Intel IA-32 - Manual (Page 97)

Vol. 3A 3-17
PROTECTED-MODE MEMORY MANAGEMENT
3.5 SYSTEM DESCRIPTOR TYPES
When the S (descriptor type) flag in a segment descriptor is clear, the descriptor type is a system
descriptor. The processor recognizes the following types of system descriptors:
•
Local descriptor-table (LDT) segment descriptor.
•
Task-state segment (TSS) descriptor.
•
Call-gate descriptor.
•
Interrupt-gate descriptor.
•
Trap-gate descriptor.
•
Task-gate descriptor.
These descriptor types fall into two categories: system-segment descriptors and gate descriptors.
System-segment descriptors point to system segments (LDT and TSS segments). Gate descrip-
tors are in themselves “gates,” which hold pointers to procedure entry points in code segments
(call, interrupt, and trap gates) or which hold segment selectors for TSS’s (task gates).
Table 3-2 shows the encoding of the type field for system-segment descriptors and gate descrip-
tors. Note that system descriptors in IA-32e mode are 16 bytes instead of 8 bytes.
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 byte 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