 286 or later,
or NEC) for which you are assembling. The instructions not
common to all 86 processors are marked with a # or a * in
Chapter 6. See Chapter 3 for the discussion of the P switch,
that allows you to specify the processor for which you are
assembling.

ERROR 97: Object Overflow

This is reported when the assembler runs out of room in its

output object-code segment (which also holds records used to
resolve forward references). This will happen only if your
object output nears the object capacity, which is 64K if a
full amount of memory (about 300K) is available to the
assembler. If you have a limited amount of memory, you should
increase the memory available to A86, by buying another board,
or by having fewer memory-resident programs installed when you
run A86. If you are assembling OBJ files, you can break the
program into smaller assembled modules.

It's conceivable that this error could result in a D86

session, when you are using patch-memory mode to type in an
extremely complicated program. In that case, you should type
the program into a text file instead, and use A86 to assemble
the text file.

ERROR 98: Undefined Symbol Not Allowed

This error should occur only during a D86 debugging session,

when you type an immediate-execution assembly language line
containing a symbol not in the table (typically a mistyping on
your part). D86 allows you to add symbols to the table only
when you are in patch-memory mode (reached by pressing the F7
key).

ERROR 99: Symbol Table Overflow

 This is reported when the symbol table runs out of space. It's
unlikely that you'll ever run into this error, since A86's
capacity is thousands of symbols. If you do, you'll need to
reduce the number of symbols in your program. One way to do
so is to replace all place-marker symbols with local labels in
a limited range (like L0--L9). See Chapter 5 for a
description of A86's local label facility.

CHAPTER 15 RELEASE HISTORY OF A86

Virtually all releases of A86 include bug fixes. If I don't say

anything about a release, then it was essentially only bug fixes.

V2.10 June 1986. Initial public release of the MSDOS version of

A86. The last previous version ran under the Xenix
operating system on the Altos series of computers. For
this "public offering", I cleaned up the invocation
syntax, upgraded the error-reporting facility, and started
adding compatbility features.

V2.11 June 1986. Added RADIX command.

V2.13 July 1986. Reduced memory requirements.

V2.15 August 1986. Implemented COMMENT directive for

compatibility; added floating point instruction set and DQ
and DT directives.

V2.16 August 1986. Made internal changes to accommodate forward

referencing in D86's patch-memory mode.

V2.18 November 1986.

V2.90 March 1987. Test release for .OBJ support.

V3.00 April 1987. Major upgrade. Added added support for

linkable .OBJ files, long constants and floating-point
constants, A86LIB library tool and A86LIB support,
ability to forward-reference variables, 286
protected-mode and NEC-specific instructions, options not
to insert errors in source, long forward JMP for local
labels, and default decimal, "=" equate compatibility
feature, double-quoted strings, and parentheses no longer
required for most DUP right operands.

V3.01 April 1987. Added "S" suppress-symtab and "C"

case-sensitivity switches

V3.05 June 1987. Added recognition of SEGMENT AT in non-OBJ

mode, and ignore END directive in non-OBJ mode

V3.07 July 1987. Added features necessary for Turbo C support

(+c, +f, +F switches; ignore DGROUP:). Generalized the
environment variable to include macro files. Added the
 ampersand feature. Made = compatible with MASM.

V3.09 August 1987. Legalized MOV segreg,immediate. Duplicated

MASM functionality for case-sensitive mode (A86's +C
switch).

V3.10 September 1987. Added a printed version of the manual.

Added +c switch, reinstating case sensitivity during
assembly, but this time without sensitivity in built-in
symbols.
15-2

V3.11 November 1987. Added the SEG operator for compatibility

with Turbo C, and made it possible to define relocatable
segments called CODE, DATA, or STACK, for compatibility
with Turbo Pascal.

V3.12 February 1988. Changed the format of SYM files, so that

they are much smaller yet hold more information. Allowed
an arbitrary expression to appear in a conditional
assembly (#IF) directive. Added macro features: #V value
operator, #S size operator, #N number operator, #EX exit
directive, string comparison of operands, and large
operand numbers (up to 255).

V3.13 March 1988. Made memory management more flexible, to

allow A86 to run with less available memory.

V3.15 May 1988. Allowed up-arrow in place of equals-sign in

invocation equates. Allowed MOV mem,mem and XCHG of a
variety of new forms, generating sequences of instructions
to implement the unavailable forms.

V3.17 June 1988.

V3.18 July 1988. For compatibility: allowed OFFSET segname, and

implicitly converted a constant with a segment override
into a memory type.

V3.19 August 1988.

V3.20 July 1989. Made internal redesign of handling of size-

override operators (B, W, D, F, etc.) so they are handled
more consistently. Outlawed first DATA SEGMENT without a
starting ORG statement, forcing an explicit ORG 0 for
future compatibility.

V3.21 August 1989.

V3.22 January 1990. Added support for additional coprocessors:

the 80387 and the IIT-2C87. Made numerous minor changes
to enhance MASM-compatibility.

V3.70 January 1994. Test release for INCLUDE support, forward

ORGs, default ORG for DATA SEGMENT to the end of the
program, listing files, macros in A86LIB, K numeric base,
the DEF and REF operators, numeric operands to MOVx and
 STOSx, enforcement of processor-specific instructions,
forward references in complicated expressions, and symbols
beginning with a period.

V4.00 December 1994. "Official" public release with all the new
features mentioned in V3.70 above. Added COMPAT.8 file to
implement some MASM directives as A86 macros.

