Intel® Architecture Instruction Set Extensions Programming Reference

ADDITIONAL NEW INSTRUCTIONS
STAC—Set AC Flag in EFLAGS Register
Opcode/
Instruction
0F 01 CB
STAC
Description
Instruction Operand Encoding
STAC sets the AC flag bit in EFLAGS/RFLAGS without affecting other bits. Attempt to execute STAC when CPL > 0
will cause #UD.
This instruction's operation is the same in non-64-bit modes and 64-bit mode.
Operation
EFLAGS.AC ← 1;
Flags Affected
AC set, All other flags are unchanged
C/C++ Compiler Intrinsic Equivalent
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
Op/
En
64/32 bit
Mode
Support
CPUID
Feature
Flag
Description
A V/V SMAP Set only the AC flag in the EFLAGS register.
Op/En Operand 1 Operand 2 Operand 3 Operand 4
A NA NA NA NA
#UD If the LOCK prefix is used.
If the CPL > 0.
If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0
9-14 Ref. # 319433-014