User's Manual

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'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 cache with descriptor
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