1st Computer – ENIAC

ENIAC was amongst the earliest electronic general-purpose computers made. It was Turing-complete, digital and able to solve “a large class of numerical problems” through reprogramming.

Although ENIAC was designed and primarily used to calculate artillery firing tables for the United States Army‘s Ballistic Research Laboratory, its first programs included a study of the feasibility of the thermonuclear weapon.

ENIAC was formally dedicated at the University of Pennsylvania on February 15, 1946 and was heralded as a “Giant Brain” by the press. It had a speed on the order of one thousand times faster than that of electro-mechanical machines; this computational power, coupled with general-purpose programmability, excited scientists and industrialists alike. This combination of speed and programmability allowed for thousands more calculations for problems, as ENIAC calculated a trajectory that took a human 20 hours in 30 seconds (a 2400× increase in speed).


1st Computer - ENIAC

1st Computer – ENIAC



ENIAC’s design and construction was financed by the United States Army, Ordnance Corps, Research and Development Command, led by Major General Gladeon M. Barnes. The total cost was about $487,000, equivalent to $6,740,000 in 2016. The construction contract was signed on June 5, 1943; work on the computer began in secret at the University of Pennsylvania‘s Moore School of Electrical Engineering the following month, under the code name “Project PX”, with John Grist Brainerd as principal investigator. Herman Goldstine persuaded the Army to fund the project which put him in charge to oversee it for them.

ENIAC was designed by John Mauchly and J. Presper Eckert of the University of Pennsylvania, U.S. The team of design engineers assisting the development included Robert F. Shaw (function tables), Jeffrey Chuan Chu (divider/square-rooter), Thomas Kite Sharpless (master programmer), Frank Mural (master programmer), Arthur Burks (multiplier), Harry Huskey (reader/printer) and Jack Davis (accumulators). In 1946, the researchers resigned from the University of Pennsylvania and formed the Eckert-Mauchly Computer Corporation.

ENIAC was a modular computer, composed of individual panels to perform different functions. Twenty of these modules were accumulators which could not only add and subtract, but hold a ten-digit decimal number in memory. Numbers were passed between these units across several general-purpose buses (or trays, as they were called). In order to achieve its high speed, the panels had to send and receive numbers, compute, save the answer and trigger the next operation, all without any moving parts. Key to its versatility was the ability to branch; it could trigger different operations, depending on the sign of a computed result.


Women with 1st Computer

During World War II an unassuming fraternity house at the University of Pennsylvania was converted into an office building. On the door hung a sign reading: “WOMEN ONLY.”



By the end of its operation in 1956, ENIAC contained 20,000 vacuum tubes, 7200 crystal diodes, 1500 relays, 70,000 resistors, 10,000 capacitors and approximately 5,000,000 hand-soldered joints. It weighed more than 30 short tons (27 t), was roughly 2.4 m × 0.9 m × 30 m (8 × 3 × 100 feet) in size, occupied 167 m2 (1,800 ft2) and consumed 150 kW of electricity. This power requirement led to the rumor that whenever the computer was switched on, lights in Philadelphia dimmed. Input was possible from an IBM card reader and an IBM card punch was used for output. These cards could be used to produce printed output offline using an IBM accounting machine, such as the IBM 405. While ENIAC had no system to store memory in its inception, these punch cards could be used for external memory storage. In 1953, a 100-word magnetic-core memory built by the Burroughs Corporation was added to ENIAC.

ENIAC used ten-position ring counters to store digits; each digit required 36 vacuum tubes, 10 of which were the dual triodes making up the flip-flops of the ring counter. Arithmetic was performed by “counting” pulses with the ring counters and generating carry pulses if the counter “wrapped around,” the idea being to electronically emulate the operation of the digit wheels of a mechanical adding machine.

ENIAC had 20 ten-digit signed accumulators, which used ten’s complement representation and could perform 5000 simple addition or subtraction operations between any of them and a source (e.g., another accumulator or a constant transmitter) every second. It was possible to connect several accumulators to run simultaneously, so the peak speed of operation was potentially much higher, due to parallel operation.

It was possible to wire the carry of one accumulator into another accumulator to perform double precision arithmetic, but the accumulator carry circuit timing prevented the wiring of 3+ for even higher precision. ENIAC used 4 of the accumulators (controlled by a special multiplier unit) to perform up to 385 multiplication operations/second; 5 of the accumulators were controlled by a special divider/square-rooter unit to perform up to 40 division operations/second or 3 square root operations/second.

