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Instruction Set of 8086

Instructions are classified on the basis of functions they perform. They are categorized into the following main types:

Data Transfer instruction

All the instructions which perform data movement come under this category. The source data may be a register, memory location, port etc. the destination may be a register, memory location or port. The following instructions come under this category:

Instruction Description
MOV Moves data from register to register, register to memory, memory to register, memory to accumulator, accumulator to memory, etc.
LDS Loads a word from the specified memory locations into specified register. It also loads a word from the next two memory locations into DS register.
LES Loads a word from the specified memory locations into the specified register. It also loads a word from next two memory locations into ES register.
LEA Loads offset address into the specified register.
LAHF Loads low order 8-bits of the flag register into AH register.
SAHF Stores the content of AH register into low order bits of the flags register.
XLAT/XLATB Reads a byte from the lookup table.
XCHG Exchanges the contents of the 16-bit or 8-bit specified register with the contents of AX register, specified register or memory locations.
PUSH Pushes (sends, writes or moves) the content of a specified register or memory location(s) onto the top of the stack.
POP Pops (reads) two bytes from the top of the stack and keeps them in a specified register, or memory location(s).
POPF Pops (reads) two bytes from the top of the stack and keeps them in the flag register.
IN Transfers data from a port to the accumulator or AX, DX or AL register.
OUT Transfers data from accumulator or AL or AX register to an I/O port identified by the second byte of the instruction.

Arithmetic Instructions

Instructions of this group perform addition, subtraction, multiplication, division, increment, decrement, comparison, ASCII and decimal adjustment etc.

The following instructions come under this category:

Instruction Description
ADD Adds data to the accumulator i.e. AL or AX register or memory locations.
ADC Adds specified operands and the carry status (i.e. carry of the previous stage).
SUB Subtract immediate data from accumulator, memory or register.
SBB Subtract immediate data with borrow from accumulator, memory or register.
MUL Unsigned 8-bit or 16-bit multiplication.
IMUL Signed 8-bit or 16-bit multiplication.
DIV Unsigned 8-bit or 16-bit division.
IDIV Signed 8-bit or 16-bit division.
INC Increment Register or memory by 1.
DEC Decrement register or memory by 1.
DAA Decimal Adjust after BCD Addition: When two BCD numbers are added, the DAA is used after ADD or ADC instruction to get correct answer in BCD.
DAS Decimal Adjust after BCD Subtraction: When two BCD numbers are added, the DAS is used after SUB or SBB instruction to get correct answer in BCD.
AAA ASCII Adjust for Addition: When ASCII codes of two decimal digits are added, the AAA is used after addition to get correct answer in unpacked BCD.
AAD Adjust AX Register for Division: It converts two unpacked BCD digits in AX to the equivalent binary number. This adjustment is done before dividing two unpacked BCD digits in AX by an unpacked BCD byte.
AAM Adjust result of BCD Multiplication: This instruction is used after the multiplication of two unpacked BCD.
AAS ASCII Adjust for Subtraction: This instruction is used to get the correct result in unpacked BCD after the subtraction of the ASCII code of a number from ASCII code another number.
CBW Convert signed Byte to signed Word.
CWD Convert signed Word to signed Doubleword.
NEG Obtains 2's complement (i.e. negative) of the content of an 8-bit or 16-bit specified register or memory location(s).
CMP Compare Immediate data, register or memory with accumulator, register or memory location(s).

Logical Instructions

Instruction of this group perform logical AND, OR, XOR, NOT and TEST operations. The following instructions come under this category:

Instruction Description
AND Performs bit by bit logical AND operation of two operands and places the result in the specified destination.
OR Performs bit by bit logical OR operation of two operands and places the result in the specified destination.
XOR Performs bit by bit logical XOR operation of two operands and places the result in the specified destination.
NOT Takes one's complement of the content of a specified register or memory location(s).
TEST Perform logical AND operation of a specified operand with another specified operand.

Rotate Instructions

The following instructions come under this category:

Instruction Description
RCL Rotate all bits of the operand left by specified number of bits through carry flag.
RCR Rotate all bits of the operand right by specified number of bits through carry flag.
ROL Rotate all bits of the operand left by specified number of bits.
ROR Rotate all bits of the operand right by specified number of bits.

