Instruction Set of
8086
An instruction is a binary pattern
designed inside a microprocessor to
perform a specific function.
The entire group of instructions that
a microprocessor supports is called
Instruction Set.
8086 has more than 20,000
instructions.
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Classification of
Instruction Set
Data Transfer Instructions
Arithmetic Instructions
Bit Manipulation Instructions
Program Execution Transfer
Instructions
String Instructions
Processor Control Instructions
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Data Transfer
Instructions
These instructions are used to
transfer data from source to
destination.
The operand can be a constant,
memory location, register or I/O port
address.
Data Transfer
Instructions
MOV Des, Src:
Src operand can be register, memory location or
immediate operand.
Des can be register or memory operand.
Both Src and Des cannot be memory location at
the same time.
E.g.:
MOV CX, 037A H
MOV AL, BL
MOV BX, [0301 H]
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Data Transfer
Instructions
PUSH Operand:
It pushes the operand into top of stack.
E.g.: PUSH BX
POP Des:
It pops the operand from top of stack to Des.
Des can be a general purpose register, segment
register (except CS) or memory location.
E.g.: POP AX
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Data Transfer
Instructions
XCHG Des, Src:
This instruction exchanges Src with Des.
It cannot exchange two memory locations directly.
E.g.: XCHG DX, AX
Data Transfer
Instructions
IN Accumulator, Port Address:
It transfers the operand from specified port to
accumulator register.
E.g.: IN AX, 0028 H
OUT Port Address, Accumulator:
It transfers the operand from accumulator to specified
port.
E.g.: OUT 0028 H, AX
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Data Transfer
Instructions
LEA Register, Src:
It loads a 16-bit register with the
offset address of the data specified
by the Src.
E.g.: LEA BX, [DI]
This
instruction loads the contents of DI
(offset) into the BX register.
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Data Transfer Instructions
LDS Des, Src:
It loads 32-bit pointer from memory source to
destination register and DS.
The offset is placed in the destination register
and the segment is placed in DS.
To use this instruction the word at the lower
memory address must contain the offset and
the word at the higher address must contain
the segment.
E.g.: LDS BX, [0301 H]
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Data Transfer
Instructions
LES Des, Src:
It loads 32-bit pointer from memory source to
destination register and ES.
The offset is placed in the destination register
and the segment is placed in ES.
This instruction is very similar to LDS except
that it initializes ES instead of DS.
E.g.: LES BX, [0301 H]
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Data Transfer
Instructions
LAHF:
It copies the lower byte of flag register to AH.
SAHF:
It copies the contents of AH to lower byte of flag register.
PUSHF:
Pushes flag register to top of stack.
POPF:
Pops the stack top to flag register.
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Arithmetic
Instructions
ADD Des, Src:
It adds a byte to byte or a word to word.
It effects AF, CF, OF, PF, SF, ZF flags.
E.g.:
ADD
AL, 74H
ADD
DX, AX
ADD
AX, [BX]
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Arithmetic
Instructions
ADC Des, Src:
It adds the two operands with CF.
It effects AF, CF, OF, PF, SF, ZF flags.
E.g.:
ADC
AL, 74H
ADC
DX, AX
ADC
AX, [BX]
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Arithmetic
Instructions
SUB Des, Src:
It subtracts a byte from byte or a word from
word.
It effects AF, CF, OF, PF, SF, ZF flags.
For subtraction, CF acts as borrow flag.
E.g.:
SUB
AL, 74H
SUB
DX, AX
SUB
AX, [BX]
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Arithmetic
Instructions
SBB Des, Src:
It subtracts the two operands and also
the borrow from the result.
It effects AF, CF, OF, PF, SF, ZF flags.
E.g.:
SBB
AL, 74H
SBB
DX, AX
SBB
AX, [BX]
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Arithmetic
Instructions
INC Src:
It increments the byte or word by one.
The operand can be a register or
memory location.
It effects AF, OF, PF, SF, ZF flags.
CF is not effected.