V4.01 March 1995.

V4.02 September 1995.

CHAPTER 16 A86 RESERVED SYMBOLS

$ Current assembly pointer

.287 Directive
.LIST Listing control
.NOLIST Listing control
.RADIX Directive
? Uninitialized memory slot
A4 Address override prefix
A2 Address override prefix
AAA ASCII adjust addition
AAD ASCII adjust division
AAM ASCII adjust multiply
AAS ASCII adjust subtract
ABS EXTRN specifier
ADC Add with carry
ADD Instruction
ADD4S NEC Instruction
AH Byte register
AL Byte register
AND Instruction/operator
ARPL 286 Prot Instruction
ASSUME Ignored, compatibility
AT SEGMENT specifier
AX Word register

B Byte memory specifier

BH Byte register
BIT Bit-mask operator
BL Byte register
BOUND Instruction
BP Word register
BSF Bit Scan Forward
BSR Bit Scan Reverse
BSWAP Byte Swap
BT Bit Test
BTC Bit Test Complement
BTR Bit Test Reset
BTS Bit Test Set
BX Word register
BY Bytes-combine operator
BYTE Byte memory specifier
CALL Instruction
CALL80 NEC Instruction
CBW Convert byte to word
CDQ Convert Double to Quad
CH Byte register
CL Byte register
CLC Clear carry
CLD Clear direction
CLI Clear interrupt
CLRBIT NEC Instruction
CLTS 286 Prot Instruction
16-2

CMC Complement carry

CMP Compare
CMP4S NEC Instruction
CMPS Compare string
CMPSB Compare string byte
CMPSD Compare string dword
CMPSW Compare string word
CMPX8 Compare Xchg 8 bytes
CMPXCHG Compare and Exchange
CODE Segment name
COMMENT Directive
COMMON SEGMENT specifier
CPUID Identify CPU
CRn Control registers 0234
CS Segment register
CWD Convert word to dword
CWDE CWD into EAX
CX Word register

D Dword specifier
DAA Decimal adjust add
DAS Decimal adjust sub
DATA Segment name
DB Define bytes
DD Define dwords
DEC Decrement
DEF Defined operator
DH Byte register
DI Word register
DIV Divide

DL Byte register
DQ Define Qwords
DRn Debug registers 0--7
DS Segment register
DT Define Twords
DUP Duplicate operator
DW Define words
DWORD Memory specifier
DX Word register

EAX Doubleword register

EBP Doubleword register
EBX Doubleword register
ECX Doubleword register
EDI Doubleword register
EDX Doubleword register
ELSE Conditional term
ELSEIF Conditional term
16-3

END Start specifier

ENDIF Conditional term
ENDM Ignored, future use
ENDP End of procedure
ENDS End of segment
ENTER Instruction
EQ Equals operator
EQU Equate directive
ES Segment register
ESI Doubleword register
ESP Doubleword register
EVEN Coerce to even address
EXTRN Declare type of a symbol

F Far specifier
F2XM1 87 Instruction
F4X4 IIT-2C87 Instruction
FABS 87 Instruction
FADD 87 Instruction
FADDP 87 Instruction
FAR Far specifier
FBANK IIT-2C87 Instruction
FBLD 87 Instruction
FBSTP 87 Instruction
FCHS 87 Instruction
FCLEX 87 Instruction
FCOM 87 Instruction
FCOMP 87 Instruction
FCOMPP 87 Instruction
FCOS 387 Instruction

FDECSTP 87 Instruction
FDISI 87 Instruction
FDIV 87 Instruction
FDIVP 87 Instruction
FDIVR 87 Instruction
FDIVRP 87 Instruction
FENI 87 Instruction
FFREE 87 Instruction
FIADD 87 Instruction
FICOM 87 Instruction
FICOMP 87 Instruction

FIDIV 87 Instruction
FIDIVR 87 Instruction
FILD 87 Instruction
FIMUL 87 Instruction
FINCSTP 87 Instruction
FINIT 87 Instruction
FIST 87 Instruction
FISTP 87 Instruction
FISUB 87 Instruction
FISUBR 87 Instruction
16-4

FLD 87 Instruction
FLD1 87 Instruction
FLDCW 87 Instruction
FLDENV 87 Instruction
FLDL2E 87 Instruction
FLDL2T 87 Instruction
FLDLG2 87 Instruction
FLDLN2 87 Instruction
FLDPI 87 Instruction
FLDZ 87 Instruction
FMUL 87 Instruction
FMULP 87 Instruction

FNCLEX 87 Instruction
FNDISI 87 Instruction
FNENI 87 Instruction
FNINIT 87 Instruction
FNOP 87 Instruction
FNSAVE 87 Instruction
FNSTCW 87 Instruction
FNSTENV 87 Instruction
FNSTSW 87 Instruction

FPATAN 87 Instruction
FPREM 87 Instruction
FPREM1 387 Instruction
FPTAN 87 Instruction
FRNDINT 87 Instruction
FRSTOR 87 Instruction
FS Segment register
FSAVE 87 Instruction
FSCALE 87 Instruction
FSETPM 87 Instruction

FSIN 387 Instruction

FSINCOS 387 Instruction
FSQRT 87 Instruction
FST 87 Instruction
FSTCW 87 Instruction
FSTENV 87 Instruction
FSTP 87 Instruction
FSTSW 87 Instruction

