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3-20 Vol. 3
PROTECTED-MODE MEMORY MANAGEMENT
See also: Section 3.5.1, “Segment Descriptor Tables”, and Section 7.2.2, “TSS
Descriptor” (for more information on the system-segment descriptors); see Section
5.8.3, “Call Gates”, Section 6.11, “IDT Descriptors”, and Section 7.2.5, “Task-Gate
Descriptor” (for more information on the gate descriptors).
3.5.1 Segment Descriptor Tables
A segment descriptor table is an array of segment descriptors (see Figure 3-10). A
descriptor table is variable in length and can contain up to 8192 (2
13
) 8-byte descrip-
tors. There are two kinds of descriptor tables:
• The global descriptor table (GDT)
• The local descriptor tables (LDT)
Figure 3-10. Global and Local Descriptor Tables
Segment
Selector
Global
Descriptor
T
First Descriptor in
GDT is Not Used
TI = 0
I
56
40
48
32
24
16
8
0
TI = 1
56
40
48
32
24
16
8
0
Table (GDT)
Local
Descriptor
Table (LDT)
Base Address
Limit
GDTR Register LDTR Register
Base Address
Seg. Sel.
Limit