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The Atanasoff-Berry computer (ABC) was the first automatic electronic digital computer, an early electronic digital computing device that has remained somewhat obscure. The ABC's priority is debated among historians of computer technology, because it was neither programmable, nor Turing-complete.

Atanasoff and Berry's computer work was not widely known until it was rediscovered in the 1960s, amidst conflicting claims about the first instance of an electronic computer. At that time ENIAC, that had been created by John Mauchly and J. Presper Eckert, was considered to be the first computer in the modern sense, but in 1973 a U.S. District Court invalidated the ENIAC patent and concluded that the ENIAC inventors had derived the subject matter of the electronic digital computer from Atanasoff (see Patent dispute). When, in the mid-1970s, the secrecy surrounding the British World War II development of the Colossus computers that pre-dated ENIAC, was lifted and Colossus was described at a conference in Los Alamos, New Mexico in June 1976, John Mauchly and Konrad Zuse were reported to have been astonished.

The ABC was built by Atanasoff and Berry in the basement of the physics building at Iowa State College during 1939–42. 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. 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 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.

The memory of the Atanasoff-Berry Computer was a system called regenerative capacitor memory, which consisted of 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 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 numbers. The electronics of the memory and arithmetic units could store and operate on 60 such numbers at a time (3000 bits).

The arithmetic 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. The control logic functions, which only needed to operate once per drum rotation and therefore did not require electronic speed, were electromechanical, implemented with relays.

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.)

The ABC was designed for a specific purpose, the solution of systems of simultaneous linear equations. It could handle systems with up to twenty-nine equations, a difficult problem for the time. Problems of this scale were becoming common in physics, the department in which John Atanasoff worked. The machine could be fed two linear equations with up to twenty-nine variables and a constant term and eliminate one of the variables. This process would be repeated manually for each of the equations, which would result in a system of equations with one fewer variable. Then the whole process would be repeated to eliminate another variable.

George W. Snedecor, the head of Iowa State's Statistics Department, was very likely the first user of an electronic digital computer to solve real-world mathematics problems. He submitted many of these problems to Atanasoff.

The original ABC was eventually dismantled in 1948, when the University converted the basement to classrooms, and all of its pieces except for one memory drum were discarded.

In 1997, a team of researchers led by John Gustafson from Ames Laboratory (located on the Iowa State campus) finished building a working replica of the Atanasoff¥Berry Computer at a cost of $350,000 (equivalent to $546,000 in 2018). The replica ABC is now on permanent display in the first floor lobby of the Durham Center for Computation and Communication at Iowa State University. As of May 2012, it is on loan to the Computer History Museum in Mountain View, California for a major exhibition.