STAC

Set AC Flag in EFLAGS Register

stableVMJITAOTinstruction

Encodings

Measured cost

Flags named

• AC Alignment check

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.

Exceptions

Sources

• This instruction in the Intel manual
• Latency and throughput per microarchitecture