clac

Clear AC Flag in EFLAGS

CLAC

Clears Alignment Check flag (SMAP prevention).

Pseudocode Operation

EFLAGS.AC ← 0;

Encoding

Binary Layout

Format Legacy

Opcode NP 0F 01 CA

Extension SMAP

Operands

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
NP 0F 01 CA	CLAC	ZO	V/V	SMAP	Clear the AC flag in the EFLAGS register.

Description

Clears the AC flag bit in EFLAGS register. This disables any alignment checking of user-mode data accesses. If the SMAP bit is set in the CR4 register, this disallows explicit supervisor-mode data accesses to user-mode pages. This instruction's operation is the same in non-64-bit modes and 64-bit mode. Attempts to execute CLAC when CPL > 0 cause #UD.

Operation

EFLAGS.AC := 0;

Flags Affected

AC cleared. Other flags are unaffected.

Exceptions

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 CLAC instruction is not recognized in virtual-8086 mode.

Compatibility Mode Exceptions