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18 CHAPTER 2 / COMPUTER EVOLUTION AND PERFORMANCE

John Mauchly, a professor of electrical engineering at the University of
Pennsylvania, and John Eckert, one of his graduate students, proposed to build a
general-purpose computer using vacuum tubes for the BRL’s application. In 1943,
the Army accepted this proposal, and work began on the ENIAC. The resulting
machine was enormous, weighing 30 tons, occupying 1500 square feet of floor
space, and containing more than 18,000 vacuum tubes. When operating, it con-
sumed 140 kilowatts of power. It was also substantially faster than any electro-
mechanical computer, capable of 5000 additions per second.
The ENIAC was a decimal rather than a binary machine. That is, numbers
were represented in decimal form, and arithmetic was performed in the decimal sys-
tem. Its memory consisted of 20 “accumulators,” each capable of holding a 10-digit
decimal number. A ring of 10 vacuum tubes represented each digit. At any time,
only one vacuum tube was in the ON state, representing one of the 10 digits. The
major drawback of the ENIAC was that it had to be programmed manually by set-
ting switches and plugging and unplugging cables.
The ENIAC was completed in 1946, too late to be used in the war effort. In-
stead, its first task was to perform a series of complex calculations that were used to
help determine the feasibility of the hydrogen bomb. The use of the ENIAC for a
purpose other than that for which it was built demonstrated its general-purpose
nature.The ENIAC continued to operate under BRL management until 1955, when
it was disassembled.

THE VON NEUMANN MACHINE The task of entering and altering programs for the
ENIAC was extremely tedious.The programming process could be facilitated if the
program could be represented in a form suitable for storing in memory alongside
the data.Then, a computer could get its instructions by reading them from memory,
and a program could be set or altered by setting the values of a portion of memory.
This idea, known as the stored-program concept, is usually attributed to the
ENIAC designers, most notably the mathematician John von Neumann, who was a
consultant on the ENIAC project.Alan Turing developed the idea at about the same
time.The first publication of the idea was in a 1945 proposal by von Neumann for a
new computer, the EDVAC (Electronic Discrete Variable Computer).
In 1946, von Neumann and his colleagues began the design of a new stored-
program computer, referred to as the IAS computer, at the Princeton Institute for
Advanced Studies.The IAS computer, although not completed until 1952, is the pro-
totype of all subsequent general-purpose computers.
Figure 2.1 shows the general structure of the IAS computer (compare to mid-
dle portion of Figure 1.4). It consists of
• A main memory, which stores both data and instructions 1
• An arithmetic and logic unit (ALU) capable of operating on binary data

1 In this book, unless otherwise noted, the term instruction refers to a machine instruction that is
directly interpreted and executed by the processor, in contrast to an instruction in a high-level lan-
guage, such as Ada or C++, which must first be compiled into a series of machine instructions before
being executed.