THE
80286
INSTRUCTION SET
If Nested Task Flag=O then
INTERRUPT RETURN
ON
STACK:
Second word on stack must be within stack limits else #SS(O)
Return CS selector RPL must be
;:::
CPL else
#GP
(Return selector)
If return selector RPL = CPL then
Else
INTERRUPT RETURN TO SAME LEVEL:
Top 6 bytes on stack must be within limits else #SS(O)
Return CS selector (at
SP+2)
must be non-null else #GP(O)
Selector index must be within its descriptor table limits else
#GP(
Return selector)
AR byte mustJndicate code segment else
#GP
(Return selector)
If
non-conforming then code segment OPL must = CPL else
#GP
(Return selector)
If
conforming then code segment OPL must be
::s;
CPL else
#GP
(Return selector)
Segment must be PRESENT else
#NP
(Return selector)
IP
must be in code segment limit else #GP(O)
Load CS:IP from stack
Load CS-cache with new code segment descriptor
Load flags with third word on stack
Increment SP by 6
INTERRUPT RETURN TO OUTER PRIVILEGE LEVEL:
Top 10 bytes on stack must be within limits else #SS(O)
Examine return
'CS
selector (at
SP+2)
and associated descriptor:
Selector must be non-null else #GP(O)
Selector index must be within its descriptor table limits else
#GP
(Return selector)
AR byte must indicate code segment else
#GP
(Return selector)
If non-conforming then code segment OPL must =
CS
selector RPL else
#GP
(Return
selector)
If
conforming then code segment OPL must be > CPL else
#GP
(Return selector)
Segment must be PRESENT else
#NP
(Return selector)
Examine return SS selector (at
SP+8)
and associated descriptor:
Selector must be non-nUll else #GP(O)
Selector index must be within its descriptor table limits else
#GP
(SS selector)
Selector RPL must equal the RPL
of
the return CS selector else
#GP
(SS selector)
AR byte must indicate a writable data segment else
#GP
(SS
selector)
Stack segment OPL must equal the RPL
of
the return CS selector else
#GP
(SS selector)
SS must be PRESENT else
#SS
(SS selector)
IP must be in code segment limit else #GP(O)
Load C5:IP from stack
Load flags with values at
(SP+4)
Load SS:SP from stack
Set CPL to the RPL
of
the return CS selector
Load the CS-cache with the CS descriptor
Load the 5S-cache with the SS descriptor
For each
of
ES
and
OS:
If the current register setting is
not
valid
for
the outer level, then zero the register and
clear the valid flag
To
be valid, the register setting must satisfy the following properties:
Selector index must be within descriptor table limits
AR byte must indicate data
or
readable code segment
If segment is data
or
non-conforming code, then:
DPL must be
;:::
CPL.
or
OPL must be
;:::
RPL.
8-52
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