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

Because the I/O operation may take a relatively long time to complete, the
I/O program is hung up waiting for the operation to complete; hence, the user
program is stopped at the point of the WRITE call for some considerable period
of time.
INTERRUPTS AND THE INSTRUCTION CYCLE With interrupts, the processor can
be engaged in executing other instructions while an I/O operation is in progress.
Consider the flow of control in Figure 3.7b. As before, the user program reaches a
point at which it makes a system call in the form of a WRITE call.The I/O program
that is invoked in this case consists only of the preparation code and the actual I/O
command. After these few instructions have been executed, control returns to the
user program. Meanwhile, the external device is busy accepting data from computer
memory and printing it. This I/O operation is conducted concurrently with the exe-
cution of instructions in the user program.
When the external device becomes ready to be serviced—that is, when it is
ready to accept more data from the processor,—the I/O module for that external
device sends an interrupt request signal to the processor. The processor responds by
suspending operation of the current program, branching off to a program to service
that particular I/O device, known as an interrupt handler, and resuming the original
execution after the device is serviced. The points at which such interrupts occur are
indicated by an asterisk in Figure 3.7b.
From the point of view of the user program, an interrupt is just that: an inter-
ruption of the normal sequence of execution.When the interrupt processing is com-
pleted, execution resumes (Figure 3.8). Thus, the user program does not have to
contain any special code to accommodate interrupts; the processor and the operat-
ing system are responsible for suspending the user program and then resuming it at
the same point.
To accommodate interrupts, an interrupt cycle is added to the instruction cycle,
as shown in Figure 3.9. In the interrupt cycle, the processor checks to see if any

User program Interrupt handler

1

2
• •
• •
• •

i
Interrupt
occurs here
i 1
•
•
•

M
Figure 3.8 Transfer of Control via Interrupts