FSUB 87 Instruction
FSUBP 87 Instruction
FSUBR 87 Instruction
FSUBRP 87 Instruction
FTST 87 Instruction
16-5

FUCOM 387 Instruction

FUCOMP 387 Instruction
FUCOMPP 387 Instruction
FWAIT 87 Instruction
FXAM 87 Instruction
FXCH 87 Instruction
FXTRACT 87 Instruction
FYL2X 87 Instruction
FYL2XP1 87 Instruction

GE Greater/equal operator
GROUP Group of segments
GS Segment register
GT Greater than operator
HIGH High byte of word op
HLT Halt
IDIV Integer divide
IF Skip/conditional term
IMUL Integer multiply

IN Input from port

INC Increment
INCLUDE Include another file
INS Input string
INSB Input string byte
INSD Input string doubleword
INSW Input string word

INT Interrupt
INTO Interrupt on overflow
INVD Invalidate data cache
INVLPG Invalidate TLB entry
IRET Interrupt return
IRETD Interrupt return 32-bit

JA Jump on above
JAE Jump above equal
JB Jump on below
JBE Jump below equal
JC Jump on carry

JCXZ Jump on CX zero

JE Jump on equal
JECXZ Jump on ECX zero
JG Jump on greater
JGE Jump greater equal

JL Jump on less
JLE Jump less equal
JMP Jump unconditional
JNA Jump not above
JNAE Jump not above equal
JNB Jump not below
JNBE Jump not below equal
JNC Jump not carry
16-6

JNE Jump not equal

JNG Jump not greater
JNGE Jump not greater equ
JNL Jump not less
JNLE Jump not less equal
JNO Jump not overflow
JNP Jump not parity
JNS Jump not sign
JNZ Jump not zero

JO Jump overflow
JP Jump parity
JPE Jump parity even
JPO Jump parity odd
JS Jump on sign
JZ Jump on zero

L2E Real constant

L2T Real constant
LABEL Declaration
LAHF Load AH flags
LAR 286 Prot Instruction
LDS Load into DS
LE Less equal operator
LEA Load eff address
LEAVE Instruction

LES Load into ES

LFS Load into FS
LG2 Real constant
LGDT 286 Prot Instruction
LGS Load into GS

LIDT 286 Prot Instruction

LLDT 286 Prot Instruction
LMSW 286 Prot Instruction
LN2 Real constant
LOCK Instruction
LODBITS NEC Instruction

LODS Load string

LODSB Load string byte
LODSD Load string doubleword
LODSW Load string word
LONG Operator

LOOP Instruction
LOOPE Loop on equal
LOOPNE Loop not equal
LOOPNZ Loop not zero
LOOPZ Loop on zero
LOW Operator
16-7

LSL 286 Prot Instruction

LSS Load into SS
LT Less than operator
LTR 286 Prot Instruction
MACRO Directive
MAIN Program strating label
MEMORY Segment specifier

MOD Operator
MOV Instruction
MOVS Move string
MOVSB Move string byte
MOVSD Move string doubleword
MOVSW Move string word
MOVSX Move sign extended
MOVZX Move zero extended
MUL Multiply

NAME .OBJ module name

NE Not equals operator
NEAR Operator
NEG Instruction
NIL No code instruction
NOP No operation
NOT Instruction/operator
NOTBIT NEC Instruction

O2 Operand override prefix

O4 Operand override prefix
OFFSET Operator
OR Instruction/operator
ORG Directive
OUT Output to port
OUTS Output string
OUTSB Output string byte
OUTSD Output string doubleword
OUTSW Output string word

PAGE Listing page control

PARA Segment specifier
PI Real Constant
POP Instruction
POPA Pop all
POPAD Pop all doublewords
POPF Pop flags
POPFD Pop doubleword flags
PROC Procedure Directive

PTR Ignored, compatibility

PUBLIC Specify public symbols
PUSH Instruction
PUSHA Push all
PUSHAD Push all doublewords
PUSHF Push flags
PUSHFD Push doubleword flags
Q Qword specifier
QWORD Memory specifier
16-8

RADIX Directive
RCL Rotate carry left
RCR Rotate carry right
RDMSR Read model specific reg
RDTSC Read time stamp counter
REF Referenced operator

REP Repeat prefix

REPC NEC Instruction
REPE Repeat while equal
REPNC NEC Instruction
REPNE Repeat not equal
REPNZ Repeat while zero
REPT Ignored, future use
REPZ Repeat non zero

RET Return
RETF Far Return
ROL Rotate left
ROL4 NEC Instruction
ROR Rotate right
ROR4 NEC Instruction
RSM Resume from sys mgmt.

