STAC (Set AC Flag in EFLAGS Register)

Opcodes

Opcode Instruction Op/En 64-bit Mode Compat/Leg Mode Description
0F 01 CB STAC NP Valid Valid Set the AC flag in the EFLAGS register.

Instruction Operand Encoding

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

Description

Sets the AC flag bit in EFLAGS register. This may enable alignment checking of user-mode data accesses. This allows explicit supervisor-mode data accesses to user-mode pages even if the SMAP bit is set in the CR4 register.

This instruction's operation is the same in non-64-bit modes and 64-bit mode. Attempts to execute STAC when CPL 0 cause #UD.

Operation

EFLAGS.AC ← 1;

Flags Affected

AC set. Other flags are unaffected.

Protected Mode Exceptions

#UD If the LOCK prefix is used. If the CPL 0. If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used. If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.

Virtual-8086 Mode Exceptions

#UD The STAC instruction is not recognized in virtual-8086 mode.

Compatibility Mode Exceptions

#UD If the LOCK prefix is used. If the CPL 0. If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.

64-Bit Mode Exceptions

#UD If the LOCK prefix is used. If the CPL 0. If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.