8086 INSTRUCTION SET

Stack Implementation in Memory

Original SP In Use Stack grows in

In Use direction
of decreasing
In Use memory
In Use addresses
Direction of
increasing In Use
memory In Use
addresses SS:SP
FREE
FREE
FREE
FREE
SS
 Stack Implementation
in Memory (cont.)

• SS - Stack Segment
• SP (stack pointer) always points to the top of the stack
– SP initially points to top of the stack (high memory address).
– SP decreases as data is PUSHed
PUSH AX ==> SUB SP, 2 ; MOV [SS:SP], AX
– SP increases as data is POPed
POP AX ==> MOV AX, [SS:SP] ; ADD SP, 2
• BP (base pointer) can point to any element on the stack

EENG4005
 PUSH Instruction
Example
To
address
12FFF
Register Array
PUSH BX 03800
AX
6A 037FF
BX 6AB3 6AB3
B3 037FE
CX
DX SH
PU

SH
re

U
fo

P
BX Be

fter
SP

A
SP 0800 SP
BX

SS 0300 03000
STACK segment
EENG4005
 POP Instruction
Example
To
address
0FFFF
Register Array
01008
AX POP BX
39 01007
BX 392F 392F
2F 01006
CX
DX
OP
rPe
BX Aft

POP
SP

fo re
Be
SP 1006 SP
BX

SS 0000 00000
STACK segment

EENG4005
LOGICAL INSTRUCTIONS
 Assembler Directives
Assume
• Used to tell the assembler the name of the logical segment it
should use for a specified segment.
• You must tell the assembler that what to assume for any
segment you use in the program.
• Example, ASSUME: CODE tells the assembler that the
instructions for the program are in segment named CODE.
DB – Defined Byte
• Used to declare a byte type variable or to set aside one or
more locations of type byte in memory.
• Example, PRICES DB 49H, 98H, 29H: Declare array of 3 bytes
named PRICES and initialize 3 bytes as shown.
 Assembler Directives(contd.)
DD – Define Double Word
• Used to declare a variable of type doubleword or to reserve a
memory location which can be accessed as doubleword.
DQ – Define Quadword
• Used to tell the assembler to declare the variable as 4 words of
storage in memory.
DT – Define Ten bytes
• Used to tell the assembler to declare the variable which is 10
bytes in length or reserve 10 bytes of storage in memory.
DW – Define Word
• Used to tell the assembler to define a variable type as word or
reserve word in memory.
 Assembler Directives(contd.)
END – End the program
• To tell the assembler to stop fetching the instruction and
end the program execution.
• ENDP – it is used to end the procedure.
• ENDS – used to end the segment.
EQU – Equate
• Used to give name to some value or symbol.
EVEN – Align on Even memory address
• Tells the assembler to increment the location counter to the
next even address if it is not already at an even address.
 Assembler Directives(contd.)
EXTRN
• Used to tell the assembler that the name or labels following
the directive are in some other assembly module.
GLOBAL – Declares symbols as PUBLIC or EXTRN
• Used to make the symbol available to other modules.
• It can be used in place of EXTRN or PUBLIC keyword.
GROUP – Group related segment
• Used to tell the assembler to group the logical segments
named after the directive into one logical segment.
• This allows the content of all the segments to be accessed
from the same group.
 Assembler Directives(contd.)
INCLUDE – include source code from file
• Used to tell the assembler to insert a block of source code from
the named file into the current source module. This shortens the
source code.
LABEL
• Used to give the name to the current value in the location
counter.
• The LABEL directive must be followed by a term which specifies
the type you want associated with that name.
LENGTH
• Used to determine the number of items in some data such as
string or array.
 Assembler Directives(contd.)
NAME
• Used to give a specific name to a module when the programs consisting of
several modules.
OFFSET
• It is an operator which tells the assembler to determine the offset or
displacement of named data item or procedure from the start of the
segment which contains it.
ORG – Originate
• Tells the assembler to set the location counter value.
• Example, ORG 7000H sets the location counter value to point to 7000H
location in memory.
• $ is often used to symbolically represent the value of the location counter. It
is used with ORG to tell the assembler to change the location according to
the current value in the location counter. E.g. ORG $+100.
 Assembler Directives(contd.)
PROC – Procedure
• Used to identify the start of the procedure.
PTR – Pointer
• Used to assign a specific type to a variable or a label.
• It is necessary to do this in any instruction where the type of
the operand is not clear.
Public
• It is used to tell the assembler that the specified label or
variable is accessible by other modules.
• This is useful in large programs which are generally written
in modules.
8086 Interrupts
8086 Interrupts and Interrupt Responses

• The meaning of ‘interrupts’ is to break the sequence of

operation

• While the CPU is executing a program, an interrupt

breaks the normal sequence of execution of instructions,
diverts its execution to some other program called
Interrupt Service Routine (ISR)