Shift Instructions

The following instructions come under this category:

Instruction Description
SAL or SHL Shifts each bit of operand left by specified number of bits and put zero in LSB position.
SAR Shift each bit of any operand right by specified number of bits. Copy old MSB into new MSB.
SHR Shift each bit of operand right by specified number of bits and put zero in MSB position.

Branch Instructions

It is also called program execution transfer instruction. Instructions of this group transfer program execution from the normal sequence of instructions to the specified destination or target. The following instructions come under this category:

Instruction Description
JA or JNBE Jump if above, not below, or equal i.e. when CF and ZF = 0
JAE/JNB/JNC Jump if above, not below, equal or no carry i.e. when CF = 0
JB/JNAE/JC Jump if below, not above, equal or carry i.e. when CF = 0
JBE/JNA Jump if below, not above, or equal i.e. when CF and ZF = 1
JCXZ Jump if CX register = 0
JE/JZ Jump if zero or equal i.e. when ZF = 1
JG/JNLE Jump if greater, not less or equal i.e. when ZF = 0 and CF = OF
JGE/JNL Jump if greater, not less or equal i.e. when SF = OF
JL/JNGE Jump if less, not greater than or equal i.e. when SF ≠ OF
JLE/JNG Jump if less, equal or not greater i.e. when ZF = 1 and SF ≠ OF
JMP Causes the program execution to jump unconditionally to the memory address or label given in the instruction.
CALL Calls a procedure whose address is given in the instruction and saves their return address to the stack.
RET Returns program execution from a procedure (subroutine) to the next instruction or main program.
IRET Returns program execution from an interrupt service procedure (subroutine) to the main program.
INT Used to generate software interrupt at the desired point in a program.
INTO Software interrupts to indicate overflow after arithmetic operation.
LOOP Jump to defined label until CX = 0.
LOOPZ/LOOPE Decrement CX register and jump if CX ≠ 0 and ZF = 1.
LOOPNZ/LOOPNE Decrement CX register and jump if CX ≠ 0 and ZF = 0.

Here, CF = Carry Flag
ZF = Zero Flag
OF = Overflow Flag
SF = Sign Flag
CX = Register

Flag Manipulation and Processor Control Instructions

Instructions of this instruction set are related to flag manipulation and machine control. The following instructions come under this category:

Instruction Description
CLC Clear Carry Flag: This instruction resets the carry flag CF to 0.
CLD Clear Direction Flag: This instruction resets the direction flag DF to 0.
CLI Clear Interrupt Flag: This instruction resets the interrupt flag IF to 0.
CMC This instruction take complement of carry flag CF.
STC Set carry flag CF to 1.
STD Set direction flag to 1.
STI Set interrupt flag IF to 1.
HLT Halt processing. It stops program execution.
NOP Performs no operation.
ESC Escape: makes bus free for external master like a coprocessor or peripheral device.
WAIT When WAIT instruction is executed, the processor enters an idle state in which the processor does no processing.
LOCK It is a prefix instruction. It makes the LOCK pin low till the execution of the next instruction.

String Instructions

String is series of bytes or series of words stored in sequential memory locations. The 8086 provides some instructions which handle string operations such as string movement, comparison, scan, load and store.

The following instructions come under this category:

Instruction Description
MOVS/MOVSB/MOVSW Moves 8-bit or 16-bit data from the memory location(s) addressed by SI register to the memory location addressed by DI register.
CMPS/CMPSB/CMPSW Compares the content of memory location addressed by DI register with the content of memory location addressed by SI register.
SCAS/SCASB/SCASW Compares the content of accumulator with the content of memory location addressed by DI register in the extra segment ES.
LODS/LODSB/LODSW Loads 8-bit or 16-bit data from memory location addressed by SI register into AL or AX register.
STOS/STOSB/STOSW Stores 8-bit or 16-bit data from AL or AX register in the memory location addressed by DI register.
REP Repeats the given instruction until CX ≠ 0
REPE/ REPZ Repeats the given instruction till CX ≠ 0 and ZF = 1
REPNE/REPNZ Repeats the given instruction till CX ≠ 0 and ZF = 0

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