Intel IA-32 Manuale Utente

Vol. 3A 3-17

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.

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