Vol. 3A 5-13
INTERRUPT AND EXCEPTION HANDLING
5.11 IDT DESCRIPTORS
The IDT may contain any of three kinds of gate descriptors:
• Task-gate descriptor
• Interrupt-gate descriptor
• Trap-gate descriptor
Figure 5-2 shows the formats for the task-gate, interrupt-gate, and trap-gate descriptors. The
format of a task gate used in an IDT is the same as that of a task gate used in the GDT or an LDT
(see Section 6.2.5, “Task-Gate Descriptor”). The task gate contains the segment selector for a
TSS for an exception and/or interrupt handler task.
Interrupt and trap gates are very similar to call gates (see Section 4.8.3, “Call Gates”). They
contain a far pointer (segment selector and offset) that the processor uses to transfer program
execution to a handler procedure in an exception- or interrupt-handler code segment. These gates
differ in the way the processor handles the IF flag in the EFLAGS register (see Section 5.12.1.2,
“Flag Usage By Exception- or Interrupt-Handler Procedure”).
Figure 5-1. Relationship of the IDTR and IDT
IDT Limit
IDT Base Address
+
Interrupt
Descriptor Table (IDT)
Gate for
0
IDTR Register
Interrupt #n
Gate for
Interrupt #3
Gate for
Interrupt #2
Gate for
Interrupt #1
151647
0
31
0
8
16
(n−1)∗8
