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 

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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 7 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

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AAA : ASCII Adjust After Addition 

The AAA instruction is executed aftr 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 

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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)