Intel 80286, 80287 manual Pop Ss

THE 80286 INSTRUCTION SET

POP-Pop a Word from the Stack

Opcode	Instruction		Clocks	Description
1F	POP	OS	5,pm=20	Pop top of stack into OS
07	POP	ES	5,pm=20	Pop top of stack into ES
17	POP SS		5,pm=20	Pop top of stack into SS
SF /0	POP	mw	5	Pop top of stack into memory word
58+rw	POP	rw	5	Pop top of stack into word register

FLAGS MODIFIED

None

FLAGS UNDEFINED

None

OPERATION

The word on the top of the 80286 stack, addressed by SS:SP, replaces the previous contents of the memory, register, or segment register operand. The stack pointer SP is incremented by 2 to point to the new top of stack.

If the destination operand is another segment register (DS, ES, or SS), the value popped must be a selector. In protected mode, loading the selector initiates automatic loading of the descriptor informa- tion 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 loaded into the DS or ES register without causing a protection excep- tion. Attempts to reference memory using a segment register with a null value will cause #GP(O) exception. No memory reference will occur. The saved value of the segment register will be null.

A POP SS instruction will inhibit all interrupts, including NMI, until after the execution of the next instruction. This permits a POP SP instruction to be performed first.

Following is a listing of the protected-mode checks and actions taken in the loading of a segment register:

If SS is loaded:

If selector is null then #GP(O)

Selector index must be within its descriptor table limits else #GP (selector)

Selector'sRPL 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 cache with descriptor

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