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
8-83
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