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2.1 / A BRIEF HISTORY OF COMPUTERS 19

Central Processing Unit (CPU)

Arithmetic-
logic
unit (CA)

I/O
Main
memory Equip-
(M) ment
(I, O)
Program
control
unit (CC)

Figure 2.1 Structure of the IAS Computer

• A control unit, which interprets the instructions in memory and causes them to
be executed
• Input and output (I/O) equipment operated by the control unit

This structure was outlined in von Neumann’s earlier proposal, which is worth
quoting at this point [VONN45]:

2.2 First: Because the device is primarily a computer, it will
have to perform the elementary operations of arithmetic most fre-
quently. These are addition, subtraction, multiplication and divi-
sion. It is therefore reasonable that it should contain specialized
organs for just these operations.
It must be observed, however, that while this principle as
such is probably sound, the specific way in which it is realized re-
quires close scrutiny. At any rate a central arithmetical part of the
device will probably have to exist and this constitutes the first spe-
cific part: CA.
2.3 Second: The logical control of the device, that is, the
proper sequencing of its operations, can be most efficiently carried
out by a central control organ. If the device is to be elastic, that is, as
nearly as possible all purpose, then a distinction must be made be-
tween the specific instructions given for and defining a particular
problem, and the general control organs which see to it that these
instructions—no matter what they are—are carried out. The for-
mer must be stored in some way; the latter are represented by def-
inite operating parts of the device. By the central control we mean
this latter function only, and the organs which perform it form the
second specific part: CC.