.so mnx.mac

.TH AS 1x

.\" unchecked (kjb)

.CD "as \(en assembler"

.SE "AS\(emASSEMBLER [IBM]"

.SP 1

.PP

This document describes the language accepted by the 80386 assembler

that is part of the Amsterdam Compiler Kit. Note that only the syntax is

described, only a few 386 instructions are shown as examples.

.SS "Tokens, Numbers, Character Constants, and Strings"

.PP

The syntax of numbers is the same as in C.

The constants 32, 040, and 0x20 all represent the same number, but are

written in decimal, octal, and hex, respectively.

The rules for character constants and strings are also the same as in C.

For example, \(fma\(fm is a character constant.

A typical string is "string".

Expressions may be formed with C operators, but must use [ and ] for

parentheses. (Normal parentheses are claimed by the operand syntax.)

.SS "Symbols"

.PP

Symbols contain letters and digits, as well as three special characters:

dot, tilde, and underscore.

The first character may not be a digit or tilde.

.PP

The names of the 80386 registers are reserved. These are:

.HS

~~~al, bl, cl, dl

.br

~~~ah, bh, ch, dh

.br

~~~ax, bx, cx, dx, eax, ebx, ecx, edx

.br

~~~si, di, bp, sp, esi, edi, ebp, esp

.br

~~~cs, ds, ss, es, fs, gs

.HS

The xx and exx variants of the eight general registers are treated as

synonyms by the assembler. Normally "ax" is the 16-bit low half of the

32-bit "eax" register. The assembler determines if a 16 or 32 bit

operation is meant solely by looking at the instruction or the

instruction prefixes. It is however best to use the proper registers

when writing assembly to not confuse those who read the code.

.HS

The last group of 6 segment registers are used for selector + offset mode

addressing, in which the effective address is at a given offset in one of

the 6 segments.

.PP

Names of instructions and pseudo-ops are not reserved.

Alphabetic characters in opcodes and pseudo-ops must be in lower case.

.SS "Separators"

.PP

Commas, blanks, and tabs are separators and can be interspersed freely

between tokens, but not within tokens.

Commas are only legal between operands.

.SS "Comments"

.PP

The comment character is \*(OQ!\*(CQ.

The rest of the line is ignored.

.SS "Opcodes"

.PP

The opcodes are listed below.

Notes: (1) Different names for the same instruction are separated by \*(OQ/\*(CQ.

(2) Square brackets ([]) indicate that 0 or 1 of the enclosed characters

can be included.

(3) Curly brackets ({}) work similarly, except that one of the

enclosed characters \fImust\fR be included.

Thus square brackets indicate an option, whereas curly brackets indicate

that a choice must be made.

.sp

.if t .ta 0.25i 1.2i 3i

.if n .ta 2 10 24

.nf

.B "Data Transfer"

.HS

mov[b] dest, source ! Move word/byte from source to dest

pop dest ! Pop stack

push source ! Push stack

xchg[b] op1, op2 ! Exchange word/byte

xlat ! Translate

o16 ! Operate on a 16 bit object instead of 32 bit

.B "Input/Output"

.HS

in[b] source ! Input from source I/O port

in[b] ! Input from DX I/O port

out[b] dest ! Output to dest I/O port

out[b] ! Output to DX I/O port

.B "Address Object"

.HS

lds reg,source ! Load reg and DS from source

les reg,source ! Load reg and ES from source

lea reg,source ! Load effect address of source to reg and DS

{cdsefg}seg ! Specify seg register for next instruction

a16 ! Use 16 bit addressing mode instead of 32 bit

.B "Flag Transfer"

.HS

lahf ! Load AH from flag register

popf ! Pop flags

pushf ! Push flags

sahf ! Store AH in flag register

.B "Addition"

.HS

aaa ! Adjust result of BCD addition

add[b] dest,source ! Add

adc[b] dest,source ! Add with carry

daa ! Decimal Adjust after addition

inc[b] dest ! Increment by 1

.B "Subtraction"

.HS

aas ! Adjust result of BCD subtraction

sub[b] dest,source ! Subtract

sbb[b] dest,source ! Subtract with borrow from dest

das ! Decimal adjust after subtraction

dec[b] dest ! Decrement by one

neg[b] dest ! Negate

cmp[b] dest,source ! Compare

.B "Multiplication"

.HS

aam ! Adjust result of BCD multiply

imul[b] source ! Signed multiply

mul[b] source ! Unsigned multiply

.B "Division"

.HS

aad ! Adjust AX for BCD division

o16 cbw ! Sign extend AL into AH

o16 cwd ! Sign extend AX into DX

cwde ! Sign extend AX into EAX

cdq ! Sign extend EAX into EDX

idiv[b] source ! Signed divide

