SAHF (Store AH into Flags)

Opcodes

Opcode* Instruction Op/En 64-Bit Mode Compat/Leg Mode Description
9E SAHF NP Invalid* Valid Loads SF, ZF, AF, PF, and CF from AH into EFLAGS register.

NOTES: * Valid in specific steppings. See Description section.

Instruction Operand Encoding

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

Description

Loads the SF, ZF, AF, PF, and CF flags of the EFLAGS register with values from the corresponding bits in the AH register (bits 7, 6, 4, 2, and 0, respectively). Bits 1, 3, and 5 of register AH are ignored; the corresponding reserved bits (1, 3, and 5) in the EFLAGS register remain as shown in the “Operation” section below.

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

Flags Affected

The SF, ZF, AF, PF, and CF flags are loaded with values from the AH register. Bits 1, 3, and 5 of the EFLAGS register are unaffected, with the values remaining 1, 0, and 0, respectively.

Protected Mode Exceptions

None.

Real-Address Mode Exceptions

None.

Virtual-8086 Mode Exceptions

None.

Compatibility Mode Exceptions

None.

64-Bit Mode Exceptions

#UD If CPUID.80000001H.ECX[0] = 0. If the LOCK prefix is used.