Atanasoff Berry Computer

From Academic Kids

Atanasoff Berry Computer is the name given, long after the fact, to an experimental machine for solving systems of simultaneous linear equations, developed in 1938-42 at Iowa State University by Dr. John Vincent Atanasoff and Clifford E. Berry. It is sometimes referred to by its initials, ABC.

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The Atanasoff-Berry Computer, constructed in the basement of the Physics building at Iowa State University, took over two years to complete due to lack of funds. The prototype was first demonstrated in November of 1939. The computer weighed more than seven hundred pounds (320 kg). It contained approximately 1 mile (1.6 km) of wire, 280 dual-triode vacuum tubes, 31 thyratrons, and was about the size of a desk.

Because of the machine's innovative use of electronics for arithmetical calculation, it has been described as the first "electronic digital computer". However, it was not a stored program machine, which distinguishes it from later, more general machines, such as the Z3, ENIAC, the Harvard Mark I, EDVAC, the University of Manchester designs, or Alan Turing's post-War designs at NPL and elsewhere.

The machine was, however, the first to implement three ideas that are still part of every modern computer:

  1. Using binary digits to represent all numbers and data
  2. Performing all calculations using electronics rather than wheels, ratchets, or mechanical switches
  3. Organizing a system in which computation and memory are separated.

In addition, the computer pioneered the use of regenerative capacitor memory, as in the DRAM still widely used today.

The machine was seen by John Mauchly in 1941, and is alleged to have influenced his later work on ENIAC. Mauchly denied this, but it was the basis for a court decision invalidating the ENIAC patents.

This court case was brought by Honeywell against Sperry Rand in 1967. The ENIAC system was found to be derived from the ABC and the patents were invalidated. The court released its final judgement on October 19, 1973. The decision was not appealed.

The memory of the Atanasoff-Berry Computer was a pair of drums, each containing 1600 capacitors that rotated on a common shaft once per second. The capacitors on each drum were organized into 32 "bands" of 50 (30 active bands and 2 spares in case a capacitor failed), giving the machine a speed of 30 additions/subtractions per second. Data was represented as 50-bit binary fixed point numbers. The electronics of the memory and arithmetic units could store and operate on 60 such numbers at a time (3000 bits).

The AC power line frequency of 60 Hz was the primary clock rate for the lowest level operations.

The logic functions were fully electronic, implemented with vacuum tubes. The family of logic gates ranged from inverters to two and three input gates. The input and output levels and operating voltages were compatible between the different gates. Each gate consisted of one inverting vacuum tube amplifier, preceded by a resistor divider input network that defined the logical function.

Although the Atanasoff-Berry Computer was an important step up from earlier computing machines, it was not the first stored program computer. An operator was needed to operate the control switches to set up its program, much the way boot programs would be entered in later computers. Selection of the operation to be performed, reading, writing, converting to or from binary to decimal, or reducing a set of equations was made by front panel switches and in some cases jumpers.

There were two forms of input and output. Primary user input and output and an intermediate results output and input. The intermediate results storage allowed operation on problems too large to be handled entirely within the electronic memory.

Intermediate results were written onto paper sheets by electrostatically modifying the resistance at 1500 locations to represent 30 of the 50 bit numbers. Each sheet could be written or read in one second. The reliability of the system was limited to about 1 error in 100,000 calculations by these units, primarily attributed to lack of control of the sheets' material characteristics. In retrospect a solution could have been to add a parity bit to each number as written. This problem was not solved by the time Atanasoff left the university for war-related work.

Primary user input was via standard punched cards and output via a front panel display.

The ABC was designed for a fairly specific purpose, the solution of systems of simultaneous linear equations. It could handle systems with 29 equations, which was large for the time. Problems of this scale were becoming common in physics, the department in which John Atanasoff worked. The initial funds to start development and demonstrate the circuits involved was from the Agronomy department which was also interersted in such problems for economic and research analysis. Further funding to complete the machine came from Research Corporation of America, in New York.


Anthony Ralston and Edwin D. Reilly (ed), Encyclopedia of Computer Science, 3rd Ed. , 1993, Van Nostrand Reinhold, New York ISBN 0442276796

Clark R. Mollenhoff, Atanasoff: Forgotten Father of the Computer, 1988, ISBN 0-8138-0032-3

See also: History of computing hardware

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