STAC - Set AC Flag in EFLAGS Register

Opcode/ Instruction

Op / En

64/32 bit Mode Support

CPUID Feature Flag

Description

NP 0F 01 CB

STAC

ZO

V/V

SMAP

Set the AC flag in the EFLAGS register.

Instruction Operand Encoding

Op/En

Operand 1

Operand 2

Operand 3

Operand 4

ZO

N/A

N/A

N/A

N/A

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.