inter80386

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

32 -

BIT LIMIT

 

 

OTHER ATIRIBUTES

 

 

(UPDATED DURING SELECTOR

(FIXED)

 

 

(FIXED)

 

 

 

LOAD INTO SEGMENT REGISTER)

 

 

 

 

 

 

 

 

CONFORMING PRIVILEGE ----------------------

 

 

 

 

 

,

 

STACK SIZE -----------------------

 

 

 

 

.

 

 

EXECUTABLE ----------------------

 

 

 

,

 

 

 

WRITEABLE ---------------------

 

 

 

,

 

 

 

READABLE --------------------

 

 

 

,

 

 

 

EXPANSION DIRECTION

 

 

 

 

1

 

 

 

GRANULARITY

 

 

 

 

 

 

 

ACCESSED

 

 

 

 

1

 

 

 

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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

-

 

OS

16X CURRENT OS SELECTOR

OOOOFFFFH

Y

3

Y B U Y Y N

- -

 

ES

16X CURRENT ES SELECTOR

OOOOFFFFH

Y

3

Y B U Y Y N -

-

 

FS

16X CURRENT FS SELECTOR

OOOOFFFFH

Y

3

Y R U Y Y N

- -

 

GS

16X CURRENT GS SELECTOR

OOOOFFFFH

Y

3

Y B U Y Y N -

-

 

 

 

 

 

 

 

 

 

231630-62

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·bitwords

 

 

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 4-13. Segment Descriptor Caches for Virtual 8086 Mode within Protected Mode

(Segment Limit and Attributes are Fixed)

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Intel 80386 manual ~~?~~~~EL~~E~B~~E ~I~I~ tJ1