Page 21 - Studio-International-January-1974

instruction it must first run a programme of its
own to translate it into its own numerical code.
A single line of code — a 'statement' — in any
higher-level language will normally break down
into a large number of machine instructions,
and these are executed electronically, literally
by switching electrical currents, with
consequent speeds measured in millionths of a
second per instruction.
Yet the computer's phenomenal speed is
probably less significant in accounting for its
versatility than the fact that it can break down
any user's programme into the same instruction
set. While the machine is running a user's
programme it can't do anything else, so that you
might say the machine is identified by the
programme. But it can take on a new identity
in the time it takes to clear one programme from
memory and load a new one, and in a single day
a moderately sized computer installation may
run a thousand different programmes. A
thousand different tasks, a thousand 'different'
machines.
The man-machine relationship I am
describing here is a very curious one, and not
quite like any other I can think of. Nor is it
possible to deal meaningfully with questions
relating to what the machine can do except in
terms of that relationship. It is true that the
machine can do nothing not determined by the
user's programme; that the programme
literally gives the machine its identity. But it is
true also that once it has been given that
identity, it functions as independently and as
autonomously as if it had been built to perform
that task and no other. Whatever is being done,
it is being done by the machine.
When we talk of the computer doing
something, it is implied that it is doing it, or
controlling the doing of it, in the outside world.
For the computer this outside world consists of
any or all of a large number of special purpose
devices to which it may be connected through
The Central Processing Unit is responsible Figure 3 its Input/Output Unit, varying widely in their
for moving these words around, and for `Binary' counting is illustrated here by hand, using functions from typing or punching cards, to
each successive finger in its 'on' or 'off' positions to
performing certain operations upon them. count successive powers of two. The total is given in monitoring heart beats or controlling flow-
Ingeniously, it knows from the words themselves each case by adding the 'on' fingers together valves. Some of these 'peripheral' devices serve
what it is to do, since several bits of each word the computer in the very direct sense that they
are actually reserved for instruction codes. provide communication channels to the user,
Thus part 'A' of a word might tell the CPU, allowing him both to get his programme into the
`put the number shown in part "B" into machine and receive its response to it. The
memory'; or, 'get the number which is in the ubiquitous teletype, and its many more
cell in memory specified by the number in part sophisticated modern equivalents, serve both
"B" '; or, 'add the number in part "B" to the needs : combinations of punched-card reader
number you are now holding, and put the result and line-printer, or paper-tape reader and
back in memory'. A machine might recognize punch, do the same. Several peripherals
and act upon as many as fifty or sixty such function as extra memory for the machine, but
instructions, but in fact most of them will be then memory simply means storage, and a deck
concatenations of simpler instructions, like 'add', of punched cards, or a punched paper tape, is as
`subtract', 'multiply', 'divide', 'compare', 'move much a storage medium as is magnetic tape or
this into memory', 'move this out of memory'. the more recently developed magnetic disc.
The user sees nothing of all this going on. Figure 4 Once a programme has been entered via the
This memory module taken from the Hewlett
Sitting in the outside world, the set of Packard 2 too A computer illustrates the development teletype or the card reader, the computer can
instructions he composes for the machine will of miniaturization in recent technology. The permanently record it in any of these media, and
almost certainly be written in a 'higher level' module holds 800o sixteen-bit words — 128,000 reload it from them when required to do so.
switches in all. The switches are minute
language, like Fortran or Algol, and before the doughnut-shaped ferrite 'cores' strung on wires. Obviously, these media can be used also for
machine can execute that programme of Courtesy Hewlett Packard storing large quantities of information.
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