Intel 80286, 80287 manual Protection Exceptions and Reserved Vectors

INTERRUPTS AND EXCEPTIONS

9.6 PROTECTION EXCEPTIONS AND RESERVED VECTORS

A protection violation will cause an exception, i.e., a non-maskable interrupt. Such a fault can be handled by the task that caused it if an interrupt or trap gate is used, or by a different task if a task gate is used (in the IDT).

Protection exceptions can be classified into program errors or implicit requests for service. The latter include stack overflow and not-present faults. Examples of program errors include attempting to write into a read-only segment, or violating segment limits.

Requests for service may use different interrupt vectors, but many diverse types of protection violation use the same general protection fault vector. Table 9-3 shows the reserved exceptions and interrupts. Interrupts 0-31 are reserved by Intel.

When simultaneous external interrupt requests occur, they are processed in the fixed order shown in table 9-4. For each interrupt serviced, the machine state is saved. The new CS:IP is loaded from the gate or TSS. If other interrupts remain enabled, they are processed before the first instruction of the current interrupt handler, i.e., the last interrupt processed is serviced first.

Table 9-3. Reserved Exceptions and Interrupts

Vector	Description	Restartable	Error Code
Number			on Stack
0	Divide Error Exception	Yes	No
1	Single Step Interrupt	Yes	No
2	NMI Interrupt	Yes	No
3	Breakpoint Interrupt	Yes	No
4	INTO Detected Overflow Exception	Yes	No
5	BOUND Range Exceeded Exception	Yes	No
6	Invalid Opcode Exception	Yes	No
7	Processor Extension Not Available Exception	Yes	No
8	Double Exception Detected	No	Yes (Always 0)
9	Processor Extension Segment Overrun Interrupt	No	No
10	Invalid Task State Segment	Yes	Yes
11	Segment Not Present	Yes	Yes
12	Stack Segment Overrun or Not Present	Yes	Yes
13	General Protection	Yes'	Yes

• Except for writes into read-only segments (see section 9.6)

	Table 9-4. Interrupt Processing Order
Order	Interrupt
1	Instruction exception
2	Single step
3	NMI
4	Processor extension segment overrun
5	INTR

9-9