Intel 80386 ttl , _ , __

inter
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