Intel 80386 manual ~~~~~~~EL~~E~ ~A~~ ~I~I! ttl

inter80386

4.3.4.10SEGMENT 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		32 - BIT LIMIT		OTHER ATIRIBUTES
(UPDATED DURING SELECTOR			(FIXED)		(FIXED)
LOAD INTO SEGMENT REGISTER)
CONFORMING PRIVILEGE -----------------------						,
STACKSIZE ----------------------------						,
EXECUTABLE ------------------------					,
WRITEABLE -----------------------					,
R~DABLE-----------------------					,
EXPANSION DIRECTION					1
GRANULARITY
ACCESSED					1
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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

'Exceptthe 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	D	= expand down
N	= no	B	= byte granularity
o	= privilege level 0	P	= page granularity
1	= privilege level 1	W	= push/pop 16-bit words
2	= privilege level 2	F	= push/pop 32·bitdwords
3	= privilege level 3	-	= does not apply to that segment cache register
U	= expand up

Figure 4-11. Segment Descriptor Caches for Real Address Mode

(Segment Limit and Attributes are Fixed)

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