Editing Atanasoff–Berry computer (section)

==Design and construction==
[[File:Atanasoff Berry Computer.gif|thumb|right|Diagram of the ABC pointing out its various components]]
According to Atanasoff's account, several key principles of the Atanasoff–Berry computer were conceived in a sudden insight after a long nighttime drive to [[Rock Island, Illinois]], during the winter of 1937–38. The ABC innovations included electronic computation, binary arithmetic, [[Parallel computing|parallel processing]], [[regenerative capacitor memory]], and a separation of memory and computing functions.<ref>{{cite web |url=http://mason.gmu.edu/~montecin/computer-hist-web.htm |title=The History of Computing |website=mason.gmu.edu |access-date=6 April 2018}}</ref> The mechanical and logic design was worked out by Atanasoff over the next year. A grant application to build a [[proof of concept]] prototype was submitted in March 1939 to the [[Agronomy]] department, which was also interested in speeding up computation for economic and research analysis. $5,000 of further funding ({{Inflation|US|5000|1939|fmt=eq|r=-3}}) to complete the machine came from the nonprofit [[Research Corporation]] of New York City.{{citation needed|date=July 2016}}

The ABC was built by Atanasoff and Berry in the basement of the physics building at [[Iowa State College]] from 1939 to 1942. The initial funds were released in September, and the 11-tube prototype was first demonstrated in October 1939. A December demonstration prompted a grant for construction of the full-scale machine.<ref>{{Citation | last = Mollenhoff | first = Clark R. | year = 1988 | title = Atanasoff: Forgotten Father of the Computer | pages = [https://archive.org/details/atanasoffforgott0000moll/page/47 47, 48] | isbn = 0-8138-0032-3 | publisher = Iowa State University Press | location = Ames | url = https://archive.org/details/atanasoffforgott0000moll/page/47 }}</ref><ref>{{Cite book |url=https://books.google.com/books?id=v-cdBfPrxfUC |title=The Biographical Dictionary of Iowa |last1=Hudson |first1=David |last2=Bergman |first2=Marvin |last3=Horton |first3=Loren |date=2009 |publisher=University of Iowa Press |isbn=9781587297243 |pages=22 |language=en}}</ref> The ABC was built and tested over the next two years. A January 15, 1941, story in the ''[[Des Moines Register]]'' announced the ABC as "an electrical computing machine" with more than 300 vacuum tubes that would "compute complicated algebraic equations" (but gave no precise technical description of the computer). The system weighed more than {{convert|700|lb|kg|spell=in}}. It contained approximately {{convert|1|mi|km|adj=on}} of wire, 280 dual-triode [[vacuum tube]]s, 31 [[thyratron]]s, and was about the size of a desk.

It was not programmable, which distinguishes it from more general machines of the same era, such as [[Konrad Zuse]]'s 1941 [[Z3 (computer)|Z3]] (or earlier iterations) and the [[Colossus computer]]s of 1943–1945. Nor did it implement the [[stored-program architecture]], first implemented in the [[Manchester Baby]] of 1948, required for fully general-purpose practical computing machines.

[[File:Atanasoff-Berry add-subtract module.agr.jpg|thumb|Add-subtract module (reconstructed) from Atanasoff–Berry computer]]
The machine was, however, the first to implement:
#Using vacuum tubes, rather than wheels, ratchets, mechanical switches, or telephone relays, allowing for greater speed than previous computers
#Using capacitors for memory, rather than mechanical components, allowing for greater speed and density

The memory of the Atanasoff–Berry computer was a system called ''regenerative capacitor memory'', which consisted of a pair of drums, each containing 1600 [[capacitor]]s 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 two 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 arithmetic|fixed-point]] numbers. The electronics of the memory and arithmetic units could store and operate on 60 such numbers at a time (3000&nbsp;bits).

The [[alternating current]] [[utility frequency|power-line frequency]] of 60&nbsp;Hz was the primary clock rate for the lowest-level operations.

The [[arithmetic logic unit|arithmetic logic functions]] were fully electronic, implemented with vacuum tubes. The family of [[logic gate]]s 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. The control logic functions, which only needed to operate once per drum rotation and therefore did not require electronic speed, were electromechanical, implemented with [[relay]]s.

The ALU operated on only [[Serial computer|one bit of each number at a time]]; it kept the carry/borrow bit in a capacitor for use in the next AC cycle.<ref>
John Gustafson.
[http://johngustafson.net/pubs/pub57/ABCPaper.htm "Reconstruction of the Atanasoff-Berry Computer"].
Quote: "the total vacuum tube count was very low: about 300 for the entire machine. Much of this economy is the result of operating on only one bit of each number at a time, keeping the carry/borrow bit in a capacitor for use in the next cycle."
</ref>

Although the Atanasoff–Berry computer was an important step up from earlier calculating machines, it was not able to run entirely automatically through an entire problem. An operator was needed to operate the control switches to set up its functions, much like the electro-mechanical calculators and [[unit record equipment]] of the time. 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. (The largest problem that could be solved without the use of the intermediate output and input was two [[simultaneous equations]], a trivial problem.)

Intermediate results were binary, written onto paper sheets by electrostatically modifying the resistance at 1500 locations to represent 30 of the 50-bit numbers (one equation). 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 decimal, via standard [[IBM]] 80-column [[punched card]]s, and output was decimal, via a front-panel display.

[[File:Display inside of rest stop at mile 100 northbound on I-35 in Iowa, Commemorating the ABC Computer.jpg|thumb|Inside display on [[Interstate 35|I-35]] rest stop 100 north of Des Moines honoring the ABC Computer]]

[[File:Outside wall of rest stop at mile 100 northbound on I-35 in Iowa, Commemorating the ABC Computer.jpg|thumb|Outside display on I-35 rest stop 100 north of Des Moines honoring the ABC Computer]]