STAC

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.07H.00H:EBX.SMAP[20] = 0.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used. If CPUID.07H.00H:EBX.SMAP[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.07H.00H:EBX.SMAP[20] = 0.

64-Bit Mode Exceptions

#UD If the LOCK prefix is used. If the CPL > 0. If CPUID.07H.00H:EBX.SMAP[20] = 0.

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Source: Intel® 64 and IA-32 Architectures Software Developer's Manual, Combined Volumes (Order Number 325462-091US, March 2026)
Generated: 7-6-2026