Condition Operations

Overview

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

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.

Bibliography

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