Instruction set and assembler directives of 8086 Microprocessor

INSTRUCTION SET OF 8086

The 8086 instructions are categorized into the following main types.

1.     Data Copy / Transfer Instructions

2.     Arithmetic and Logical Instructions

3.     Shift and Rotate Instructions

4.     Loop Instructions

5.     Branch Instructions

6.     String Instructions

7.     Flag Manipulation Instructions

8.     Machine Control Instructions

1 Data Copy / Transfer Instructions:

MOV:

This instruction copies a word or a byte of data from some source to a destination. The destination can be a register or a memory location. The source can be a register, a memory location, or an immediate number.

MOV AX, BX MOV AX, 5000H MOV AX, [SI] MOV AX, [2000H] MOV AX, 50H[BX] MOV [734AH], BX

MOV DS, CX MOV CL, [357AH]

Direct loading of the segment registers with immediate data is not permitted.

PUSH: Push to Stack

This instruction pushes the contents of the specified register/memory location on to the stack. The stack pointer is decremented by 2, after each execution of the instruction.

E.g. PUSH AX

• PUSH DS

• PUSH [5000H]

POP: Pop from Stack

This instruction when executed, loads the specified register/memory location with the

contents of the memory location of which the address is formed using the current stack

segment and stack pointer.

The stack pointer is incremented by 2 Eg. POP AX

POP DS POP [5000H]

XCHG: Exchange byte or word

This instruction exchange the contents of the specified source and destination operands Eg. XCHG [5000H], AX

XCHG BX, AX

XLAT:

Translate byte using look-up table

Eg. LEA BX, TABLE1

MOV AL, 04H

XLAT

Input and output port transfer instructions:

IN:

Copy a byte or word from specified port to accumulator.

Eg. IN AL,03H

IN AX,DX

OUT:

Copy a byte or word from accumulator specified port.

Eg. OUT 03H, AL

OUT DX, AX

LEA:

Load effective address of operand in specified register. [reg] offset portion of address in DS

Eg. LEA reg, offset

LDS:

Load DS register and other specified register from memory. [reg] [mem]

[DS] [mem + 2] Eg. LDS reg, mem

LES:

Load ES register and other specified register from memory. [reg] [mem]

[ES] [mem + 2] Eg. LES reg, mem

Flag transfer instructions:

LAHF:

Load (copy to) AH with the low byte the flag register. [AH] [ Flags low byte]

Eg. LAHF

SAHF:

Store (copy) AH register to low byte of flag register. [Flags low byte] [AH]

Eg. SAHF

PUSHF:

Copy flag register to top of stack. [SP] [SP] – 2

[[SP]] [Flags] Eg. PUSHF

POPF:

Copy word at top of stack to flag register. [Flags] [[SP]]

[SP] [SP] + 2

2 Arithmetic Instructions:

The 8086 provides many arithmetic operations: addition, subtraction, negation, multiplication and comparing two values.

ADD:

The add instruction adds the contents of the source operand to the destination operand. Eg. ADD AX, 0100H

ADD AX, BX

ADD AX, [SI] ADD AX, [5000H]

ADD [5000H], 0100H ADD 0100H

ADC: Add with Carry

This instruction performs the same operation as ADD instruction, but adds the carry flag to the result.

Eg. ADC 0100H

ADC AX, BX

ADC AX, [SI] ADC AX, [5000] ADC [5000], 0100H

SUB: Subtract

The subtract instruction subtracts the source operand from the destination operand and the result is left in the destination operand.

Eg. SUB AX, 0100H

SUB AX, BX SUB AX, [5000H]

SUB [5000H], 0100H

SBB: Subtract with Borrow

The subtract with borrow instruction subtracts the source operand and the borrow flag (CF) which may reflect the result of the previous calculations, from the destination operand

Eg. SBB AX, 0100H

SBB AX, BX SBB AX, [5000H]

SBB [5000H], 0100H

INC: Increment

This instruction increases the contents of the specified Register or memory location by 1. Immediate data cannot be operand of this instruction.

Eg. INC AX INC [BX] INC [5000H]

DEC: Decrement

The decrement instruction subtracts 1 from the contents of the specified register or memory location.

