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20 CHAPTER 2 / COMPUTER EVOLUTION AND PERFORMANCE

2.4 Third: Any device which is to carry out long and compli-
cated sequences of operations (specifically of calculations) must
have a considerable memory . . .
(b) The instructions which govern a complicated problem
may constitute considerable material, particularly so, if the code is
circumstantial (which it is in most arrangements). This material
must be remembered.
At any rate, the total memory constitutes the third specific
part of the device: M.
2.6 The three specific parts CA, CC (together C), and M cor-
respond to the associative neurons in the human nervous system. It
remains to discuss the equivalents of the sensory or afferent and the
motor or efferent neurons.These are the input and output organs of
the device.
The device must be endowed with the ability to maintain
input and output (sensory and motor) contact with some specific
medium of this type. The medium will be called the outside record-
ing medium of the device: R.
2.7 Fourth: The device must have organs to transfer . . . infor-
mation from R into its specific parts C and M. These organs form
its input, the fourth specific part: I. It will be seen that it is best to
make all transfers from R (by I) into M and never directly from C.
2.8 Fifth: The device must have organs to transfer . . . from its
specific parts C and M into R.These organs form its output, the fifth
specific part: O. It will be seen that it is again best to make all trans-
fers from M (by O) into R, and never directly from C.

With rare exceptions, all of today’s computers have this same general structure
and function and are thus referred to as von Neumann machines. Thus, it is worth-
while at this point to describe briefly the operation of the IAS computer [BURK46].
Following [HAYE98], the terminology and notation of von Neumann are changed
in the following to conform more closely to modern usage; the examples and illus-
trations accompanying this discussion are based on that latter text.
The memory of the IAS consists of 1000 storage locations, called words, of
2
40 binary digits (bits) each. Both data and instructions are stored there. Numbers
are represented in binary form, and each instruction is a binary code. Figure 2.2
illustrates these formats. Each number is represented by a sign bit and a 39-bit value.
A word may also contain two 20-bit instructions, with each instruction consisting of
an 8-bit operation code (opcode) specifying the operation to be performed and a
12-bit address designating one of the words in memory (numbered from 0 to 999).
The control unit operates the IAS by fetching instructions from memory and
executing them one at a time. To explain this, a more detailed structure diagram is

2 There is no universal definition of the term word. In general, a word is an ordered set of bytes or bits that
is the normal unit in which information may be stored, transmitted, or operated on within a given com-
puter. Typically, if a processor has a fixed-length instruction set, then the instruction length equals the
word length.