80386

 

 

 

 

 

 

 

 

 

 

 

31

 

a FFFFFFFF

 

 

 

 

 

15

a

 

 

RESET ROUTINES

 

 

 

 

 

 

 

tFtFtFto

 

 

 

 

 

SS §ITI

 

 

INITIALIZATION

 

 

 

 

 

 

 

 

 

 

 

 

 

GS §ITI

 

 

ROUTINES

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FS §ITI

 

 

 

 

 

 

 

 

 

 

ES §ITI

 

 

 

 

 

 

 

 

 

 

 

 

 

 

USER MEMORY

 

 

 

 

 

 

CS

 

 

 

 

 

 

 

 

 

 

 

GDTR

 

 

 

CODE DESCRIPTOR

"""'"1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DATA DESCRIPTOR

00000110

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NULL SELECTOR

00000 108

GDT

 

 

 

 

 

 

 

00000100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

INTERRUPT

t

 

 

 

 

 

 

 

 

 

DESCRIPTORS (32)

IDT

 

 

 

 

 

 

 

 

 

t

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

00000000

 

231630-66

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4-17. Simple Protected System

 

 

 

 

DATA

SEGMENT BASE 15 ... 0

 

 

SEGMENT LIMIT 15 ... 0

 

DESCRIPTOR

0118 (H)

 

 

 

 

FFFF (H)

 

 

 

 

 

BASE 31 ... 24

G

o 0

 

LIMIT

 

 

o 0 1

o BASE 23 ... 16

2

0

19.16

1

001

 

00 (H)

1

1

 

F(H)

 

 

 

 

 

00 (H)

 

 

 

 

 

 

 

 

 

 

 

CODE

SEGMENT BASE 15 ... 0

 

 

SEGMENT LIMIT 15 ... 0

 

DESCRIPTOR

0118 (H)

 

 

 

 

FFFF (H)

 

 

 

 

 

BASE 31 ... 24

G

0 o 0

LIMIT

 

 

 

o 1

 

BASE 23 ... 16

 

19.16

1

001

1

0

 

00 (H)

1

1

 

F(H)

 

 

 

 

 

00 (H)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NULL

DESCRIPTOR

 

 

 

o

 

 

 

 

 

 

 

 

 

 

 

 

31

 

24

 

16

15

 

 

8

 

o

 

Figure 4-18. GOT Descriptors for Simple System

 

 

 

4.4.8 Tools for Building Protected

 

4.5

PAGING

 

 

 

 

Systems

 

 

 

 

 

 

 

 

 

 

In order to simplify the design of a protected multi· tasking system, Intel provides a tool which allows the system designer an easy method of constructing the data structures needed for a Protected Mode 80386 system. This tool is the builder BLD·386TM. BLD·386lets the operating system writer specify all of the segment descriptors discussed in the previous sections (LOTs, lOTs, GDTs, Gates, and TSSs) in a high·level language.

4.5.1 Paging Concepts

Paging is another type of memory management use· ful for virtual memory multitasking operating sys- tems. Unlike segmentation which modularizes pro- grams and data into variable length segments, pa-

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Intel 80386 manual Tools for Building Protected, Systems, Paging Concepts