Intel 64 and IA-32 Architectures Software Developers Manual Volume 3A, System Programming Guide, Part 1

Vol. 3A 3-19
PROTECTED-MODE MEMORY MANAGEMENT
• 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 descriptors are in themselves “gates,” which hold pointers to proce-
dure 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 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 6.2.2, “TSS
Descriptor” (for more information on the system-segment descriptors); see Section
4.8.3, “Call Gates”, Section 5.11, “IDT Descriptors”, and Section 6.2.5, “Task-Gate
Descriptor” (for more information on the gate descriptors).
Table 3-2. System-Segment and Gate-Descriptor Types
Type Field Description
Decimal 11 10 9 8 32-Bit Mode IA-32e Mode
0 0000Reserved Upper 8 byte of an 16-
byte descriptor
1 000116-bit TSS (Available) Reserved
2 0010LDT LDT
3 001116-bit TSS (Busy) Reserved
4 010016-bit Call Gate Reserved
5 0101Task Gate Reserved
6 011016-bit Interrupt Gate Reserved
7 011116-bit Trap Gate Reserved
8 1000Reserved Reserved
9 100132-bit TSS (Available) 64-bit TSS (Available)
10 1010Reserved Reserved
11 101132-bit TSS (Busy) 64-bit TSS (Busy)
12 110032-bit Call Gate 64-bit Call Gate
13 1101Reserved Reserved
14 111032-bit Interrupt Gate 64-bit Interrupt Gate
15 111132-bit Trap Gate 64-bit Trap Gate