3-14 Vol. 3A
PROTECTED-MODE MEMORY MANAGEMENT
Table 3-2 shows the encoding of the type field for system-segment descriptors and gate descriptors. Note that
system descriptors in IA-32e mode are 16 bytes instead of 8 bytes.
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 descriptors. There are two kinds of descriptor tables:
•
The global descriptor table (GDT)
•
The local descriptor tables (LDT)
Table 3-2. System-Segment and Gate-Descriptor Types
Type Field
Description
Decimal
11
10
9
8
32-Bit Mode
IA-32e Mode
0
0
0
0
0
Reserved
Upper 8 bytes of an 16-byte
descriptor
1
0
0
0
1
16-bit TSS (Available)
Reserved
2
0
0
1
0
LDT
LDT
3
0
0
1
1
16-bit TSS (Busy)
Reserved
4
0
1
0
0
16-bit Call Gate
Reserved
5
0
1
0
1
Task Gate
Reserved
6
0
1
1
0
16-bit Interrupt Gate
Reserved
7
0
1
1
1
16-bit Trap Gate
Reserved
8
1
0
0
0
Reserved
Reserved
9
1
0
0
1
32-bit TSS (Available)
64-bit TSS (Available)
10
1
0
1
0
Reserved
Reserved
11
1
0
1
1
32-bit TSS (Busy)
64-bit TSS (Busy)
12
1
1
0
0
32-bit Call Gate
64-bit Call Gate
13
1
1
0
1
Reserved
Reserved
14
1
1
1
0
32-bit Interrupt Gate
64-bit Interrupt Gate
15
1
1
1
1
32-bit Trap Gate
64-bit Trap Gate