LAHF

LAHF — Load Status Flags into AH Register

Opcode	Instruction	Op/ En	64-Bit Mode	Compat/ Leg Mode	Description
9F	LAHF	ZO	Invalid*	Valid	Load: AH ← EFLAGS(SF:ZF:0:AF:0:PF:1:CF).

*Valid in specific steppings. See Description section.

Instruction Operand Encoding

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
ZO	NA	NA	NA	NA

Description

This instruction executes as described above in compatibility mode and legacy mode. It is valid in 64-bit mode only if CPUID.80000001H:ECX.LAHF-SAHF[bit 0] = 1.

Operation

IF 64-Bit Mode
    THEN
        IF CPUID.80000001H:ECX.LAHF-SAHF[bit 0] = 1;
            THEN AH ← RFLAGS(SF:ZF:0:AF:0:PF:1:CF);
            ELSE #UD; 
        FI;
    ELSE
        AH ← EFLAGS(SF:ZF:0:AF:0:PF:1:CF);
FI;

Flags Affected

None. The state of the flags in the EFLAGS register is not affected.

Protected Mode Exceptions

#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#UD If CPUID.80000001H:ECX.LAHF-SAHF[bit 0] = 0. If the LOCK prefix is used.

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Source: Intel® Architecture Software Developer's Manual (May 2018)
Generated: 5-6-2018