Eg. DEC AX

DEC [5000H]

NEG: Negate

The negate instruction forms 2’s complement of the specified destination in the instruction. The destination can be a register or a memory location. This instruction can be implemented by inverting each bit and adding 1 to it.

Eg. NEG AL

AL = 0011 0101 35H Replace number in AL with its 2’s complement

AL = 1100 1011 = CBH

CMP: Compare

This instruction compares the source operand, which may be a register or an immediate data or a memory location, with a destination operand that may be a register or a memory location

Eg. CMP BX, 0100H CMP AX, 0100H CMP [5000H], 0100H CMP BX, [SI]

CMP BX, CX

MUL:Unsigned Multiplication Byte or Word

This instruction multiplies an unsigned byte or word by the contents of AL. Eg. MUL BH; (AX) (AL) x (BH)

MUL CX; (DX)(AX) (AX) x (CX)

MUL WORD PTR [SI]; (DX)(AX) (AX) x ([SI])

IMUL:Signed Multiplication

This instruction multiplies a signed byte in source operand by a signed byte in AL or a signed word in source operand by a signed word in AX.

Eg. IMUL BH

IMUL CX

IMUL [SI]

CBW: Convert Signed Byte to Word

This instruction copies the sign of a byte in AL to all the bits in AH. AH is then said to be sign extension of AL.

Eg. CBW

AX= 0000 0000 1001 1000 Convert signed byte in AL signed word in AX. Result in AX = 1111 1111 1001 1000

CWD: Convert Signed Word to Double Word

This instruction copies the sign of a byte in AL to all the bits in AH. AH is then said to be sign extension of AL.

Eg. CWD

Convert signed word in AX to signed double word in DX: AX DX= 1111 1111 1111 1111

Result in AX = 1111 0000 1100 0001

DIV: Unsigned division

This instruction is used to divide an unsigned word by a byte or to divide an unsigned double word by a word.

Eg. DIV CL; Word in AX / byte in CL; Quotient in AL, remainder in AH

DIV CX; Double word in DX and AX / word; in CX, and Quotient in AX; remainder in

DX

AAA: ASCII Adjust After Addition

The AAA instruction is executed after an ADD instruction that adds two ASCII coded operand to give a byte of result in AL. The AAA instruction converts the resulting contents of Al to a unpacked decimal digits.

Eg. ADD CL, DL; [CL] = 32H = ASCII for 2; [DL] = 35H = ASCII for 5; Result [CL] = 67H

MOV AL, CL; Move ASCII result into AL since; AAA adjust only [AL] AAA; [AL]=07, unpacked BCD for 7

AAS: ASCII Adjust AL after Subtraction

This instruction corrects the result in AL register after subtracting two unpacked ASCII operands. The result is in unpacked decimal format. The procedure is similar to AAA instruction except for the subtraction of 06 from AL.

AAM: ASCII Adjust after Multiplication

This instruction, after execution, converts the product available In AL into unpacked BCD format.

Eg. MOV AL, 04; AL = 04 MOV BL ,09; BL = 09

MUL BL; AX = AL*BL; AX=24H AAM; AH = 03, AL=06

AAD: ASCII Adjust before Division

This instruction converts two unpacked BCD digits in AH and AL to the equivalent binary number in AL. This adjustment must be made before dividing the two unpacked BCD digits in AX by an unpacked BCD byte. In the instruction sequence, this instruction appears Before DIV instruction.

Eg. AX 05 08

AAD result in AL 00 3A 58D = 3A H in AL

The result of AAD execution will give the hexadecimal number 3A in AL and 00 in AH where 3A is the hexadecimal Equivalent of 58 (decimal).

DAA: Decimal Adjust Accumulator

This instruction is used to convert the result of the addition of two packed BCD numbers to a valid BCD number. The result has to be only in AL.

Eg. AL = 53 CL = 29

ADD AL, CL; AL (AL) + (CL); AL 53 + 29; AL 7C

DAA; AL 7C + 06 (as C>9); AL 82

DAS: Decimal Adjust after Subtraction

This instruction converts the result of the subtraction of two packed BCD numbers to a valid BCD number. The subtraction has to be in AL only.

