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	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.