Intel 80286, 80287 manual Infer, Interrupt and Gate Interactions

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INTERRUPTS AND EXCEPTIONS

Table 9-2 illustrates how the interrupt enable flag and interrupt type interact with the type of gate used.

9.5 TASK GATES AND INTERRUPT TASKS

The 80286 allows interrupts to directly cause a task switch. When an interrupt vector selects an entry in the IDT which is a task gate, a task switch occurs. The format of a task gate is described in section

8.5.If a task gate is used to handle an exception that passes an error code, the error code will be pushed onto the new task's stack.

A task gate offers two advantages over interrupt gates:

1.It automatically saves all of the processor registers as part of the task-switch operation, whereas an interrupt gate saves only the flag register and CS:IP.

2.The new task is completely isolated from the task that was interrupted. Address spaces are isolated and the interrupt-handling task is unaffected by the privilege level of the interrupted task.

An interrupt task switch works like any other task switch once the TSS selector is fetched from the task gate. Like a trap or an interrupt gate, privilege and presence rules are applied to accessing a task gate during an interrupt.

Interrupts that cause a task switch set the NT bit in the flags of the new task. The TSS selector of the interrupted task is saved in the back link field of the new TSS. The interrupting task executes IRET to perform a task switch to return to the interrupted task because.NT was previously set. The interrupt task state is saved in its TSS before returning control .to the task that was interrupted; NT is restored to its original value in the interrupted task.

Since the interrupt handler state after executing IRET is saved, a re-entry of the interrupt service task will result in the execution of the instruction that follows IRET. Therefore, when the next interrupt occurs, the machine state will be the same as that when the IRET instruction was executed.

Note that an interrupt task resumes execution each time it is re-invoked, whereas an interrupt proce- dure starts executing at the beginning of the procedure each time. The interrupted task restarts execu- tion.at the point of interruption because interrupts occur before the execution of an instruction.

Table 9-2. Interrupt and Gate Interactions

Type of	Type of	Further	Further	Further	Further software
Interrupt	Gate	NMls?	INTRs?	Exceptions?	Interrupts?
NMI	Trap	No	Yes	Yes	Yes
NMI	Interrupt	No	No	Yes	Yes
INTR	Trap	Yes	Yes	Yes	Yes
INTR	Interrupt	Yes	No	Yes	Yes
Software	Trap	Yes	Yes	Yes	Yes
Software	Interrupt	Yes	No	Yes	Yes
Exception	Trap	Yes	Yes	Yes	Yes
Exception	Interrupt	Yes	No	Yes	Yes

9-7