Condition Operations

Overview

A number of instructions operate with respect to the condition code registers.

The conventional way to implement conditional operationsis through a conditional transfer of control, where the program follows one execution path when a condition holds and another when it does not.

An alternate strategy is through a conditional transfer of data. This approach computes both outcomes of a conditional operation and then selects one based on whether or not the condition holds.

Instructions

CMP and TEST

Instruction	Operands	Based On	Description
cmp[bwlq]	$S_1,S_2$	$S_2-S_1$	Compare
test[bwlq]	$S_1,S_2$	$S_1\&S_2$	Test

SET

The description of the SET commands apply to the case of a comparison instruction. That is, the condition codes are set according to computation t = a - b, where t, a, and b may be interpreted as signed or unsigned depending on the SET instruction invoked.

Instruction	Synonym	Evaluation	Description
sete	setz	ZF	Equal / zero
setne	setnz	~ZF	Not equal / not zero
sets		SF	Negative
setns		~SF	Nonnegative
setl	setnge	SF ^ OF	Less (signed <)
setle	setng	(SF ^ OF) | ZF	Less or equal (signed <=)
setg	setnle	~(SF ^ OF) & ~ZF	Greater (signed >)
setge	setnl	~(SF ^ OF)	Greater or equal (signed >=)
setb	setnae	CF	Below (unsigned <)
setbe	setna	CF | ZF	Below or equal (unsigned <=)
seta	setnbe	~CF & ~ZF	Above (unsigned >)
setae	setnb	~CF	Above or equal (unsigned >=)

Note how the other condition code evaluations are easy to derive from setl and setb.

JMP

Jump instructions are categorized as either direct or indirect. Direct jump instructions specify a label whereas indirect jump instructions specify a * followed by an memory operand.

Instruction	Synonym	Jump Condition	Description
jmp Label	-	1	Direct jump
jmp *Operand	-	1	Indirect jump
je Label	jz	ZF	Equal / zero
jne Label	jnz	~ZF	Not equal / not zero
js Label	-	SF	Negative
jns Label	-	~SF	Nonnegative
jl Label	jnge	SF ^ OF	Less (signed <)
jle Label	jng	(SF ^ OF) | ZF	Less or equal (signed <=)
jg Label	jnle	~(SF ^ OF) & ~ZF	Greater (signed >)
jge Label	jnl	~(SF ^ OF)	Greater or equal (signed >=)
jb Label	jnae	CF	Below (unsigned <)
jbe Label	jna	CF | ZF	Below or equal (unsigned <=)
ja Label	jnbe	~CF & ~ZF	Above (unsigned >)
jae Label	jnb	~CF	Above or equal (unsigned >=)

CMOV

Like MOV instructions, but with the data transfer only happening if the move condition is satisfied.

Instruction	Synonym	Move Condition	Description
cmove	cmovz	ZF	Equal / zero
cmovene	cmovnz	~ZF	Not equal / not zero
cmovs	-	SF	Negative
cmovns	-	~SF	Nonnegative
cmovl	cmovnge	SF ^ OF	Less (signed <)
cmovle	cmovng	(SF ^ OF) | ZF	Less or equal (signed <=)
cmovg	cmovenle	~(SF ^ OF) & ~ZF	Greater (signed >)
cmovge	cmovnl	~(SF ^ OF)	Greater or equal (signed >=)
cmovb	cmovnae	CF	Below (unsigned <)
cmovbe	cmovna	CF | ZF	Below or equal (unsigned <=)
cmova	cmovnbe	~CF & ~ZF	Above (unsigned >)
cmovae	cmovnb	~CF	Above or equal (Unsigned >=)

Bibliography

• Bryant, Randal E., and David O’Hallaron. Computer Systems: A Programmer’s Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.