Intel IA-32 Assembly Tutorial - A Guide to the Basics of x86 Assembly - Page 10

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Intel IA-32 Assembly Tutorial - A Guide to the Basics of x86 Assembly - Page 10

Language Elements (CPU Flags, More on Instructions: Conditional Jumps).

Setting and Clearing CPU Flags

	and al,0   ;this sets the Zero Flag (ZF=1)
	or  al,1   ;this clears the Zero Flag (ZF=0)

	or  al,80h ;this sets the Sign Flag (10000000b) (SF=1)
	and al,7Fh ;this clears the Sign Flag (01111111b) (SF=0)

	stc        ;this sets the Carry Flag (CF=1)
	clc        ;this clears the Carry Flag (CF=0)

	mov al,7Fh ;AL = 127
	inc al     ;AL = -128 (80h) this sets the Overflow Flag (OF=1)
	or  eax,0  ;this clears the Overflow Flag (OF=0)

Conditional Jumps

One can implement any logic structure, no matter how complex, using a combination of comparisons and jumps. Two steps are involved in executing a conditional statement; first, an operation such as CMP, AND or SUB modify the CPU flags. Seconds, a conditional jump instruction tests the flags and causes a branch to a new address.

	cmp al,0         ;compare AL to zero
	jz  label1       ;jump if zero (ZF=1)
	.
	.
label1:
	and dl,10110000b
	jnz label2       ;jump if NOT zero (ZF=0)
	.
	.
label2:

A conditional jump instruction branches to a destination label when a flag condition is true. If the flag condition is false then the instruction immediately following the conditional jump is executed.

Jumps Based On Flag Values
Mnemonic	Description	Flags
JZ	jump if zero	ZF = 1
JNZ	jump if not zero	ZF = 0
JC	jump if carry	CF = 1
JNC	jump if not carry	CF = 0
JO	jump if overflow	OF = 1
JNO	jump if not overflow	OF = 0
JS	jump if signed	SF = 1
JNS	jump if not signed	SF = 0
JP	jump if parity (even)	PF = 1
JNP	jump if not parity (odd)	PF = 0

Jumps Based On Equality
Mnemonic	Description
JE	jump if equal
JNE	jump if not equal
JCXZ	jump if CX = 0
JECXZ	jump if ECX = 0

Jumps Based On Unsigned Comparisons
Mnemonic	Description
JA	jump if above
JNBE	jump if not below or equal (same as JA)
JAE	jump if above or equal
JNB	jump if not below (same as JAE)
JB	jump if below
JNAE	jump if not above or equal (same as JB)
JBE	jump if below or equal
JNA	jump if not above (same as JBE)

Jumps Based On Signed Comparisons
Mnemonic	Description
JG	jump if greater
JNLE	jump if not less than or equal (same as JG)
JGE	jump if greater than or equal
JNL	jump if not less (same as JGE)
JL	jump if less
JNGE	jump if not greater than or equal (same as JL)
JLE	jump if less than or equal
JNG	jump if not greater (same as JLE)

The LOOPZ (loop if zero) instruction permits a loop to continue while the Zero Flag is set and the unsigned value of ECX is greater than zero. The destination label must be between -128 and +127 bytes from the location of the following instruction. The LOOPE (loop if equal) instruction is equivalent to LOOPZ.

The LOOPNZ (loop if not zero) instruction is the counter part to LOOPZ. The loop continues while the unsigned value of ECX is greater than zero and the Zero Flag is clear. The LOOPNE (loop if not equal) instruction is equivalent to LOOPNZ.

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