The Atanasoff-Berry Computer (ABC), constructed between 1939 and 1942 at Iowa State College, is widely regarded as the world's first electronic digital computer. The ABC incorporated forward-looking features such as binary arithmetic, regenerative memory, and separation of memory and computing functions. While not all these ideas were new, the ABC combined more of these innovative concepts than any previous computer. Most significantly, the Atanasoff-Berry computer was the first computer to perform its computations purely electronically (using vacuum tubes), in contrast with the moving parts relied upon by all previous computers.
The ABC was the brainchild of John Atanasoff, a physics professor at Iowa State. Atanasoff's early interests and later research experiences moved him to construct the ABC. As a youngster Atanasoff borrowed his father's slide rule to practice computations. Upon entering college Atanasoff pursued degrees in mathematics and physics. While working on his doctoral thesis in physics, he had to perform extensive calculations. These took some eight weeks of intensive labor using a desktop mechanical calculator, a tedious and error-prone effort. The difficulties Atanasoff encountered started him thinking about an automated machine to execute complex calculations.
After obtaining his Ph.D. in physics in 1930, Atanasoff accepted a position at Iowa State College (now Iowa State University). In the course of supervising graduate students in their research, Atanasoff again encountered the need for an automated computing machine, particularly with regard to solving systems of differential equations. By the early 1930s Atanasoff was considering how he might design and build a computer, identifying two sorts of computing machines: analog and digital. The fundamental idea behind an analog computer (a term which Atanasoff is supposed to have coined) is that some physical property of an object or system can be used as an analogy to a property of something else. For instance, the compression of a spring-operated bathroom scale, indicated by a graduated dial, corresponds to a person's weight pressing on the scale; the spring's degree of compression thus is an analog of a person's weight. While at Iowa State Atanasoff constructed analog computers, but he concluded they were too task-specific and would not possess the necessary precision for the computations he had in mind.
By the mid-1930s Atanasoff opted to construct a digital, rather than analog, computer. There were many aspects of this new computer to be considered: the input/output mechanism for data; digital calculation via moving parts (like an old-fashioned adding machine) versus calculation by electronics (e.g., vacuum tubes); and more. Atanasoff struggled with these problems for several years. This struggle reached a climax in the winter of 1937. Out of sheer frustration, Atanasoff aimlessly drove one evening to clear his mind. He eventually crossed from Iowa into Illinois, at which point he stopped at a roadhouse. After relaxing with a few drinks, he was able to picture the form of his machine. His digital computer would perform arithmetic in the base 2 (binary) number system, would possess regenerative memory, incorporate logic circuits and parallel processing, and have separate memory and computing functions.
In 1939 Atanasoff recruited electrical engineering student Clifford Berry and was ready to proceed. The two men constructed a prototype machine to test various hardware schemes, then began building the Atanasoff-Berry computer.
The ABC was designed to accept numerical input in decimal form, convert it into binary form, perform the appropriate arithmetic computations, and then convert back from binary to digital format. Input to and output from the ABC was accomplished with stiff paper cards. Data was represented by the absence or presence of holes in the cards. Holes were created using a 5,000-volt spark. The "punched" cards were then read electrically. The peak input/output rate of the ABC was a surprisingly fast 1,800 bits per second.
Atanasoff decided he needed numbers with 15 decimal places of precision, which translates to a binary representation of roughly 50 bits per number. As the ABC performed calculations using these 50-bit numbers, the results would be stored in what Atanasoff called "fast memory" (today called "main" or "primary" memory). Atanasoff considered using mechanical relay switches, magnetic-core memory, vacuum tubes, and charged capacitors for fast memory storage, ultimately choosing capacitors on a cost-versus-performance basis. These "memory" capacitors were placed on the surfaces of two rotating drums, each possessing 32 rings of 50 capacitors each. The result was a "fast" memory storing 3,200 bits. The drums completed one revolution per second and supplied the timing for the entire system. The capacitors required periodic recharging to maintain their configuration, similar to the "refresh" required by today's dynamic RAM (DRAM) chips, which also store information on capacitors (albeit far smaller ones).
The ABC performed calculations electronically using 210 vacuum tubes arranged in 30 add/subtract units. Parallel processing resulted in 30 simultaneous additions or subtractions being performed each second. Multiplication was performed by repeatedly shifting and adding, requiring 15 one-second cycles. The peak speed of the ABC averaged 3.75 calculations per second.
Atanasoff estimated that a person working by hand would require at least 8 hours to solve 8 simultaneous equations (i.e., 8 equations in 8 unknowns; all numbers to be 15 digits or less). The ABC would be able to perform about 15,000 multiplications and 15,000 additions in 8 hours, sufficient to solve 35 simultaneous equations.
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