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Processor Programming (Continued)

3.5.3.3

Task, Gate, Interrupt, and Application and

System Descriptors

Besides segment descriptors there are descriptors used in

task switching, switching between tasks with different prior-

ity and those used to control interrupt functions:

• Interrupt Descriptors

• Application and System Segment Descriptors

• Gate Descriptors

• Task State Segment Descriptors

All descriptors have some things in common. They are all

eight bytes in length and have three fields (BASE, LIMIT,

and TYPE). The BASE field defines the starting location for

the table or segment. The LIMIT field defines the size and

theTYPE fielddepends on thetypeofdescriptor. Oneof

the main functions of the TYPE field is to define the access

rights to the associated segment or table.

Interrupt Descriptors

The Interrupt Descriptor Table (IDT) is an array of 256 8-

byte (4-byte for real mode) interrupt descriptors, each of

which is used to point to an interrupt service routine. Every

interrupt that may occur in the system must have an asso-

ciated entry in the IDT. The contents of the IDTR are com-

pletely visible to the programmer through the use of the

SIDT instruction.

The IDT is defined by the Interrupt Descriptor Table Regis-

ter (IDTR). Some texts refer to this register as an IDT

descriptor.

Thefollowing instructions areused inconjunctionwiththe

IDTR:

• LIDT - Load memory to IDTR

• SIDT - Store IDTR to memory

The IDTR is set up in real mode using the LIDT instruction.

This is possible as the LIDT instruction is only one of two

instructions that directly load a linear address (instead of a

segment relative address) in protected mode (the other

instructions is LGDT).

As previously shown in Table 3-20 on page 70, the IDTR

contains a BASE ADDRESS field and a LIMIT field that

define the IDT.

Application and System Segment Descriptors

The bit structure and bit definitions for segment descriptors

are shown in Table 3-21 and Table 3-22 on page 72,

respectively. The explanation of the TYPE field is shown in

Table 3-23 on page 73.

Table 3-21. Application and System Segment Descriptors

31 31 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10

Memory Offset +4

BASE[31:24]

LIMIT[19:16]

DPL

TYPE

BASE[23:16]

Memory Offset +0

BASE[15:0]

LIMIT[15:0]

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