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 `>=`)