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80386
31
a
FFFFFFFF
15 a
RESET
ROUTINES
SS
§ITI
tFtFtFto
INITIALIZATION
GS
§ITI
ROUTINES
FS
§ITI
ES
§ITI
USER
MEMORY
CS
GDTR
"""'"
1
DATA
DESCRIPTOR
00000110
CODE
DESCRIPTOR
00000
108
GDT
NULL
SELECTOR
00000100
INTERRUPT
t
DESCRIPTORS
(32)
IDT
t
00000000
231630-66
Figure 4-17. Simple Protected System
DATA
DESCRIPTOR
2
CODE
DESCRIPTOR
o
SEGMENT BASE 15
...
0
0118
(H)
BASE
31
...
24 G
o 0
00
(H)
1
0
1
SEGMENT
BASE 15
...
0
0118
(H)
BASE
31
...
24 G 0
00
(H)
1 1
o 0
31
24
SEGMENT
LIMIT 15
...
0
FFFF
(H)
LIMIT
o BASE 23
...
16
19.16
1
001
o 0
1
F(H)
00
(H)
SEGMENT LIMIT 15
...
0
FFFF
(H)
LIMIT
BASE
23
...
16
19.16
1
001
1 o 1 0
00
(H)
F(H)
NULL DESCRIPTOR
16 15 8
o
Figure 4-18. GOT Descriptors
for
Simple System
4.4.8 Tools for Building Protected
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·386 lets 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.
51
4.5 PAGING
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-