Eg. AL = 75, BH = 46

SUB AL, BH; AL 2 F = (AL) - (BH) ; AF = 1

DAS; AL 2 9 (as F>9, F - 6 = 9)

Logical instructions

AND: Logical AND

This instruction bit by bit ANDs the source operand that may be an immediate register or a memory location to the destination operand that may a register or a memory location. The result is stored in the destination operand.

Eg. AND AX, 0008H

AND AX, BX

OR: Logical OR

This instruction bit by bit ORs the source operand that may be an immediate, register or a memory location to the destination operand that may a register or a memory location. The result is stored in the destination operand.

Eg. OR AX, 0008H

OR AX, BX

NOT: Logical Invert

This instruction complements the contents of an operand register or a memory location, bit by bit.

Eg. NOT AX

NOT [5000H]

OR: Logical Exclusive OR

This instruction bit by bit XORs the source operand that may be an immediate, register or a memory location to the destination operand that may a register or a memory location. The result is stored in the destination operand.

Eg. XOR AX, 0098H

XOR AX, BX

TEST: Logical Compare Instruction

The TEST instruction performs a bit by bit logical AND operation on the two operands. The result of this ANDing operation is not available for further use, but flags are affected.

Eg. TEST AX, BX

TEST [0500], 06H

3 Shift and Rotate Instructions

SAL/SHL: SAL / SHL destination, count.

SAL and SHL are two mnemonics for the same instruction. This instruction shifts each bit in the specified destination to the left and 0 is stored at LSB position. The MSB is shifted into the carry flag. The destination can be a byte or a word. It can be in a register or in a memory location. The number of shifts is indicated by count.

Eg. SAL CX, 1

SAL AX, CL

SHR: SHR destination, count

This instruction shifts each bit in the specified destination to the right and 0 is stored at MSB position. The LSB is shifted into the carry flag. The destination can be a byte or a word.

It can be a register or in a memory location. The number of shifts is indicated by count. Eg. SHR CX, 1

MOV CL, 05H

SHR AX, CL

SAR: SAR destination, count

This instruction shifts each bit in the specified destination some number of bit positions to the right. As a bit is shifted out of the MSB position, a copy of the old MSB is put in the MSB position. The LSB will be shifted into CF.

Eg. SAR BL, 1

MOV CL, 04H

SAR DX, CL

ROL Instruction: ROL destination, count

This instruction rotates all bits in a specified byte or word to the left some number of bit positions. MSB is placed as a new LSB and a new CF.

Eg. ROL CX, 1

MOV CL, 03H

ROL BL, CL

ROR Instruction: ROR destination, count

This instruction rotates all bits in a specified byte or word to the right some number of bit positions. LSB is placed as a new MSB and a new CF.

Eg. ROR CX, 1

MOV CL, 03H

ROR BL, CL

RCL Instruction: RCL destination, count

This instruction rotates all bits in a specified byte or word some number of bit positions to the left along with the carry flag. MSB is placed as a new carry and previous carry is place as new LSB.

Eg. RCL CX, 1

MOV CL, 04H

RCL AL, CL

RCR Instruction: RCR destination, count

This instruction rotates all bits in a specified byte or word some number of bit positions to the right along with the carry flag. LSB is placed as a new carry and previous carry is place as new MSB.

Eg. RCR CX, 1

MOV CL, 04H

RCR AL, CL

ROR Instruction: ROR destination, count

This instruction rotates all bits in a specified byte or word to the right some number of bit positions. LSB is placed as a new MSB and a new CF.

Eg. ROR CX, 1

MOV CL, 03H

ROR BL, CL

RCL Instruction: RCL destination, count

This instruction rotates all bits in a specified byte or word some number of bit positions to the left along with the carry flag. MSB is placed as a new carry and previous carry is place as new LSB.

Eg. RCL CX, 1

MOV CL, 04H

RCL AL, CL

RCR Instruction: RCR destination, count

This instruction rotates all bits in a specified byte or word some number of bit positions to the right along with the carry flag. LSB is placed as a new carry and previous carry is place as new MSB.

Eg. RCR CX, 1

MOV CL, 04H

RCR AL, CL

4 Loop Instructions:

Unconditional LOOP Instructions

LOOP: LOOP Unconditionally

This instruction executes the part of the program from the Label or address specified in the instruction upto the LOOP instruction CX number of times. At each iteration, CX is decremented automatically and JUMP IF NOT ZERO structure.

