POPF/POPFD/POPFQ - Pop Stack Into EFLAGS Register

Opcode	Instruction	Op/ En	64-Bit Mode	Compat/ Leg Mode	Description
9D	POPF	ZO	Valid	Valid	Pop top of stack into lower 16 bits of EFLAGS.
9D	POPFD	ZO	N.E.	Valid	Pop top of stack into EFLAGS.
9D	POPFQ	ZO	Valid	N.E.	Pop top of stack and zero-extend into RFLAGS.

Instruction Operand Encoding

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
ZO	N/A	N/A	N/A	N/A

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/PUSHFQ 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. See Table 4-16 and the key below for details.

When operating in protected, compatibility, or 64-bit mode at privilege level 0 (or in real-address mode, the equiv-

alent to privilege level 0), all non-reserved flags in the EFLAGS register except RF¹, VIP, VIF, and VM may be modi- fied. VIP, VIF, and VM remain unaffected.

When operating in protected, compatibility, or 64-bit 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 RF, IF, VIP, VIF, and VM; these remain unaffected. The AC and ID flags can only be modified if the operand-size attribute is 32. 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 (EFLAGS.VM = 1) without the virtual-8086 mode extensions (CR4.VME = 0), the POPF/POPFD instructions can be used only if IOPL = 3; otherwise, a general-protection exception (#GP) occurs. If the virtual-8086 mode extensions are enabled (CR4.VME = 1), POPF (but not POPFD) can be executed in virtual-8086 mode with IOPL < 3.

(The protected-mode virtual-interrupt feature - enabled by setting CR4.PVI - affects the CLI and STI instructions in the same manner as the virtual-8086 mode extensions. POPF, however, is not affected by CR4.PVI.)

In 64-bit mode, the mnemonic assigned is POPFQ (note that the 32-bit operand is not encodable). POPFQ pops 64 bits from the stack. Reserved bits of RFLAGS (including the upper 32 bits of RFLAGS) are not affected.

See Chapter 3 of the Intel^® 64 and IA-32 Architectures Software Developer's Manual, Volume 1, for more informa- tion about the EFLAGS registers.

RF is always zero after the execution of POPF. This is because POPF, like all instructions, clears RF as it begins to execute.

Table 4-16. Effect of POPF/POPFD on the EFLAGS Register

NOTES:

1. #GP fault - no flag update

2. #GP fault with no flag update if VIP=1 in EFLAGS register and IF=1 in FLAGS value on stack

3. #GP fault with no flag update if TF=1 in FLAGS value on stack

	Key
S	Updated from stack
SV	Updated from IF (bit 9) in FLAGS value on stack
N	No change in value
X	No EFLAGS update
0	Value is cleared