SAHF Store AH to flags

SAL Shift arith left
SAR Shift arith right
SBB Subtract with borrow

SCAS Scan string

SCASB Scan string byte
SCASD Scan string doubleword
SCASW Scan string word
SEG Operator
SEGMENT Directive

SETcond Set condition

(see J, all but eCXZ)
SETBIT NEC Instruction
SGDT 286 Prot Instruction
SHL Instruction/operator
SHLD Shift left double
SHORT Operator
SHR Instruction/operator
SHRD Shift right double

SI Word register
SIDT 286 Prot Instruction
SLDT 286 Prot Instruction
SMSW 286 Prot Instruction
SP Word register
SS Segment register
16-9

ST EQU 0 for compatibility

STACK Segment specifier
STC Set carry
STD Set direction
STI Set interrupts
STOBITS NEC Instruction
STOS Store string
STOSB Store string byte
STOSD Store string doubleword
STOSW Store string word

STR 286 Prot Instruction

STRUC Structure directive
SUB Instruction
SUB4S NEC Instruction
SUBTTL Listing subtitle

T Tbyte specifier
TBYTE Memory specifier
TEST Instruction
TESTBIT NEC Instruction
THIS This-location specifier
TITLE Listing title
TRn Test registers 6--7
TYPE Operator
VERR 286 Prot Instruction
VERW 286 Prot Instruction

W Word specifier
WAIT Instruction
WBINVD Write back, invalidate
WORD Word specifier
WRMSR Write model specific reg

XADD Exchange and add

XCHG Instruction
XLAT Translate byte
XLATB Translate byte
XOR Instruction/operator