E.g.: INC AX
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Arithmetic
Instructions
DEC Src:
It decrements the byte or word by one.
The operand can be a register or
memory location.
It effects AF, OF, PF, SF, ZF flags.
CF is not effected.
E.g.: DEC AX
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Arithmetic
Instructions
AAA (ASCII Adjust after Addition):
The data entered from the terminal is in ASCII format.
In ASCII, 0 9 are represented by 30H 39H.
This instruction allows us to add the ASCII codes.
This instruction does not have any operand.
Other ASCII Instructions:
AAS (ASCII Adjust after Subtraction)
AAM (ASCII Adjust after Multiplication)
AAD (ASCII Adjust Before Division)
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Arithmetic
Instructions
DAA (Decimal Adjust after Addition)
It is used to make sure that the result of adding two
BCD numbers is adjusted to be a correct BCD
number.
It only works on AL register.
DAS (Decimal Adjust after Subtraction)
It is used to make sure that the result of subtracting
two BCD numbers is adjusted to be a correct BCD
number.
It only works on AL register.
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Arithmetic
Instructions
NEG Src:
It creates 2s complement of a
given number.
That means, it changes the sign of
a number.
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Arithmetic
Instructions
CMP Des, Src:
It compares two specified bytes or words.
The Src and Des can be a constant, register or
memory location.
Both operands cannot be a memory location at
the same time.
The comparison is done simply by internally
subtracting the source from destination.
The value of source and destination does not
change, but the flags are modified to indicate the
result.
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Arithmetic
Instructions
MUL Src:
It is an unsigned multiplication instruction.
It multiplies two bytes to produce a word or two words
to produce a double word.
AX = AL * Src
DX : AX = AX * Src
This instruction assumes one of the operand in AL or AX.
Src can be a register or memory location.
IMUL Src:
It is a signed multiplication instruction.
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Arithmetic
Instructions
DIV Src:
It is an unsigned division instruction.
It divides word by byte or double word by
word.
The operand is stored in AX, divisor is Src and
the result is stored as:
AH
= remainder
AL = quotient
IDIV Src:
It is a signed division instruction.
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Arithmetic
Instructions
CBW (Convert Byte to Word):
This instruction converts byte in AL to word in AX.
The conversion is done by extending the sign bit
of AL throughout AH.
CWD (Convert Word to Double Word):
This instruction converts word in AX to double
word in DX : AX.
The conversion is done by extending the sign bit
of AX throughout DX.
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Bit Manipulation
Instructions
These instructions are used at the bit level.
These instructions can be used for:
Testing a zero bit
Set or reset a bit
Shift bits across registers
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Bit Manipulation
Instructions
NOT Src:
It complements each bit of Src to produce 1s
complement of the specified operand.
The operand can be a register or memory
location.
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Bit Manipulation
Instructions
AND Des, Src:
It performs AND operation of Des and Src.
Src can be immediate number, register or
memory location.
Des can be register or memory location.
Both operands cannot be memory locations at
the same time.
CF and OF become zero after the operation.
PF, SF and ZF are updated.
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Bit Manipulation
Instructions
OR Des, Src:
It performs OR operation of Des and Src.
Src can be immediate number, register or
memory location.
Des can be register or memory location.
Both operands cannot be memory locations at
the same time.
CF and OF become zero after the operation.
PF, SF and ZF are updated.
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Bit Manipulation
Instructions
XOR Des, Src:
It performs XOR operation of Des and Src.
Src can be immediate number, register or
memory location.
Des can be register or memory location.
Both operands cannot be memory locations at
the same time.
CF and OF become zero after the operation.
PF, SF and ZF are updated.
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Bit Manipulation
Instructions
SHL Des, Count:
It shift bits of byte or word left, by count.
It puts zero(s) in LSBs.
MSB is shifted into carry flag.
If the number of bits desired to be shifted is 1,
then the immediate number 1 can be written in
Count.
However, if the number of bits to be shifted is
more than 1, then the count is put in CL register.
