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J? Jump on Condition
Flags affected:
O D I T S Z A P C OF: Overflow flag TF: Trap flag AF: Aux carry
F F F F F F F F F DF: Direction flag SF: Sign flag PF: Parity flag
<none> IF: Interrupt flag ZF: Zero flag CF: Carry flag
Legal forms:
Descriptions Jump if flags are
JA/JNBE d (Jump If Above/Jump If Not Below or Equal) CF=0 AND ZF=0
JAE/JNB d (Jump If Above or Equal/Jump If Not Below) CF=0
JB/JNAE d (Jump If Below/Jump If Not Above or Equal) CF=1
JBE/JNA d (Jump If Below or Equal/Jump If Not Above) CF=1 OR ZF=1
JE/JZ d (Jump If Equal/Jump If Zero) ZF=1
JNE/JNZ d (Jump If Not Equal/Jump If Not Zero) ZF=0
JG/JNLE d (Jump If Greater/Jump If Not Less or Equal) ZF=0 OR SF=OF
JGE/JNL d (Jump If Greater or Equal/Jump If Not Less) SF=OF
JL/JNGE d (Jump If Less/Jump If Not Greater or Equal) SFOF
JLE/JNG d (Jump If Less or Equal/Jump If Not Greater) ZF=1 OR SFOF
JC d (Jump If Carry flag set) CF=1
JNC d (Jump If Carry flag Not set) CF=0
JO d (Jump If Overflow flag set) OF=1
JNO d (Jump If Overflow flag Not set) OF=0
JP/JPE d (Jump If PF set/Jump if Parity Even) PF=1
JNP/JPO d (Jump If PF Not set/Jump if Parity Odd) PF=0
JS d (Jump If Sign flag set) SF=1
JNS d (Jump If Sign flag Not set) SF=0
d without NEAR = 8-bit signed displacement; use NEAR before d to specify
segment-wide displacement.
Examples:
JB HalfSplit ;Jumps if CF=1
JLE TooLow ;Jumps if either ZF=1 or SFOF
JG NEAR WayOut ;Jumps if greater to 16-bit displacement
; in real mode or 32-bit displacement in
; 32-bit protected mode.
Notes:
By default all these instructions make a short jump (127 bytes forward or 128 bytes back) if some condition is true, or fall through if the condition is not true. The conditions all involve flags, and the flag conditions in question are given to the right of the mnemonic and its description.The mnemonics incorporating "above" or "below" are for use after unsigned comparisons, whereas the mnemonics incorporating "less" or "greater" are for use after signed comparisons. "Equal" and "Zero" may be used after unsigned or signed comparisons.
NASM allows use of the segmentwide form by inserting the NEAR keyword after the instruction mnemonic. In real mode this allows the use of a 16-bit signed displacement, and in 32-bit protected mode this allows the use of a 32-bit signed displacement. Use of NEAR is only supported with 386 and newer CPUs.
r8 = AL AH BL BH CL CH DL DH r16 = AX BX CX DX BP SP SI DI
sr = CS DS SS ES FS GS r32 = EAX EBX ECX EDX EBP ESP ESI EDI
m8 = 8-bit memory data m16 = 16-bit memory data
m32 = 32-bit memory data i8 = 8-bit immediate data
i16 = 16-bit immediate data i32 = 32-bit immediate data
d8 = 8-bit signed displacement d16 = 16-bit signed displacement
d32 = 32-bit unsigned displacement