INDEX 17-1
287 directive, 7-2
386 indexing, 6-2
387 support, 7-2
A-after operator in macros, 11-6
A86 environment variable, 3-7
A86.LIB file, 13-5
A86.LIB library catalog, 2-1
A86LIB environment variable, 13-6
A86LIB library tool, 13-5
AAD with operand, 5-4
AAM with operands, 5-4
about the author, 1-6
ABS operator in EXTRN, 10-6
absolute segments in OBJ mode, 10-10
address listing control, 3-4
address override byte, 6-5
address, my, 1-1
align operand list, 10-9
align specification, 10-9
alignment using EVEN, 9-3
allocation directives, 9-3
ampersand, use to specify standard input, 3-8
AND expression operator, 8-6
angle brackets in MASM, 12-4
arithmetic on floating-point numbers, 7-4
arithmetic, 32-bit with LEA, 6-3
ASP Ombudsman, 1-7
assembler variables, 9-10
assertion checking, 9-9
ASSUME directive, 6-4
asterisk multiplication operator, 8-5
AT combine type, 10-10
at-sign @, in symbols, 4-1
attribute operators/specifiers, 8-8
AUTOEXEC.BAT, 3-7
automatic paging control, 3-6
automatic paging controls, 13-2
B operator in EXTRN, 10-6
B override expression operator, 8-8
B-before operator in macros, 11-6
base registers, 6-2
base, default, 12-2
based structure example, 6-2
based structures, 9-6
bases for numbers, 8-1
bases, ambiguous, 8-2
batch file controls, 11-14
BCD numbers, 7-5
benefits of registration, 1-3
BIN extension for object files, 9-2
BIN extension, 3-2
binary base, 8-1
Binary Coded Decimal numbers, 7-5
biography, 1-6
BIOS interface, books on, 3-1
BIT expression operator, 8-6
block-structure in MASM, 12-4
17-2
books on assembler, recommended, 3-1
Boolean negation operator, 8-6
BP indexing size anomaly, 6-5
brackets, 8-9
British contact, 1-1
bugs, reporting, 1-7
built-in constant names, 7-4
built-in symbols, 16-1
built-in symbols, equates to, 9-8
BY operator, 8-4
BYTE align type, 10-9
BYTE override expression operator, 8-8
C programming language, linking to, 10-1
C switch, 3-2
C-loops in macros, 11-5
capacity, 1-5
capacity, source file, 3-9
case sensitivity, 3-2
case-insensitive comparisons, 8-8
catalog file A86.LIB, 13-5
categories of A86 elements, 4-1
cb specifier, 6-7
cd specifier, 6-7
changing the default base, 8-2
character loops in macros, 11-5
characters allowable in symbols, 4-1
characters recognized in A86 language, 4-2
choices for 87 operands, 7-6
class name, specifying, 10-11
classes, 10-4
clear-register macro, 11-1
clear-register macro, 11-5
closing of macro loops, 11-10
CODE ENDS directive, 9-2
code generation of forward references, 9-7
code label specifier, 6-7
CODE SEGMENT directive, 9-1
CODE segment, link to Pascal, 10-11
colon operator, 8-10
colon, deciding when to use, 4-4
columnar output, 2-2
COM extension, 3-2
COM programs, how to detect, 12-1
combine operand list, 10-9
combine specification, 10-9
combine types, 10-9
combining switches, 3-7
COMMENT directive, 4-2
comments in macros, removal of, 11-2
comments, 4-2
COMMON combine type, 10-10
comparison of strings, 8-7
COMPAT.8 macro file, 12-2
compatibility, 12-1
compression of macro text, 11-2
Compuserve section, 1-7
computation models, 10-2
17-3
concatenating terms in an expression, 8-5
conditional assembly and macros, 11-13
conditional assembly, 11-11
conditional calls, see IF, 5-1
conditional jump, far, see IF, 5-1
conditional line filtering, 13-1
conditional returns, 5-2
conditionals, list control, 3-4
constant operand to FLD, 7-4
constants, floating, 8-3
constants, format of, 8-1
constants, large, 9-5
constants, overview, 4-4
contacting me, 1-7
contents, 0-3
control-character notation, 8-5
controls, invocation, user-definable, 11-14
converting MASM programs, 12-1
CPU-specific instructions, 3-5
crashes, system, on lack of FWAIT, 7-2
creating programs to assemble, 3-1
credit cards, 1-1
cross reference demo, 2-2
cross reference listing, 13-3
cross-reference output switch, 3-7
cw specifier, 6-7
D operator in EXTRN, 10-6
D override expression operator, 8-8
D switch, 3-2
data allocation statements, samples, 4-1
DATA ENDS directive, 9-2
DATA SEGMENT directive, 9-1
DATA segment, link to Pascal, 10-11
DB directive, 9-3
DD directive, 9-3
DD examples, 9-5
DEC, multiple and numeric operands, 5-1
decimal base, 8-1
decimal output of macro operands, 11-8
DEF operator, 8-10
default base, changing, 8-2
default base, decimal, 3-2
default bases, 8-1
default forward references, 3-3
default output file name, 3-9
default segment registers, 6-3
default segment, OBJ mode, 10-12
defined symbols, testing for, 8-7
defining macros, 11-1
demonstration, 2-1
description of 87 instructions, 7-6
description of instructions, 6-9
Dettmann, Terry, 3-1
digits in file names, 3-9
digits, hex, 8-1
directives in a86, 9-1
directives, samples, 4-1
17-4
displacement field, 6-5
displaying macro expansions, 11-11
division operator, 8-5
dollar sign $, in symbols, 4-1
dollar sign operator, 8-11
DOS interface, books on, 3-1
double hash ## signs in macros, 11-2
double hash signs ## in macros, 11-13
double-precision, 7-5
double-quotes in strings, 9-5
doubleword indexing, 6-2
doubleword pointer initialization, 9-5
DQ directive, 9-3
DQ example, 9-5
DT directive, 9-3
DT example, 9-5
DUP construct, 9-4
duplicate definitions, 9-9
DW directive, 9-3
DWORD override expression operator, 8-8
E switch, 3-2
e-mail address, 1-7
EA byte, 6-5
eb specifier, 6-7
EBP indexing size anomaly, 6-6
ed specifier, 6-7
editing programs, 3-1
effective addresses, 6-1
effective addresses, encoding, 6-5
electronic mail, 1-7
ELSE, 11-12
ELSEIF, 11-12
EM end-of-macro symbol, 11-1
emulation, floating-point, 7-2
encoding of effective addresses, 6-5
