external equipment or the status of an external
equipment, in response to a computer control word.
Some examples of control words used by computers
include the following:
Function (command) control words Function
control words are sent by the computer to an external
equipment to specify the type of operation it is to
perform. The signals used for the control words are
often referred to as handshaking. An example of a
control word would be a function code word telling a
printer to print the contents of a specific accumulator
register at the location specified by the address in the
instruction. Computers that have a control memory use
a control memory word to transfer data for I/O buffer
External interrupt words External interrupt
words are sent to the computer to specify that an error
or special condition exists in an external equipment or
the status of an external equipment. Review chapter 5
of this volume for a detailed discussion of interrupts:
their classification, types (micro, mini, and mainframe),
priorities (micro, mini, and mainframe), codes, and
There are two formats of information exchanged by
a computer: parallel and serial. The type of interface
will dictate the format of the information exchanged.
PARALLEL. When the computer exchanges
information using a parallel configuration, all bits of
information represented by a byte or word are input or
In figure 7-9, frame A, we
illustrate how the character M is output from the
computer to a printer in parallel format.
SERIAL. When the computer exchanges
information using a serial configuration, all bits of
information are input or output one at a time. Figure
7-9, frame B, illustrates the character M being output
from the computer to a printer in serial format.
The heart of the I/O section is the input/output
processor: an IOC/IOA or I/O pcb arrangement. All
computers have I/O instructions. Computers without
an IOC/IOA arrangement have other means of
Figure 7-9.Parallel and serial configurations: A. Character M transmitted in parallel; B. Character M transmitted serially.