• After executing ISR , the control is transferred back again

to the main program
 Contd..
• An 8086 interrupt can come from any one of the three
sources

• One source is an external signal applied to the Non

Maskable Interrupt (NMI) input pin or to the Interrupt
(INTR) input pin

• An interrupt caused by a signal applied to one of these

inputs is referred to as a hardware interrupt
 Contd..
• A second source of an interrupt is execution of the
Interrupt instruction, INT

• This is referred to as Software Interrupt

• The third source of an interrupt is some error condition

produced in the 8086 by the execution of an instruction

• An example of this is the divide by zero error

Interrupt processing flow
Main program

Interrupt N
Req

Accept N
Interrupt

Get interrupt
vector

Jump to ISR
Save PC

Load PC
 Hardware Interrupts – Interrupt pins and timing
• x86 Interrupt Pins
– INTR: Interrupt Request. Activated by a peripheral device to interrupt the processor.
• Level triggered. Activated with a logic 1.
– /INTA: Interrupt Acknowledge. Activated by the processor to inform the interrupting device
the the interrupt request (INTR) is accepted.
• Level triggered. Activated with a logic 0.
– NMI: Non-Maskable Interrupt. Used for major system faults such as parity errors and power
failures.
• Edge triggered. Activated with a positive edge (0 to 1) transition.
• Must remain at logic 1, until it is accepted by the processor.
• Before the 0 to 1 transition, NMI must be at logic 0 for at least 2 clock cycles.
• No need for interrupt acknowledgement.

INTR

INTA΄

D7-D0 Vector
 Contd..

ISR procedure
Mainline Program PUSH Flags
PUSH registers
CLEAR IF , TF
PUSH CS -
PUSH IP -
FETCH ISR ADDRESS -
POP registers
POP IP
IRET
POP CS
POP FLAGS
 Contd..

1. It decrements SP by 2 and pushes the flag

register on the stack

2. Disables INTR by clearing the IF

3. It resets the TF in the flag Register

4. It decrements SP by 2 and pushes CS on the

stack
 Contd..
5. It decrements SP by 2 and pushes IP on the
stack

6. Fetch the ISR address from the interrupt

vector table
03FFH Type 255 (Available)

03FCH
Available
Interrupts
Type 32 (Available) (224)
080H

Type 31 (Reserved)
07FH
Reserved
Interrupts
(27)
Type 5
Reserved
0014H
 Type 4 POINTER
010H (OVERFLOW)
Type 3 POINTER
00CH
(BREAK POINT)
Type 2 POINTER
008H
(NON-MASKABLE)
Type 1 POINTER
004H (SINGLE STEP)
Type 0 POINTER CS base address
000H (DIVIDE ERROR) IP offset
 Interrupt Vector Table
INT Number Physical Address
INT 00 00000
INT 01 00004
INT 02 00008
: :
: :
INT FF 003FC
 Example
Find the physical address in the interrupt vector
table associated with
a) INT 04H b) INT FFH
Solution:
a) 04H * 4 = 0FH
Physical Address: 0000FH
b) FF * 4 = 3FCH
Memory Address : 003FCH
 Procedures
• Group of instructions that usually perform one task
• Reusable section of the software that is stored in memory
once, but use as often as necessary
• The stack stores the return address whenever a procedure
is called during the execution of the program
– CALL pushes the address of the instruction following it on the
stack
– RET removes an address from the stack so the program returns
to the instruction following the call
Passing parameters to and from procedures

Major ways of passing parameters to and from a

procedure:
• In register
• In dedicated memory locations accessed by
name
• With pointers passed in registers
• With the stack
 Reentrant and Recursive procedures

• Reentrant procedures: The procedure which

can be interrupted, used and “reentered”
without losing or writing over anything.
• Recursive procedure: It is the procedure
which call itself.
Writing and using Assembler
Macros
 Macros

• A macro inserts a block of statements at various

points in a program during assembly

• Text substitutions made at compile time

– NOT a procedure -- Code is literally dumped into
program
– Parameter names are substituted
– Useful for tedious programming tasks
– Instantiated within code segment.
 Macros (cont.)
• General Format
MACRO_NAME MACRO
Param1,Param2,...,ParamN
LOCAL MyLabel
Your Code ...
... Param1 ...
...Param2 ...
Your Code ...
JMP MyLabel
Your Code ...
MyLabel:
... ParamN ...
Your Code ...
ENDM
 Macros vs Procedures (cont.)
• In the example the macro and procedure produce the
same result

• The procedure definition generates code when the

assembler encounters the PROC directive

• The macro does not emit any code when processing the
statements between the MACRO and ENDM
– Upon encountering Macro_1 in the mnemonic field, MASM
assembles every statement between the MACRO and ENDM
directives and emits that code to the output file
– At run time, the processor executes these instructions without
the call/ret overhead