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Bit Manipulation
Instructions
SHR Des, Count:
It shift bits of byte or word right, by count.
It puts zero(s) in MSBs.
LSB is shifted into carry flag.
If the number of bits desired to be shifted is 1,
then the immediate number 1 can be written in
Count.
However, if the number of bits to be shifted is
more than 1, then the count is put in CL register.
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Bit Manipulation
Instructions
ROL Des, Count:
It rotates bits of byte or word left, by count.
MSB is transferred to LSB and also to CF.
If the number of bits desired to be shifted is 1,
then the immediate number 1 can be written in
Count.
However, if the number of bits to be shifted is
more than 1, then the count is put in CL
register.
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Bit Manipulation
Instructions
ROR Des, Count:
It rotates bits of byte or word right, by count.
LSB is transferred to MSB and also to CF.
If the number of bits desired to be shifted is 1,
then the immediate number 1 can be written in
Count.
However, if the number of bits to be shifted is
more than 1, then the count is put in CL
register.
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Program Execution Transfer
Instructions
These instructions cause change in the
sequence of the execution of instruction.
This change can be through a condition or
sometimes unconditional.
The conditions are represented by flags.
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Program Execution Transfer
Instructions
CALL Des:
This instruction is used to call a subroutine or
function or procedure.
The address of next instruction after CALL is
saved onto stack.
RET:
It returns the control from procedure to calling
program.
Every CALL instruction should have a RET.
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Program Execution Transfer
Instructions
JMP Des:
This instruction is used for unconditional jump
from one place to another.
Jxx Des (Conditional Jump):
All the conditional jumps follow some
conditional statements or any instruction that
affects the flag.
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Conditional Jump Table
Mnemon
ic
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Meaning
Jump
Condition
JA
Jump if Above
CF = 0 and ZF
=0
JAE
Jump if Above or Equal
CF = 0
JB
Jump if Below
CF = 1
JBE
Jump if Below or Equal
CF = 1 or ZF =
1
JC
Jump if Carry
CF = 1
JE
Jump if Equal
ZF = 1
JNC
Jump if Not Carry
CF = 0
JNE
Jump if Not Equal
ZF = 0
JNZ
Jump if Not Zero
ZF = 0
JPE
Jump if Parity Even
PF = 1
JPO
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Jump if Parity Odd
PF = 0
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Program Execution Transfer
Instructions
Loop Des:
This is a looping instruction.
The number of times looping is required is
placed in the CX register.
With each iteration, the contents of CX are
decremented.
ZF is checked whether to loop again or not.
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String Instructions
String in assembly language is just a
sequentially stored bytes or words.
There are very strong set of string instructions
in 8086.
By using these string instructions, the size of
the program is considerably reduced.
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String Instructions
CMPS Des, Src:
It compares the string bytes or words.
SCAS String:
It scans a string.
It compares the String with byte in AL or with
word in AX.
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String Instructions
MOVS / MOVSB / MOVSW:
It causes moving of byte or word from one
string to another.
In this instruction, the source string is in Data
Segment and destination string is in Extra
Segment.
SI and DI store the offset values for source and
destination index.
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String Instructions
REP (Repeat):
This is an instruction prefix.
It causes the repetition of the instruction until
CX becomes zero.
E.g.: REP MOVSB STR1, STR2
It copies byte by byte contents.
REP repeats the operation MOVSB until CX becomes
zero.
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Processor Control
Instructions
These instructions control the processor itself.
8086 allows to control certain control flags
that:
causes the processing in a certain direction
processor synchronization if more than one
microprocessor attached.
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Processor Control
Instructions
STC:
It sets the carry flag to 1.
CLC:
It clears the carry flag to 0.
CMC:
It complements the carry flag.
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Processor Control
Instructions
STD:
It sets the direction flag to 1.
If it is set, string bytes are accessed from higher
memory address to lower memory address.
CLD:
It clears the direction flag to 0.
If it is reset, the string bytes are accessed from
lower memory address to higher memory address.
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