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82 CHAPTER 3 / A TOP-LEVEL VIEW OF COMPUTER FUNCTION

Printer Communication
interrupt interrupt
User program service routine service routine
t 0

t 10 t 15

t 25
Disk
interrupt
t 40
t 25
service routine

t 35

Figure 3.14 Example Time Sequence of Multiple Interrupts

The drawback to the preceding approach is that it does not take into account
relative priority or time-critical needs. For example, when input arrives from the
communications line, it may need to be absorbed rapidly to make room for more
input. If the first batch of input has not been processed before the second batch
arrives, data may be lost.
A second approach is to define priorities for interrupts and to allow an interrupt
of higher priority to cause a lower-priority interrupt handler to be itself interrupted
(Figure 3.13b). 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, based on an example in [TANE97], illustrates a pos-
sible 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 com-
munications 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