Operation

IF EFLAGS.VM = 0 (* Not in Virtual-8086 Mode *)
   THEN IF CPL = 0 OR CR0.PE = 0
       THEN
            IF OperandSize = 32;
                THEN 
                     EFLAGS := Pop(); (* 32-bit pop *)
                     (* All non-reserved flags except RF, VIP, VIF, and VM can be modified; 
                     VIP, VIF, VM, and all reserved bits are unaffected. RF is cleared. *)
                ELSE IF (Operandsize = 64)
                     RFLAGS = Pop(); (* 64-bit pop *)
                     (* All non-reserved flags except RF, VIP, VIF, and VM can be modified; 
                     VIP, VIF, VM, and all reserved bits are unaffected. RF is cleared. *)
                ELSE (* OperandSize = 16 *)
                     EFLAGS[15:0] := Pop(); (* 16-bit pop *)
                     (* All non-reserved flags can be modified. *)
            FI;
       ELSE (* CPL > 0 *)
            IF OperandSize = 32
                THEN 
                     IF CPL > IOPL
                         THEN
                              EFLAGS := Pop(); (* 32-bit pop *)
                              (* All non-reserved bits except IF, IOPL, VIP, VIF, VM, and RF can be modified; 
                              IF, IOPL, VIP, VIF, VM, and all reserved bits are unaffected; RF is cleared. *)
                         ELSE
                              EFLAGS := Pop(); (* 32-bit pop *)
                              (* All non-reserved bits except IOPL, VIP, VIF, VM, and RF can be modified; 
                              IOPL, VIP, VIF, VM, and all reserved bits are unaffected; RF is cleared. *)
                     FI;
                ELSE IF (Operandsize = 64)
                     IF CPL > IOPL
                         THEN
                              RFLAGS := Pop(); (* 64-bit pop *)
                              (* All non-reserved bits except IF, IOPL, VIP, VIF, VM, and RF can be modified; 
                              IF, IOPL, VIP, VIF, VM, and all reserved bits are unaffected; RF is cleared. *)
                         ELSE
                              RFLAGS := Pop(); (* 64-bit pop *)
                              (* All non-reserved bits except IOPL, VIP, VIF, VM, and RF can be modified; 
                              IOPL, VIP, VIF, VM, and all reserved bits are unaffected; RF is cleared. *)
                     FI;
                ELSE (* OperandSize = 16 *)
                     EFLAGS[15:0] := Pop(); (* 16-bit pop *)
                     (* All non-reserved bits except IOPL can be modified; IOPL and all
                     reserved bits are unaffected. *)
            FI;
       FI;
   ELSE (* In virtual-8086 mode *)
       IF IOPL = 3
            THEN
                IF OperandSize = 32 
                     THEN 
                         EFLAGS := Pop();
                         (* All non-reserved bits except IOPL, VIP, VIF, VM, and RF can be modified; 
                         VIP, VIF, VM, IOPL, and all reserved bits are unaffected. RF is cleared. *)
                     ELSE 
                         EFLAGS[15:0] := Pop(); FI;
                         (* All non-reserved bits except IOPL can be modified; IOPL and all reserved bits are unaffected. *)
                FI;
            ELSE (* IOPL < 3 *)
                IF (Operandsize = 32) OR (CR4.VME = 0)
                     THEN #GP(0); (* Trap to virtual-8086 monitor. *)
                     ELSE (* Operandsize = 16 and CR4.VME = 1 *)
                         tempFLAGS := Pop();
                         IF (EFLAGS.VIP = 1 AND tempFLAGS[9] = 1) OR tempFLAGS[8] = 1
                                  THEN #GP(0);
                                  ELSE
                                           EFLAGS.VIF := tempFLAGS[9];
                                           EFLAGS[15:0] := tempFLAGS;
                                           (* All non-reserved bits except IOPL and IF can be modified;
                                  IOPL, IF, and all reserved bits are unaffected. *)
                         FI;
                FI;
       FI;
FI;

Flags Affected

All flags may be affected; see the Operation section for details.

Protected Mode Exceptions

#SS(0)	If the top of stack is not within the stack segment.
#PF(fault-code)	If a page fault occurs.
#AC(0)	If an unaligned memory reference is made while CPL = 3 and alignment checking is enabled.
#UD	If the LOCK prefix is used.

Real-Address Mode Exceptions

#SS	If the top of stack is not within the stack segment.
#UD	If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#GP(0)	If IOPL < 3 and VME is not enabled. If IOPL < 3 and the 32-bit operand size is used. If IOPL < 3, EFLAGS.VIP = 1, and bit 9 (IF) is set in the FLAGS value on the stack. If IOPL < 3 and bit 8 (TF) is set in the FLAGS value on the stack. If an attempt is made to execute the POPF/POPFD instruction with an operand-size override prefix.
#SS(0)	If the top of stack is not within the stack segment.
#PF(fault-code)	If a page fault occurs.
#AC(0)	If an unaligned memory reference is made while alignment checking is enabled.
#UD	If the LOCK prefix is used.

Compatibility Mode Exceptions

Same as for protected mode exceptions.

64-Bit Mode Exceptions

#SS(0)	If the stack address is in a non-canonical form.
#PF(fault-code)	If a page fault occurs.
#AC(0)	If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#UD	If the LOCK prefix is used.

POPF/POPFD/POPFQ - Pop Stack Into EFLAGS Register

Opcode

Instruction

Op/ En

64-Bit Mode

Compat/ Leg Mode

Description

Instruction Operand Encoding

Op/En

Operand 1

Operand 2

Operand 3

Operand 4

Description

Table 4-16. Effect of POPF/POPFD on the EFLAGS Register

NOTES:

Key

S

SV

N

X

0

Operation

Flags Affected

Protected Mode Exceptions

Real-Address Mode Exceptions

Virtual-8086 Mode Exceptions

Compatibility Mode Exceptions

64-Bit Mode Exceptions