encoding of floating-point numbers, 8-3
END directive, 10-7
end of a macro, 11-1
end of file, 10-7
END used as an operand value, 9-7
ENDIF, 11-12
ENDM, 12-3
ENDP directive, 9-11
ENDS directive, OBJ mode, 10-11
ENDS directives in COM mode, 9-2
English contact, 1-1
environment string, invocation equates in, 11-15
environment variable A86LIB, 13-6
environment variable, a86, 3-7
EQ expression operator, 8-7
EQ in comparing strings, 8-7
EQU directive, 9-7
equal-sign string compare, 8-8
equals-sign directive, 9-10
equates to built-in symbols, 9-8
equates to interrupts, 9-9
ER end-of-repeat symbol, 11-4
17-5
ERDEMO.BAT batch file, 2-1
ERR extension, 3-2
error file redirection, 3-2
error messages, 1-5
error messages, explanation, 14-1
evaluating macro operands, 11-8
EVEN directive, 9-3
ew specifier, 6-7
EX exit macro symbol, 11-10
examples of A86 statements, 4-1
examples of floating constants, 8-3
examples of numbers, 4-2
examples of type matching, 4-5
examples of useful memory accesses, 6-4
exclamation point operator, 8-6
exclusive features, 5-1
EXE programs, how to detect, 12-1
exiting from middle of macro, 11-10
EXITM simulation, 11-10
EXITM, 12-3
EXMAC, what happened to, 11-11
explicit EXTRNs, 3-4
explicit EXTRNs, forcing, 10-7
explicit OBJ specification, 10-1
explicit public names, 10-5
explicit WAITs, 7-1
exponent specifier, 8-3
expressions and forward references, 9-7
expressions in conditional assembly, 11-11
extended-precision operands, 7-5
extensions of source files, 3-9
external names and LINK, 10-3
extra coprocessor support, 7-2
EXTRN directive, 10-6
EXTRNs, explicit, 3-4
F operator in EXTRN, 10-6
F override expression operator, 8-8
F switch, 3-3
f switch, 7-3
FALSE in conditional assembly, 11-12
FALSE return value, 8-7
far label constants, 8-10
FAR override expression operator, 8-8
FBANK instruction on IIT-2C87, 7-2
FDISI instruction, 7-1
features, exclusive, 5-1
FENI instruction, 7-1
file breaks, listing control, 3-6
file in which a symbol was defined, 13-4
file lists, 3-9
file maintenance, 3-9
file names, digits in, 3-9
files, source, 3-1
filtering conditional lines, 13-1
FLD, immediate operand, 7-4
floating constants, examples of, 8-3
floating point operand types, 7-5
17-6
floating point operands, choices for, 7-6
floating point stack, 7-3
floating-point constants, format of, 8-3
floating-point emulation, 7-2
floating-point processor, 7-1
footprint, code generation, 1-3
forcing explicit EXTRNs, 10-7
forcing explicit EXTRNs, 3-4
forcing library lookup, 13-7
format of assembler source lines, 4-3
format of macros, 11-2
formfeed control, 3-6
FORTRAN, 10-10
forward references, 12-2
forward references, 9-6
forward references, default, 3-3
fragments, 10-3
FSETPM instruction, 7-1
FSTSW AX form, 7-1
FWAIT instruction, 7-1
G switch and EXTRNs, 10-7
G switch, 3-3
gaps in code, page breaks at, 13-3
GE expression operator, 8-7
Great Britain contact, 1-1
greater-mark ">" for local symbols, 5-3
GROUP directive, 10-12
groups, reason for, 10-3
GT expression operator, 8-7
H switch, 3-4
hash sign # in invocation, 13-7
hash sign #, conditional assembly, 11-11
hash signs # in macros, 11-13
hash signs # in macros, 11-2
hash signs #, literalizing in macros, 11-2
hex address listing control, 3-4
hex object lines, extra, 3-4
hexadecimal base, 8-1
HIGH operator, 8-4
high-level language computation models, 10-2
history of A86, 15-1
I switch, 3-4
ib specifier, 6-8
IBM, 12-1
IEEE standard for floating-point, 8-3
IF conditional assembly symbol, 11-11
IF statement, 5-1
IFDIF, 12-3
IFE, 12-3
IFIDN, 12-3
IIT-2C87 support, 7-2
immediate operand to FLD, 7-4
implicit public names, 10-5
INC, multiple and numeric operands, 5-1
incentives to register, 1-3
INCLUDE directive, 9-12
include file listing control, 3-6
17-7
INCLUDE with no file name, 13-7
indefinite repeats, 11-5
indentation of source listing, 3-4
indentation of wraparound lines, 3-7
index expressions, 8-4
index registers, 6-2
indexed memory, 6-2
inferior assemblers, 12-1
inferior assemblers, porting to, 12-5
initializations of floating-point, 7-3
instruction set chart, explanation, 6-7
instruction set, 87, 7-6
instruction statements, samples, 4-1
instructions on specific CPUs, 3-5
instructions, list of, 6-9
instructions, special, 6-8
integer operands to 8087, 7-5
Intel assembler, 12-1
Intel meeting, 10-3
intermediate numeric results, 7-5
Internet mail address, 1-7
interrupt equates, 9-9
interrupts, grabbing, 12-2
invocation variables in environment string, 11-15
invocation variables, 11-14
invoking A86, 3-1
IRET operand, 5-2
IRP and IRPC functionality, 11-1
IRP, 12-3
IRPC, 12-3
iw specifier, 6-8
JHASH example, 11-10
juxtaposing terms in an expression, 8-5
K base for numbers, 8-2
keyboard entry coding example, 11-3
L last-operand in macros, 11-5
L switch for listing, 13-1
L switch, 3-4
L2E and L2T constants, 7-4
LABEL directive, 9-11
labels, examples, 4-4
large constant initialization, 9-5
large macro operand numbers, 11-9
large model of segmentation, 10-2
last-operand in macros, 11-5
LE expression operator, 8-7
LEA and 32-bit arithmetic, 6-3
LEA instruction, optimizing, 3-3
LEA optimization, 5-4
leading underscore, in C, 10-1
legal terms, 1-1
length byte, generating in macro, 11-8
length of a symbol name, 4-2
LG2 constant, 7-4
library search, trigger in source, 9-12
line numbers, suppressing, 3-4
line-format, 4-3
17-8
LINES.8 library file, 2-1
LINK program, 10-3
linkage, 10-1
LIST directive (leading period), 13-2
list of instructions, 6-9
listing control directives, 13-2
listing control switches, 13-1
