Intel IA-32 Assembly Tutorial - A Guide to the Basics of x86 Assembly - Page 05

Addition and Subtraction Instructions

There are five integer addition/subtraction instructions: INC DEC ADD SUB NEG.

INC increments by one and DEC decrements by one.

	.data
	var1 DWORD 1
	.code
	xor eax,eax     ;set eax to zero
	inc eax         ;eax equals one
	dec eax         ;eax equals zero again
	dec var1        ;var1 equals zero

ADD adds two operands. The first operand is the destination and the second operand is the source. One operand can be a memory variable or both can be registers. ADC is "add with carry" and will carry from a previous ADD or ADC instruction if the Carry Flag (CF) is set.

	.data
	var1 DWORD 9
	.code
	mov eax,1       ;set eax to one
	add eax,var1    ;eax equals ten

SUB subtracts two operands. The first operand is the destination and the second operand is the source. One operand can be a memory variable or both can be registers. SBB is "subtract with borrow" and will borrow from a previous SUB or SBB instruction if the Carry Flag (CF) is set.

	.data
	var1 DWORD 9
	.code
	mov eax,19       ;set eax to 19
	sub eax,var1     ;eax equals ten

NEG reverses the sign of a number in a registry or memory location.

The zero flag (ZF) is set when the destination operand of an arithmetic instruction is assigned a value of zero.

	mov eax,1
	dec eax          ;eax = 0, ZF = 1
	mov eax,FFFFFFFFh
	inc eax          ;eax = 0, ZF = 1
	inc eax          ;eax = 1, ZF = 0

The sign flag (SF) is set when the result of an arithmetic instruction is negative.

	mov eax,0
	sub eax,1        ;eax = -1, SF = 1
	add eax,2        ;eax = 1, SF = 0

Significant only when the CPU performs an unsigned arithmetic operation, the carry flag (CF) is set if the result of an unsigned addition operation is too large or too small for the destination operand.

	mov al,FFh
	add al,1         ;AL = 0, CF = 1
	;   1 1 1 1 1 1 1 1 <-- carry

	;   1 1 1 1 1 1 1 1 <-- AL (FFh)
	;+  0 0 0 0 0 0 0 1
	;-- ------- -------
	; 1 0 0 0 0 0 0 0 0

The same applies to subtracting a larger unsigned value from a smaller one; the carry flag (CF) is set and the operand is invalid.

	mov al,1
	sub al,2         ;AL = invalid, CF = 1

INC and DEC do not affect the carry flag.

Performing signed arithmetic is the only time that the overflow flag (OF) is relevent. For example, a signed byte has a range of of -128 to +127.

	mov al,+127
	add al,1         ;OF = 1
	mov al,-128
	sub al,1         ;OF = 1

NEG produces an invalid result if, for example, moving -128 into AL, executing NEG, +128 can not be stored in AL and the overflow flag (OF) is set.

	mov al,-128
	neg al           ;OF = 1

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