div[b] source ! Unsigned divide

.B "Logical"

.HS

and[b] dest,source ! Logical and

not[b] dest ! Logical not

or[b] dest,source ! Logical inclusive or

test[b] dest,source ! Logical test

xor[b] dest,source ! Logical exclusive or

.B "Shift"

.HS

sal[b]/shl[b] dest,CL ! Shift logical left

sar[b] dest,CL ! Shift arithmetic right

shr[b] dest,CL ! Shift logical right

.B "Rotate"

.HS

rcl[b] dest,CL ! Rotate left, with carry

rcr[b] dest,CL ! Rotate right, with carry

rol[b] dest,CL ! Rotate left

ror[b] dest,CL ! Rotate right

.B "String Manipulation"

.HS

cmps[b] ! Compare string element ds:esi with es:edi

lods[b] ! Load from ds:esi into AL, AX, or EAX

movs[b] ! Move from ds:esi to es:edi

rep ! Repeat next instruction until ECX=0

repe/repz ! Repeat next instruction until ECX=0 and ZF=1

repne/repnz ! Repeat next instruction until ECX!=0 and ZF=0

scas[b] ! Compare ds:esi with AL/AX/EAX

stos[b] ! Store AL/AX/EAX in es:edi

.fi

.B "Control Transfer"

.PP

\fIAs\fR accepts a number of special jump opcodes that can assemble to

instructions with either a byte displacement, which can only reach to targets

within \(mi126 to +129 bytes of the branch, or an instruction with a 32-bit

displacement. The assembler automatically chooses a byte or word displacement

instruction.

.PP

The English translation of the opcodes should be obvious, with

\*(OQl(ess)\*(CQ and \*(OQg(reater)\*(CQ for signed comparisions, and

\*(OQb(elow)\*(CQ and \*(OQa(bove)*(CQ for unsigned comparisions. There are

lots of synonyms to allow you to write "jump if not that" instead of "jump

if this".

.PP

The \*(OQcall\*(CQ, \*(OQjmp\*(CQ, and \*(OQret\*(CQ instructions can be

either intrasegment or

intersegment. The intersegment versions are indicated with

the suffix \*(OQf\*(CQ.

.if t .ta 0.25i 1.2i 3i

.if n .ta 2 10 24

.nf

.B Unconditional

.HS

jmp[f] dest ! jump to dest (8 or 32-bit displacement)

call[f] dest ! call procedure

ret[f] ! return from procedure

.B "Conditional"

.HS

ja/jnbe ! if above/not below or equal (unsigned)

jae/jnb/jnc ! if above or equal/not below/not carry (uns.)

jb/jnae/jc ! if not above nor equal/below/carry (unsigned)

jbe/jna ! if below or equal/not above (unsigned)

jg/jnle ! if greater/not less nor equal (signed)

jge/jnl ! if greater or equal/not less (signed)

jl/jnqe ! if less/not greater nor equal (signed)

jle/jgl ! if less or equal/not greater (signed)

je/jz ! if equal/zero

jne/jnz ! if not equal/not zero

jno ! if overflow not set

jo ! if overflow set

jnp/jpo ! if parity not set/parity odd

jp/jpe ! if parity set/parity even

jns ! if sign not set

js ! if sign set

.B "Iteration Control"

.HS

jcxz dest ! jump if ECX = 0

loop dest ! Decrement ECX and jump if CX != 0

loope/loopz dest ! Decrement ECX and jump if ECX = 0 and ZF = 1

loopne/loopnz dest ! Decrement ECX and jump if ECX != 0 and ZF = 0

.B "Interrupt"

.HS

int n ! Software interrupt n

into ! Interrupt if overflow set

iretd ! Return from interrupt

.B "Flag Operations"

.HS

clc ! Clear carry flag

cld ! Clear direction flag

cli ! Clear interrupt enable flag

cmc ! Complement carry flag

stc ! Set carry flag

std ! Set direction flag

sti ! Set interrupt enable flag

.fi

.SS "Location Counter"

.PP

The special symbol \*(OQ.\*(CQ is the location counter and its value

is the address of the first byte of the instruction in which the symbol

appears and can be used in expressions.

.SS "Segments"

.PP

There are four different assembly segments: text, rom, data and bss.

Segments are declared and selected by the \fI.sect\fR pseudo-op. It is

customary to declare all segments at the top of an assembly file like

this:

.HS

~~~.sect .text; .sect .rom; .sect .data; .sect .bss

.HS

The assembler accepts up to 16 different segments, but

.MX

expects only four to be used. Anything can in principle be assembled

into any segment, but the

.MX

bss segment may only contain uninitialized data.

Note that the \*(OQ.\*(CQ symbol refers to the location in the current

segment.

.SS "Labels"

.PP

There are two types: name and numeric. Name labels consist of a name

followed by a colon (:).

.PP

The numeric labels are single digits. The nearest 0: label may be

referenced as 0f in the forward direction, or 0b backwards.

.SS "Statement Syntax"

.PP

Each line consists of a single statement.

