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80386
When operating
in
a Virtual 8086 Mode within the
Protected Mode, the segment base,
limit, and other
attributes within the segment cache registers are
de-
fined as shown
in
Figure 4-13. For compatibility with
the
8086 architecture, the base
is
set to sixteen
times the current
selector value, the limit
is
fixed at
OOOOFFFFH,
and the attributes are fixed so as to
indicate the segment is present and
fully usable. The
virtual program executes at lowest privilege level,
level
3,
to allow trapping of all IOPL-sensitive in-
structions and level-a-only instructions.
SEGMENT
DESCRIPTOR
CACHE
REGISTER
CONTENTS
32-
BIT
BASE
(UPDATED
DURING
SELECTOR
LOAD
INTO
SEGMENT
REGISTER)
32
-
BIT
LIMIT
(FIXED)
OTHER
ATIRIBUTES
(FIXED)
CONFORMING
PRIVILEGE
----------------------,
STACK
SIZE-----------------------.
EXECUTABLE----------------------,
WRITEABLE---------------------,
READABLE--------------------,
EXPANSION
DIRECTION
GRANULARITY
1
ACCESSED
1
~~?~~~~E
_
L~~E~
___
B~~E
____________
~I~I~
___
t J 1 _
__
CS
16X
CURRENT
CS
SELECTOR
OOOOFFFFH
Y 3 Y B U
Y Y
Y
-
N
SS
16X
CURRENT
SS
SELECTOR
OOOOFFFFH
Y 3
Y B U Y Y N W
-
Key:
Y = yes
N
=
no
OS
ES
FS
GS
16X
CURRENT
OS
SELECTOR
16X
CURRENT
ES
SELECTOR
16X
CURRENT
FS
SELECTOR
16X
CURRENT
GS
SELECTOR
OOOOFFFFH
Y 3
Y B U Y Y N
OOOOFFFFH
Y 3 Y
B
OOOOFFFFH
Y 3 Y R
OOOOFFFFH
Y 3
Y B
D = expand down
B
= byte granularity
P
= page granularity
U
U
U
W =
push/
pop 16·bit words
F
=
push/pop
32·bit dwords
Y Y N
Y
Y
N
Y Y N
- -
- -
- -
-
-
231630-62
o = privilege level 0
1 = privilege level 1
2
= privilege level 2
3
= privilege level 3
U
= expand up
- = does
not
apply to that segment cache register
Figure 4-13. Segment Descriptor Caches
for
Virtual 8086 Mode within Protected Mode
(Segment Limit and Attributes are Fixed)
44