1965-1980: Integrated Circuits and Multiprogramming
In the early 1960s, computer manufacturers typically made two types of
computers - word-oriented, large scale scientific computers (such as
the IBM-7094), and character-oriented commercial computers (such as the
IBM-1401), which were really better suited for I/O.
Incompatibility and a
lack of an upgrade path were the problems of the day.
IBM attempted to address both problems with the release of their
System/360, a family of software compatible machines differing only in
capacity, price and performance. The machines had the same architecture
and instruction set.
With heavy CPU-bound scientific calculations, I/O is infrequent, so the
time spent (wasted) waiting was not significant.
However, commercial processing programs in the emerging COBOL
(Computer Oriented Business Organizational Language)
often spent 80-90% of its time waiting for I/O to complete.
The advent of separate I/O processors made simultaneous I/O and CPU
execution possible.
The CPU was multiplexed (shared),
or employed multiprogramming,
amongst a number of jobs - while one job was waiting for I/O from
comparatively slow I/O devices (such as a keyboard or tape), another
job could use the CPU.
Jobs would run until their completion or until they made an I/O
request.
Advantages:
- Interactivity was restored.
- The CPU was kept busy if enough jobs were ready to run.
Disadvantages:
- The computer hardware and the operating system software became
significantly more complex (and there has been no looking back
since!).
CITS2002 Systems Programming, Lecture 3, p11, 29th July 2024.
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CITS2002