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80386
4.3.4.10 SEGMENT DESCRIPTOR REGISTER
SETTINGS
The contents of the segment descriptor cache vary
depending on the mode the
80386 is operating in.
When operating
in
Real Address Mode, the segment
base,
limit, and other attributes within the segment
cache registers are defined as shown
in
Figure
4-11
.
For compatiblity 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.
In
Real Address Mode, the internal
"privilege level"
is
always fixed to the highest level,
level
0,
so I/O and other privileged opcodes may be
executed.
SEGMENT
DESCRIPTOR
CACHE
REGISTER
CONTENTS
32
-
BIT
BASE
(UPDATED
DURING
SELECTOR
LOAD
INTO
SEGMENT
REGISTER)
32
-
BIT
LIMIT
(FIXED)
OTHER
ATIRIBUTES
(FIXED)
CONFORMING
PRIVILEGE
-----------------------,
STACKSIZE----------------------------,
EXECUTABLE------------------------,
WRITEABLE-----------------------,
R~DABLE-----------------------,
EXPANSION
DIRECTION
GRANULARITY
1
ACCESSED
1
~~~~~~~E
_L~~E~
___
~A~~
___________
~I~I!
___
ttl
_
__
CS
16X
CURRENT
CS
SELECTOR"
OOOOFFFFH
Y 0 Y B U Y
Y
Y
-
N
SS
16X
CURRENT
SS
SELECTOR
OOOOFFFFH
Y 0
Y B U Y Y N
W -
DS
16X
CURRENT
DS
SELECTOR
OOOOFFFFH
Y 0 Y B U Y
Y
N
- -
ES
16X
CURRENT
ES
SELECTOR
OOOOFFFFH
Y 0 Y
B
U
Y Y N
- -
FS
16X
CURRENT
FS
SELECTOR
OOOOFFFFH
Y 0
Y B U Y Y N
- -
GS
16X
CURRENT
GS
SELECTOR
OOOOFFFFH
Y 0 Y B U Y
Y
N
- -
231630-60
'Except the 32-bit
CS
base is initialized to
FFFFFOOOH
after reset until first intersegment control transfer (e.g. intersegment CALL. or
intersegment
JMP, or
INT).
(See Figure 4-13 Example.)
Key:
Y = yes
N = no
o
= privilege level 0
1 = privilege level 1
2
= privilege level 2
3
= privilege level 3
U
= expand up
D = expand down
B
= byte granularity
P
= page granularity
W
= push/pop 16-bit words
F
= push/pop 32·bit dwords
- = does not apply to that segment cache register
Figure 4-11. Segment Descriptor Caches for Real Address Mode
(Segment Limit and Attributes are Fixed)
42