Page 105 -

3.2 / COMPUTER FUNCTION 79

Time
1 1

4 4

Processor I/O 2
wait operation
I/O
operation
Processor
5
wait

5
2

4
4

3
I/O
Processor I/O
operation
wait operation
Processor
wait
5
5

(b) With interrupts
3

(a) Without interrupts
Figure 3.11 Program Timing: Long I/O Wait

In any case, it is possible for a communications interrupt to occur while a printer
interrupt is being processed.
Two approaches can be taken to dealing with multiple interrupts. The first is to
disable interrupts while an interrupt is being processed. A disabled interrupt simply
means that the processor can and will ignore that interrupt request signal. If an inter-
rupt occurs during this time, it generally remains pending and will be checked by the
processor after the processor has enabled interrupts.Thus, when a user program is exe-
cuting and an interrupt occurs, interrupts are disabled immediately.After the interrupt
handler routine completes, interrupts are enabled before resuming the user program,
and the processor checks to see if additional interrupts have occurred.This approach is
nice and simple, as interrupts are handled in strict sequential order (Figure 3.13a).