POP — Pop a Value from the Stack

Instruction Operand Encoding ¶

Description ¶

Loads the value from the top of the stack to the location specified with the destination operand (or explicit opcode) and then increments the stack pointer. The destination operand can be a general-purpose register, memory location, or segment register.

Address and operand sizes are determined and used as follows:

• Address size. The D flag in the current code-segment descriptor determines the default address size; it may be overridden by an instruction prefix (67H). The address size is used only when writing to a destination operand in memory.
• Operand size. The D flag in the current code-segment descriptor determines the default operand size; it may be overridden by instruction prefixes (66H or REX.W).

The operand size (16, 32, or 64 bits) determines the amount by which the stack pointer is incremented (2, 4 or 8).

• Stack-address size. Outside of 64-bit mode, the B flag in the current stack-segment descriptor determines the size of the stack pointer (16 or 32 bits); in 64-bit mode, the size of the stack pointer is always 64 bits.

The stack-address size determines the width of the stack pointer when reading from the stack in memory and when incrementing the stack pointer. (As stated above, the amount by which the stack pointer is incremented is determined by the operand size.)

If the destination operand is one of the segment registers DS, ES, FS, GS, or SS, the value loaded into the register must be a valid segment selector. In protected mode, popping a segment selector into a segment register automatically causes the descriptor information associated with that segment selector to be loaded into the hidden (shadow) part of the segment register and causes the selector and the descriptor information to be validated (see the “Operation” section below).

A NULL value (0000-0003) may be popped into the DS, ES, FS, or GS register without causing a general protection fault. However, any subsequent attempt to reference a segment whose corresponding segment register is loaded with a NULL value causes a general protection exception (#GP). In this situation, no memory reference occurs and the saved value of the segment register is NULL.

The POP instruction cannot pop a value into the CS register. To load the CS register from the stack, use the RET instruction.

If the ESP register is used as a base register for addressing a destination operand in memory, the POP instruction computes the effective address of the operand after it increments the ESP register. For the case of a 16-bit stack where ESP wraps to 0H as a result of the POP instruction, the resulting location of the memory write is processor-family-specific.

The POP ESP instruction increments the stack pointer (ESP) before data at the old top of stack is written into the destination.

A POP SS instruction inhibits all interrupts, including the NMI interrupt, until after execution of the next instruction. This action allows sequential execution of POP SS and MOV ESP, EBP instructions without the danger of having an invalid stack during an interrupt¹. However, use of the LSS instruction is the preferred method of loading the SS and ESP registers.

1. If a code instruction breakpoint (for debug) is placed on an instruction located immediately after a POP SS instruction, the breakpoint may not be triggered. However, in a sequence of instructions that POP the SS register, only the first instruction in the sequence is guaranteed to delay an interrupt.

In the following sequence, interrupts may be recognized before POP ESP executes:

POP SS POP SS POP ESP

In 64-bit mode, using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). When in 64-bit mode, POPs using 32-bit operands are not encodable and POPs to DS, ES, SS are not valid. See the summary chart at the beginning of this section for encoding data and limits.

Operation ¶

IF StackAddrSize = 32
    THEN
        IF OperandSize = 32
            THEN
                DEST ← SS:ESP; (* Copy a doubleword *)
                ESP ← ESP + 4;
            ELSE (* OperandSize = 16*)
                DEST ← SS:ESP; (* Copy a word *)

                    ESP ← ESP + 2;
        FI;
    ELSE IF StackAddrSize = 64
        THEN
                IF OperandSize = 64
                    THEN
                        DEST ← SS:RSP; (* Copy quadword *)
                        RSP ← RSP + 8;
                    ELSE (* OperandSize = 16*)
                        DEST ← SS:RSP; (* Copy a word *)
                        RSP ← RSP + 2;
                FI;
        FI;
    ELSE StackAddrSize = 16
        THEN
                IF OperandSize = 16
                    THEN
                        DEST ← SS:SP; (* Copy a word *)
                        SP ← SP + 2;
                    ELSE (* OperandSize = 32 *)
                        DEST ← SS:SP; (* Copy a doubleword *)
                        SP ← SP + 4;
                FI;
FI;
Loading a segment register while in protected mode results in special actions, as described in the following listing.
These checks are performed on the segment selector and the segment descriptor it points to.
64-BIT_MODE
IF FS, or GS is loaded with non-NULL selector;
    THEN
        IF segment selector index is outside descriptor table limits
                OR segment is not a data or readable code segment
                OR ((segment is a data or nonconforming code segment)
                    AND (both RPL and CPL > DPL))
                        THEN #GP(selector);
                IF segment not marked present
                    THEN #NP(selector);
        ELSE
                SegmentRegister ← segment selector;
                SegmentRegister ← segment descriptor;
        FI;
FI;
IF FS, or GS is loaded with a NULL selector;
        THEN
                SegmentRegister ← segment selector;
                SegmentRegister ← segment descriptor;
FI;
PREOTECTED MODE OR COMPATIBILITY MODE;
IF SS is loaded;
    THEN
        IF segment selector is NULL
                THEN #GP(0);
        FI;
        IF segment selector index is outside descriptor table limits
                or segment selector's RPL ≠ CPL
                or segment is not a writable data segment
                or DPL ≠ CPL
                    THEN #GP(selector);
        FI;
        IF segment not marked present
                THEN #SS(selector);
                ELSE
                    SS ← segment selector;
                    SS ← segment descriptor;
        FI;
FI;
IF DS, ES, FS, or GS is loaded with non-NULL selector;
    THEN
        IF segment selector index is outside descriptor table limits
                or segment is not a data or readable code segment
                or ((segment is a data or nonconforming code segment)
                and (both RPL and CPL > DPL))
                    THEN #GP(selector);
        FI;
        IF segment not marked present
                THEN #NP(selector);
                ELSE
                    SegmentRegister ← segment selector;
                    SegmentRegister ← segment descriptor;
            FI;
FI;
IF DS, ES, FS, or GS is loaded with a NULL selector
    THEN
        SegmentRegister ← segment selector;
        SegmentRegister ← segment descriptor;
FI;

Flags Affected ¶

None.

Protected Mode Exceptions ¶

Real-Address Mode Exceptions ¶

Virtual-8086 Mode Exceptions ¶

Compatibility Mode Exceptions ¶

Same as for protected mode exceptions.

64-Bit Mode Exceptions ¶