Example: MOV CX, 0004H

Conditional LOOP Instructions

LOOPZ / LOOPE Label

Loop through a sequence of instructions from label while ZF=1 and CX=0.

LOOPNZ / LOOPENE Label

Loop through a sequence of instructions from label while ZF=1 and CX=0.

5 Branch Instructions:

Branch Instructions transfers the flow of execution of the program to a new address specified in the instruction directly or indirectly. When this type of instruction is executed, the CS and IP registers get loaded with new values of CS and IP corresponding to the location to be transferred.

The Branch Instructions are classified into two types

1.     Unconditional Branch Instructions.

2.     Conditional Branch Instructions.

1.4.5.1 Unconditional Branch Instructions:

In Unconditional control transfer instructions, the execution control is transferred to the specified location independent of any status or condition. The CS and IP are unconditionally modified to the new CS and IP.

CALL: Unconditional Call

This instruction is used to call a Subroutine (Procedure) from a main program. Address of procedure may be specified directly or indirectly. There are two types of procedure depending upon whether it is available in the same segment or in another segment.

i. Near CALL i.e., ±32K displacement.

ii. For CALL i.e., anywhere outside the segment.

On execution this instruction stores the incremented IP & CS onto the stack and loads the CS & IP registers with segment and offset addresses of the procedure to be called.

RET: Return from the Procedure.

At the end of the procedure, the RET instruction must be executed. When it is executed, the previously stored content of IP and CS along with Flags are retrieved into the CS, IP and Flag registers from the stack and execution of the main program continues further.

INT N: Interrupt Type N.

In the interrupt structure of 8086, 256 interrupts are defined corresponding to the types from 00H to FFH. When INT N instruction is executed, the type byte N is multiplied by 4 and the contents of IP and CS of the interrupt service routine will be taken from memory block in 0000 segment.

INTO: Interrupt on Overflow

This instruction is executed, when the overflow flag OF is set. This is equivalent to a Type 4 Interrupt instruction.

JMP: Unconditional Jump

This instruction unconditionally transfers the control of execution to the specified address using an 8-bit or 16-bit displacement. No Flags are affected by this instruction.

IRET: Return from ISR

When it is executed, the values of IP, CS and Flags are retrieved from the stack to continue the execution of the main program.

MOV BX, 7526H

Label 1 MOV AX, CODE

OR BX, AX LOOP Label 1

Conditional Branch Instructions

When this instruction is executed, execution control is transferred to the address specified relatively in the instruction, provided the condition implicit in the Opcode is satisfied. Otherwise execution continues sequentially.

JZ/JE Label

Transfer execution control to address ‘Label’, if ZF=1.

JNZ/JNE Label

Transfer execution control to address ‘Label’, if ZF=0

JS Label

Transfer execution control to address ‘Label’, if SF=1.

JNS Label

Transfer execution control to address ‘Label’, if SF=0.

JO Label

Transfer execution control to address ‘Label’, if OF=1.

14

JNO Label

Transfer execution control to address ‘Label’, if OF=0.

JNP Label

Transfer execution control to address ‘Label’, if PF=0.

JP Label

Transfer execution control to address ‘Label’, if PF=1.

JB Label

Transfer execution control to address ‘Label’, if CF=1.

JNB Label

Transfer execution control to address ‘Label’, if CF=0.

JCXZ Label

Transfer execution control to address ‘Label’, if CX=0

6 String Manipulation Instructions

A series of data byte or word available in memory at consecutive locations, to be referred as Byte String or Word String. A String of characters may be located in consecutive memory locations, where each character may be represented by its ASCII equivalent. The 8086 supports a set of more powerful instructions for string manipulations for referring to a string, two parameters are required.

I. Starting and End Address of the String.

II. Length of the String.

The length of the string is usually stored as count in the CX register. The incrementing or decrementing of the pointer, in string instructions, depends upon the Direction Flag (DF) Status. If it is a Byte string operation, the index registers are updated by one. On the other hand, if it is a word string operation, the index registers are updated by two.