listing hex object bytes, 3-4
listing in A86, 13-1
listing indentation of source, 3-4
listing of 87 instructions, 7-6
listing of cross references, 13-3
listing, specific formats, 13-1
listings, how to activate, 3-4
LN2 constant, 7-4
loading named segments, 12-2
local labels in macros, 11-10
local labels, simulating, 12-6
local symbols, 5-3
local symbols, specifying, 9-10
location, this, operator, 8-11
logical operators, 8-6
long default jump, 3-3
LONG expression operator, 8-8
looping in macros, 11-4
loops with large index, 11-9
LOW operator, 8-4
lower case letters in symbols, 3-2
LST file, producing, 13-1
LT expression operator, 8-7
m specifier, 6-8
macro compatibility, 12-2
macro exiting from within loop, 11-10
macro expansions, displaying, 11-11
macro file, default, 3-8
macro libraries, making, 13-5
macro listing global control, 3-4
macro loops, closing, 11-10
macro loops, skipping increments, 11-6
macro operand substitution, 11-2
macro operands, computing number, 11-9
macros and conditional assembly, 11-13
macros, 11-1
macros, defining, 11-1
mailing list, 1-4
main module, 10-3
MAIN symbol, 10-7
maintenance of files, 3-9
manual, scope of, 3-1
MASM compatibility, 12-1
MASM compatible CODE, DATA, 10-11
MASM conditional assembly, simulating, 11-14
matching of types, examples, 4-5
matrix multiplication on IIT-2C87, 7-2
maximum length of a symbol name (127), 4-2
maximum source file size, 3-9
meeting at Intel, 10-3
17-9
MEMORY combine type, 10-10
memory forms, overlooked, 6-4
memory operand forms to 87 instructions, 7-5
memory requirements, 3-9
memory resident code, 12-2
memory variables, specifying, 6-1
menu systems and A86, 3-8
Microsoft, 12-1
minus operator, 8-5
MIX tool, compatibility, 3-3
mixing constant types in word inits., 8-5
mnemonics, 8086, 6-9
mnemonics, one for many instructions, 4-5
MOD modulo operator, 8-5
model of segmentation, grotesque, 10-3
ModRM byte, 6-5
module names, 10-5
modules, object, 10-3
MOV immediate into segment reg, 5-2
MOV of memory operands, 5-2
MOV of segment registers, 5-2
MOV substitute for LEA, 5-4
MOV with three operands, 5-2
move-memory macro example, 11-1
MOVSx, numeric operand to, 5-1
MSDOS.8 library file, 2-1
MTCOLS.BAT batch file, 2-2
multiple allocation using DUP, 9-4
multiple files in OBJ mode, 10-1
multiple increments in macro loops, 11-6
multiple operands to PUSH,POP,INC,DEC, 5-1
multiply by 10 coding example, 10-1
multiply operator, 8-5
NAME directive, 10-5
name of output files, 3-1
NE expression operator, 8-7
NE in comparing strings, 8-7
NEAR expression operator, 8-9
NEC chips, lack of AAD with operands, 5-4
NEC chips, special instructions, 6-8
NEC instructions, allowing, 3-5
negation, Boolean, 8-6
negative R-loops in macros, 11-7
nested IF blocks, 11-12
nested PROCs, lack of, 9-10
nesting of loops in macros, 11-8
new file listing control, 3-6
NIL prefix, 9-9
NOLIST directive (leading period), 13-2
non-combinable segments, 10-10
NOP and EVEN directive, 9-3
Norton, Peter, 3-1
NOT expression operator, 8-6
null invocation names, 11-15
null operands to macros, 11-3
number operands in expressions, 8-3
numbering, suppressing, 3-4
17-10
numbers, examples, 4-2
numbers, examples, 8-1
numbers, floating, 8-3
numbers, format of, 8-1
numeric operands to INC,DEC, 5-1
numeric operands to STOSx,MOVSx, 5-1
O switch, 10-1
O switch, 3-2
O switch, 3-5
OBJ extension, 3-2
OBJ file generation, 3-5
OBJ internal optimization, 3-3
OBJ production made easy, 10-1
object file name, 3-1
object modules, 10-3
octal base, 8-1
OFFSET expression operator, 8-9
Ombudsman, ASP, 1-7
online support, 1-7
opcodes, 8086, 6-9
opcodes, 87, 7-6
operand choices for 87 instructions, 7-6
operand number, generating, 11-9
operand override byte, 6-6
operand types to 87 instructions, 7-5
operating system requirements, 3-9
operation of A86, 3-1
operator precedence, 8-12
Optimized LEA instruction, 5-4
OR expression operator, 8-6
ORG directive, 9-2
outer segment, OBJ mode, 10-12
output files, naming, 3-1
overlooked memory forms, 6-4
overrides, 32-bit, 6-6
overrides, segment, 12-2
overrides, segment, 6-4
overview of A86, 1-4
overview of expressions, 8-3
P switch, 3-5
page breaks, automatic, 13-3
page breaks, manual, 13-3
PAGE directive, 13-2
page numbers, column control, 3-7
PAGE specifier, 10-9
PAGE.8 program, 2-1
PAGE.BAD source file, 2-1
PAGE.COM program, 2-1
pagination control switch T, 3-6
paging, automatic, 3-6
PARA specifier, 10-9
parameters, MASM local, 12-4
parenthesized operand numbers, 11-9
Pascal segment names, 10-11
Pascal, linking to, 10-2
passing macro operands by value, 11-8
Pentium instructions, 6-8
17-11
period as first character of a symbol, 4-1
period operator, 8-5
permanent switch settings, 3-7
phone number, my, 1-1
PI constant, 7-4
piping file names to A86, 3-8
plus operator, 8-5
POP, multiple operands, 5-1
POPA simulation for 8088, 3-5
port programs to inferior assemblers, 12-5
pound sign #, SEE hash sign
Power C, compatibility, 3-3
powers of ten, 8-3
precedence of operators, 8-12
prices, 1-2
printer eject program, 2-1
PROC directive in MASM, 12-4
PROC directive, 9-10
procedure-level summary listings, 13-3
procedures, 9-10
processor control, 3-5
processor-specific instructions, 6-8
program invocation, A86, 3-1
program location operator, 8-11
program size in expressions (END), 9-7
program starting location, OBJ mode, 10-7
programs, how to create, 3-1
prompt for file names, 3-8
PTR operator, 8-5
Public Brand Software, 3-1
PUBLIC combine type, 10-9
PUBLIC directive, 10-5
public names and LINK, 10-3
PUSH multiple operands, simulating, 12-5
