Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

4-136 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
POPF/POPFD/POPFQ—Pop Stack into EFLAGS Register
Description
Pops a doubleword (POPFD) from the top of the stack (if the current operand-size
attribute is 32) and stores the value in the EFLAGS register, or pops a word from the
top of the stack (if the operand-size attribute is 16) and stores it in the lower 16 bits
of the EFLAGS register (that is, the FLAGS register). These instructions reverse the
operation of the PUSHF/PUSHFD instructions.
The POPF (pop flags) and POPFD (pop flags double) mnemonics reference the same
opcode. The POPF instruction is intended for use when the operand-size attribute is
16; the POPFD instruction is intended for use when the operand-size attribute is 32.
Some assemblers may force the operand size to 16 for POPF and to 32 for POPFD.
Others may treat the mnemonics as synonyms (POPF/POPFD) and use the setting of
the operand-size attribute to determine the size of values to pop from the stack.
The effect of POPF/POPFD on the EFLAGS register changes, depending on the mode
of operation. When the processor is operating in protected mode at privilege level 0
(or in real-address mode, the equivalent to privilege level 0), all non-reserved flags
in the EFLAGS register except RF
1
, VIP, VIF, and VM may be modified. VIP, VIF and
VM remain unaffected.
When operating in protected mode with a privilege level greater than 0, but less than
or equal to IOPL, all flags can be modified except the IOPL field and VIP, VIF, and VM.
Here, the IOPL flags are unaffected, the VIP and VIF flags are cleared, and the VM
flag is unaffected. The interrupt flag (IF) is altered only when executing at a level at
least as privileged as the IOPL. If a POPF/POPFD instruction is executed with insuffi-
cient privilege, an exception does not occur but privileged bits do not change.
When operating in virtual-8086 mode, the IOPL must be equal to 3 to use
POPF/POPFD instructions; VM, RF, IOPL, VIP, and VIF are unaffected. If the IOPL is
less than 3, POPF/POPFD causes a general-protection exception (#GP).
In 64-bit mode, use REX.W to pop the top of stack to RFLAGS. The mnemonic
assigned is POPFQ (note that the 32-bit operand is not encodable). POPFQ pops 64
Opcode Instruction
64-Bit
Mode
Compat/
Leg Mode Description
9D POPF Valid Valid Pop top of stack into lower 16 bits
of EFLAGS.
9D POPFD N.E. Valid Pop top of stack into EFLAGS.
REX.W + 9D POPFQ Valid N.E. Pop top of stack and zero-extend
into RFLAGS.
1. RF is always zero after execution of POPF. This is because POPF, like all instructions, clears RF as
it begins to execute.