Time Sequence of Multiple interrupts

Seite 1

Time Sequence of Multiple interrupts

As an example of this second approach, consider a

system with three I/O devices: a printer, a disk, and a communications line, with

increasing priorities of 2, 4, and 5, respectively. Figure 3.14 illustrates a possible

sequence. A user program begins at t = 0. At t = 10, a printer interrupt occurs; user

information is placed on the system stack and execution continues at the printer

interrupt service routine (ISR). While this routine is still executing, at t = 15, a

communications interrupt occurs. Because the communications line has higher

priority than the printer, the interrupt is honored. The printer ISR is interrupted,

its state is pushed onto the stack, and execution

continues at the communications ISR. While this routine is executing, a disk interrupt occurs (t = 20). Because this interrupt is of lower priority, it is simply held, and the communications ISR runs

to completion.

When the communications ISR is complete (t = 25), the previous processor

state is restored, which is the execution of the printer ISR. However, before even a

single instruction in that routine can be executed, the processor honors the higher priority

disk interrupt and control transfers to the disk ISR. Only when that routine

is complete

(t = 35) is the printer ISR resumed. When that routine completes (t = 40),

control finally returns to the user program.