Blank or comment lines are allowed.

.SS "Instruction Statements"

.PP

The most general form of an instruction is

.HS

~~~label: opcode operand1, operand2 ! comment

.HS

.SS "Expression Semantics"

.PP

.tr ~~

The following operators can be used:

+ \(mi * / & | ^ ~ << (shift left) >> (shift right) \(mi (unary minus).

.tr ~

32-bit integer arithmetic is used.

Division produces a truncated quotient.

.SS "Addressing Modes"

.PP

Below is a list of the addressing modes supported.

Each one is followed by an example.

.HS

.ta 0.25i 3i

.nf

constant mov eax, 123456

direct access mov eax, (counter)

register mov eax, esi

indirect mov eax, (esi)

base + disp. mov eax, 6(ebp)

scaled index mov eax, (4*esi)

base + index mov eax, (ebp)(2*esi)

base + index + disp. mov eax, 10(edi)(1*esi)

.HS

.fi

Any of the constants or symbols may be replacement by expressions. Direct

access, constants and displacements may be any type of expression. A scaled

index with scale 1 may be written without the \*(OQ1*\*(CQ.

.SS "Call and Jmp"

.PP

The \*(OQcall\*(CQ and \*(OQjmp\*(CQ instructions can be interpreted

as a load into the instruction pointer.

.HS

.ta 0.25i 3i

.nf

call _routine ! Direct, intrasegment

call (subloc) ! Indirect, intrasegment

call 6(ebp) ! Indirect, intrasegment

call ebx ! Direct, intrasegment

call (ebx) ! Indirect, intrasegment

callf (subloc) ! Indirect, intersegment

callf seg:offs ! Direct, intersegment

.HS

.fi

.SP 1

.SS "Symbol Assigment"

.SP 1

.PP

Symbols can acquire values in one of two ways.

Using a symbol as a label sets it to \*(OQ.\*(CQ for the current

segment with type relocatable.

Alternative, a symbol may be given a name via an assignment of the form

.HS

~~~symbol = expression

.HS

in which the symbol is assigned the value and type of its arguments.

.SP 1

.SS "Storage Allocation"

.SP 1

.PP

Space can be reserved for bytes, words, and longs using pseudo-ops.

They take one or more operands, and for each generate a value

whose size is a byte, word (2 bytes) or long (4 bytes). For example:

.HS

.if t .ta 0.25i 3i

.if n .ta 2 24

.data1 2, 6 ! allocate 2 bytes initialized to 2 and 6

.br

.data2 3, 0x10 ! allocate 2 words initialized to 3 and 16

.br

.data4 010 ! allocate a longword initialized to 8

.br

.space 40 ! allocates 40 bytes of zeros

.HS

allocates 50 (decimal) bytes of storage, initializing the first two

bytes to 2 and 6, the next two words to 3 and 16, then one longword with

value 8 (010 octal), last 40 bytes of zeros.

.SS "String Allocation"

.PP

The pseudo-ops \fI.ascii\fR and \fI.asciz\fR

take one string argument and generate the ASCII character

codes for the letters in the string.

The latter automatically terminates the string with a null (0) byte.

For example,

.HS

~~~.ascii "hello"

.br

~~~.asciz "world\en"

.HS

.SS "Alignment"

.PP

Sometimes it is necessary to force the next item to begin at a word, longword

or even a 16 byte address boundary.

The \fI.align\fR pseudo-op zero or more null byte if the current location

is a multiple of the argument of .align.

.SS "Segment Control"

.PP

Every item assembled goes in one of the four segments: text, rom, data,

or bss. By using the \fI.sect\fR pseudo-op with argument

\fI.text, .rom, .data\fR or \fI.bss\fR, the programmer can force the

next items to go in a particular segment.

.SS "External Names"

.PP

A symbol can be given global scope by including it in a \fI.define\fR pseudo-op.

Multiple names may be listed, separate by commas.

It must be used to export symbols defined in the current program.

Names not defined in the current program are treated as "undefined

external" automatically, although it is customary to make this explicit

with the \fI.extern\fR pseudo-op.

.SS "Common"

.PP

The \fI.comm\fR pseudo-op declares storage that can be common to more than

one module. There are two arguments: a name and an absolute expression giving

the size in bytes of the area named by the symbol.

The type of the symbol becomes

external. The statement can appear in any segment.

If you think this has something to do with FORTRAN, you are right.

.SS "Examples"

.PP

In the kernel directory, there are several assembly code files that are

worth inspecting as examples.

However, note that these files, are designed to first be

run through the C preprocessor. (The very first character is a # to signal

this.) Thus they contain numerous constructs

that are not pure assembler.

For true assembler examples, compile any C program provided with

.MX

using the \fB\(enS\fR flag.

This will result in an assembly language file with a suffix with the same

name as the C source file, but ending with the .s suffix.