POPF/POPFD/POPFQ — Pop Stack Into EFLAGS Register

Instruction Operand Encoding ¶

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 equivalent to privilege level 0), all non-reserved flags in the EFLAGS register except RF¹, VIP, VIF, and VM may be modified. 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 insufficient 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 information about the EFLAGS registers.

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

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

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 ¶

Real-Address Mode Exceptions ¶

Virtual-8086 Mode Exceptions ¶

Compatibility Mode Exceptions ¶

Same as for protected mode exceptions.

64-Bit Mode Exceptions ¶