PUSH, multiple operands, 5-1
PUSHA simulation for 8088, 3-5
Q operator in EXTRN, 10-6
Q override expression operator, 8-8
question mark ?, in symbols, 4-1
question-mark operator, 9-4
quoted-string macro operands, 11-3
QWORD override expression operator, 8-8
R-loops in macros, 11-4
R-loops, negative, 11-7
RADIX directive, 8-2
rb register specifier, 6-8
red tape, 1-4
red tape, 10-1
redefinable symbols, 5-3
redefining invocation variables, 11-15
redefining symbols, 9-9
redirection of error files, 3-2
REF operator, 8-10
references of symbols, listing, 13-3
registers, 8086, 4-3
registers, general, 6-1
registration benefits, 1-3
17-12
registration benefits, 13-5
relational operators, 8-7
release history, A86, 15-1
relocation and linkage, 10-1
repeat counts to string instructions, 5-1
repeating code using DUP, 9-4
REPT directive, simulating, 11-9
requirements, system, 3-9
reserved symbols, 16-1
reserved symbols, 4-2
RET instruction, meaning of, 9-10
RET operand, 5-2
RETF instruction and PROC, 9-10
RETF operand, 5-2
REV.8 source file, 2-1
REV.COM program, 2-1
reversing strings example, 2-1
rotate immediate simulation for 8088, 3-5
rw register specifier, 6-8
S switch, 3-5
samples of A86 statements, 4-1
scaled indexing, 6-3
scientific notation, 8-3
section number control, 3-6
section numbers, controlling, 13-2
SEG operator, 10-13
SEGMENT AT, non-OMF, 12-5
SEGMENT directive, non-OBJ mode, 12-5
SEGMENT directive, OBJ mode, 10-8
segment override colon operator, 8-10
segment overrides, 12-2
segment overrides, 6-4
segment registers managed by GROUP, 10-12
segment registers, default, 6-3
segmentation and memory access, 6-3
segmentation models, 10-2
segments in A86, 9-1
segments, loading named, 12-2
selective NOLIST for macros (#H), 13-2
shareware distribution, 1-1
shift immediate simulation for 8088, 3-5
shifting expression operators, 8-6
SHL and SHR expression operator, 8-6
SHORT expression operator, 8-8
simple macro syntax, 11-1
single-precision, 7-5
size of effective addresses, 6-5
size of macro operands, 11-8
size of program in expressions (END), 9-7
size of source files, 3-9
size of structures, 8-11
skipped lines, suppressing, 3-4
slash division operator, 8-5
slash specifier, 6-5
small model of computation, 10-2
source file as default TITLE, 3-6
source files, 3-1
17-13
source libraries, 13-5
special instructions, 6-8
Specialty Software, 7-2
speed, 1-4
square brackets operator, 8-9
ST floating-point stack specifier, 8-10
STACK combine type, 10-9
STACK segment, relocatable, 10-11
stack segments in OBJ mode, 10-9
stack, floating point, 7-3
standard input command tail, 3-8
starting location, OBJ mode, 10-7
STOSx, numeric operand to, 5-1
strategies for file maintenance, 3-9
string allocation, 9-5
string comparison operators, 8-7
STRUC directive, 9-5
STRUC, implicit via SEGMENT AT, 12-5
structure initialization, 12-4
structure, based, example, 6-2
structured programming constructs, 5-1
structures and MASM, 12-4
structures, size of, 8-11
sub-directories of programs, 3-9
substitution of macro operands, 11-2
subtitle default to source file, 3-6
subtraction operator, 8-5
SUBTTL subtitle directive, 13-2
summary of procedure calls, 13-3
suppressing line numbers, 3-4
suppressing list controls, 3-4
suppressing skipped lines, 3-4
suppressing symbols file, 3-5
switches, assembler, 3-2
switches, combining, 3-7
switches, user-definable, 11-14
SYM extension, 3-2
symbol listing control, 3-5
symbol table file name, 3-2
symbols file, suppressing, 3-5
symbols, allowable characters for, 4-1
symbols, MASM local, 12-4
symbols, redefining, 9-9
symbols, reserved, 16-1
system crashes on lack of FWAIT, 7-2
system requirements, 3-9
T operator in EXTRN, 10-6
T override expression operator, 8-8
T switch, 3-6
table of contents, 0-3
tabs, recommendation against, 4-3
TBYTE override expression operator, 8-8
TCOLS.8 source file, 2-2
TCOLS.COM program, 2-2
telephone number, my, 1-1
terms, legal, 1-1
TEST with one operand, 5-4
17-14
Texas invocation switch, 11-14
Texas, 11-12
TEXT segment name, 10-12
THIS operator, 8-11
tips for memory access, 6-4
TITLE default to source file, 3-6
TITLE directive, 13-2
titling control switch T, 3-6
TO in invocation, 3-1
TRUE in conditional assembly, 11-12
TRUE return value, 8-7
truncation of listing lines, 3-7
Turbo Pascal segment names, 10-11
Turbo Pascal, linking to, 10-2
type matching, examples, 4-5
TYPE operator, 8-11
types in the a86 language, 4-3
types, assumed, 10-2
UND undefined symbols file, 10-6
undefined symbol types, assumed, 10-2
undefined symbols listing in OBJ mode, 10-6
underscore, in symbols, 4-1
underscore, leading, in C, 10-1
underscores within numbers, 8-1
unusable user symbols, 16-1
up arrow symbol and invocation equates, 11-15
USAGE.8 library file, 2-1
user symbols, 4-2
USES clause, converting, 12-4
value, passing by, 11-8
variable forward references, 9-7
variable operands in expressions, 8-3
variables declared at invocation, 11-14
variables, 9-10
variables, examples, 4-3
verbose forms, floating point, 7-3
verbose PROC, 9-10
version history, A86, 15-1
W operator in EXTRN, 10-6
W override expression operator, 8-8
W switch, 3-7
WAIT instruction, 7-1
Wettstein, Greg, 1-7
widowed listing lines, avoiding, 13-2
wild cards in source files, 3-2
wild cards, order of, 3-9
WORD align type, 10-9
WORD override expression operator, 8-8
wraparound listing control, 3-7
X specifier for numeric bases, 8-1
X switch, 3-7
XCHG of memory operands, 5-3
XCHG with segment register, 5-3
XOR expression operator, 8-6
XREF output switch, 3-7
XRF files, producing, 13-3