## ALP Programming Guide and Reference

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### Description

The POP instruction replaces the previous contents of the memory, the register, or the segment register operand with the word on the top of the processor stack, addressed by SS:SP (address-size attribute of 16 bits) or SS:ESP (address-size attribute of 32 bits). The stack pointer SP is incremented by 2 for an operand-size of 16 bits or by 4 for an operand-size of 32 bits. It then points to the new top of stack.

The POP CS instruction is not a processor instruction. Popping from the stack into the CS register is accomplished with a RET instruction.

If the destination operand is a segment register (DS, ES, FS, GS, or SS), the value popped must be a selector. In protected mode, loading the selector initiates automatic loading of the descriptor information associated with that selector into the hidden part of the segment register; loading also initiates validation of both the selector and the descriptor information.

A null value (0000-0003) may be popped into the DS, ES, FS, or GS register without causing a protection exception. An attempt to reference a segment whose corresponding segment register is loaded with a null value causes a # GP(0) exception. No memory reference occurs. The saved value of the segment register is null.

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

A POP-to-memory instruction, which uses the stack pointer (ESP) as a base register, references memory after the POP. The base used is the value of the ESP after the instruction runs.

Loading a segment register while in protected mode results in special checks and actions, as described in the following listing:

    IF SS is loaded:
       IF selector is null THEN #GP(0);
       Selector index must be within its descriptor table limits ELSE
          #GP(selector);
       Selector's RPL must equal CPL ELSE #GP(selector);
       AR byte must indicate a writable data segment ELSE #GP(selector);
       DPL in the AR byte must equal CPL ELSE #GP(selector);
       Segment must be marked present ELSE #SS(selector);
       Load SS register with selector;
       Load SS register with descriptor;

    IF DS, ES, FS or GS is loaded with non-null selector:
       AR byte must indicate data or readable code segment ELSE
          #GP(selector);
       IF data or nonconforming code
       THEN both the RPL and the CPL must be less than or equal to DPL in
       AR byte
       ELSE #GP(selector);
       FI;
       Segment must be marked present ELSE #NP(selector);
       Load segment register with selector;
       Load segment register with descriptor;

    IF DS, ES, FS, or GS is loaded with a null selector:
       Load segment register with selector
       Clear valid bit in invisible portion of register