Opcode Instruction Clocks Description 8F /0 POP m16 5 Pop top of stack into memory word 8F /0 POP m32 5 Pop top of stack into memory dword 58 + rw POP r16 4 Pop top of stack into word register 58 + rd POP r32 4 Pop top of stack into dword register 1F POP DS 7,pm=21 Pop top of stack into DS 07 POP ES 7,pm=21 Pop top of stack into ES 17 POP SS 7,pm=21 Pop top of stack into SS 0F A1 POP FS 7,pm=21 Pop top of stack into FS 0F A9 POP GS 7,pm=21 Pop top of stack into GS
IF StackAddrSize = 16 THEN IF OperandSize = 16 THEN DEST := (SS:SP); (* copy a word *) SP := SP + 2; ELSE (* OperandSize = 32 *) DEST := (SS:SP); (* copy a dword *) SP := SP + 4; FI; ELSE (* StackAddrSize = 32 * ) IF OperandSize = 16 THEN DEST := (SS:ESP); (* copy a word *) ESP := ESP + 2; ELSE (* OperandSize = 32 *) DEST := (SS:ESP); (* copy a dword *) ESP := ESP + 4; FI; FI;
POP CS is not an 80386 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 execution of the next instruction. This allows sequential execution of POP SS and POP eSP 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.
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
Chapter 17 -- 80386 Instruction Set
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