REP: Repeat Instruction Prefix

This instruction is used as a prefix to other instructions, the instruction to which the REP prefix is provided, is executed repeatedly until the CX register becomes zero (at each iteration CX is automatically decremented by one).

i. REPE / REPZ - repeat operation while equal / zero.

ii. REPNE / REPNZ - repeat operation while not equal / not zero. These are used for CMPS, SCAS instructions only, as instruction prefixes.

MOVSB / MOVSW: Move String Byte or String Word

Suppose a string of bytes stored in a set of consecutive memory locations is to be moved to another set of destination locations. The starting byte of source string is located in the memory location whose address may be computed using SI (Source Index) and DS (Data Segment) contents. The starting address of the destination locations where this string has to be relocated is given by DI (Destination Index) and ES (Extra Segment) contents.

CMPS: Compare String Byte or String Word

The CMPS instruction can be used to compare two strings of byte or words. The length of the string must be stored in the register CX. If both the byte or word strings are equal, zero Flag is set.

The REP instruction Prefix is used to repeat the operation till CX (counter) becomes zero or the condition specified by the REP Prefix is False.

SCAN: Scan String Byte or String Word

This instruction scans a string of bytes or words for an operand byte or word specified in the register AL or AX. The String is pointed to by ES: DI register pair. The length of the string s stored in CX. The DF controls the mode for scanning of the string. Whenever a match to the specified operand is found in the string, execution stops and the zero Flag is set. If no match is found, the zero flag is reset.

LODS: Load String Byte or String Word

The LODS instruction loads the AL / AX register by the content of a string pointed to by DS: SI register pair. The SI is modified automatically depending upon DF, If it is a byte transfer (LODSB), the SI is modified by one and if it is a word transfer (LODSW), the SI is modified by two. No other Flags are affected by this instruction.

STOS: Store String Byte or String Word

The STOS instruction Stores the AL / AX register contents to a location in the string pointer by ES: DI register pair. The DI is modified accordingly, No Flags are affected by this instruction.

The direction Flag controls the String instruction execution, The source index SI and Destination Index DI are modified after each iteration automatically. If DF=1, then the execution follows auto decrement mode, SI and DI are decremented automatically after each iteration. If DF=0, then the execution follows auto increment mode. In this mode, SI and DI are incremented automatically after each iteration.

7 Flag Manipulation and a Processor Control Instructions

These instructions control the functioning of the available hardware inside the processor chip. These instructions are categorized into two types:

1. Flag Manipulation instructions.

2. Machine Control instructions.

Flag Manipulation instructions

The Flag manipulation instructions directly modify some of the Flags of 8086.

i. CLC – Clear Carry Flag.

ii. CMC – Complement Carry Flag.

iii. STC – Set Carry Flag.

iv.CLD – Clear Direction Flag.

v. STD – Set Direction Flag.

vi.CLI – Clear Interrupt Flag.

vii.STI – Set Interrupt Flag.

8 Machine Control instructions

The Machine control instructions control the bus usage and execution

i. WAIT – Wait for Test input pin to go low.

ii. HLT – Halt the process.

iii. NOP – No operation.

iv.ESC – Escape to external device like NDP

v. LOCK – Bus lock instruction prefix.

Assembler directives:

Assembler directives help the assembler to correctly understand the assembly language programs to prepare the codes. Another type of hint which helps the assembler to assign a particular constant with a label or initialize particular memory locations or labels with constants is called an operator. Rather, the operators perform the arithmetic and logical tasks unlike directives that just direct the assembler to correctly interpret the program to code it appropriately. The following directives are commonly used in the assembly language programming practice using Microsoft Macro Assembler

(MASM) or Turbo Assembler (TASM).

ü DB: Define Byte The DB directive is used to reserve byte or bytes of memory locations in the available memory. While preparing the EXE file, this directive directs the assembler to allocate the specified number of memory bytes to the said data type that may be a constant, variable, string, etc. Another option of this directive also initializes the reserved memory bytes with the ASCII codes of the characters specified as a string. The following examples show how the DB directive is used for different purposes.

Example:

LIST DB 0lH, 02H, 03H, 04H

This statement directs the assembler to reserve four memory locations for a list named LIST and initialize them with the above specified four values.