The other 9 units in ENIAC were the Initiating Unit (started and stopped the machine), the Cycling Unit (used for synchronizing the other units), the Master Programmer (controlled “loop” sequencing), the Reader (controlled an IBM punch-card reader), the Printer (controlled an IBM card punch), the Constant Transmitter and 3 function tables.


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Operation Times

The references by Rojas and Hashagen (or Wilkes) give more details about the times for operations, which differ somewhat from those stated above.

The basic machine cycle was 200 microseconds (20 cycles of the 100 kHz clock in the cycling unit), or 5,000 cycles per second for operations on the 10-digit numbers. In one of these cycles, ENIAC could write a number to a register, read a number from a register, or add/subtract two numbers.

A multiplication of a 10-digit number by a d-digit number (for d up to 10) took d+4 cycles, so a 10- by 10-digit multiplication took 14 cycles, or 2800 microseconds—a rate of 357 per second. If one of the numbers had fewer than 10 digits, the operation was faster.

Division and square roots took 13(d+1) cycles, where d is the number of digits in the result (quotient or square root). So a division or square root took up to 143 cycles, or 28,600 microseconds—a rate of 35 per second. (Wilkes 1956:20 states that a division with a 10 digit quotient required 6 milliseconds.) If the result had fewer than ten digits, it was obtained faster.


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ENIAC used common octal-base radio tubes of the day; the decimal accumulators were made of 6SN7 flip-flops, while 6L7s, 6SJ7s, 6SA7s and 6AC7s were used in logic functions. Numerous 6L6s and 6V6s served as line drivers to drive pulses through cables between rack assemblies.

Several tubes burned out almost every day, leaving ENIAC nonfunctional about half the time. Special high-reliability tubes were not available until 1948. Most of these failures, however, occurred during the warm-up and cool-down periods, when the tube heaters and cathodes were under the most thermal stress. Engineers reduced ENIAC’s tube failures to the more acceptable rate of one tube every two days. According to a 1989 interview with Eckert, “We had a tube fail about every two days and we could locate the problem within 15 minutes.” In 1954, the longest continuous period of operation without a failure was 116 hours—close to five days.


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ENIAC could be programmed to perform complex sequences of operations, including loops, branches, and subroutines. However, instead of the stored program computers that exist today, ENIAC was just a large collection of arithmetic machines, which had programs hard coded into the machines with function tables that each contained 1200 ten way switches. The task of taking a problem and mapping it onto the machine was complex, and usually took weeks. Due to the complexity of mapping programs onto the machine, programs were only changed after huge numbers of tests of the current program. After the program was figured out on paper, the process of getting the program into ENIAC by manipulating its switches and cables could take days. This was followed by a period of verification and debugging, aided by the ability to execute the program step by step. A programming tutorial for the modulo function using an ENIAC simulator gives an impression of what a program on the ENIAC looked like.

ENIAC’s six primary programmers, Kay McNultyBetty JenningsBetty SnyderMarlyn WescoffFran Bilas and Ruth Lichterman, not only determined how to input ENIAC programs, but also developed an understanding of ENIAC’s inner workings. The programmers debugged problems by crawling inside the massive structure to find bad joints and bad tubes.


1st Computer - ENIAC

1st Computer – ENIAC



Kay McNultyBetty JenningsBetty SnyderMarlyn MeltzerFran Bilas, and Ruth Lichterman were the first programmers of the ENIAC, though their work was not widely recognized for over 50 years. At the time, the hardware was seen as the primary innovation and the complexity of programming the machine was undervalued, perhaps because the first programmers were all women. Historians had at first mistaken them for “Refrigerator Ladies”, i.e. models posing in front of the machine. Most of the women did not receive recognition for their work on the ENIAC in their lifetimes. 50 years after the invention of the ENIAC most of the female programmers were not invited to the 50th anniversary event.

These early programmers were drawn from a group of about two hundred female computers who studied at the Moore School of Electrical Engineering at the University of Pennsylvania. One of the few technical job categories available to women was computing the results of mathematical formulas for science and engineering, usually with a mechanical calculator. Betty Holberton (née Snyder) continued on to invent the first sorting algorithm and help design the first commercial electronic computers, the UNIVAC and the BINAC, alongside Jean Jennings.

Herman Goldstine selected the programmers, then called “operators”, from the computers who had been calculating ballistics tables with desk calculators and a differential analyzer prior to and during the development of ENIAC. Under Herman and Adele Goldstine‘s direction, the programmers studied ENIAC’s blueprints and physical structure to determine how to manipulate its switches and cables, rather than learning a programming language. Though contemporaries considered programming a clerical task and did not publicly recognize the programmers’ impact on the successful operation and announcement of ENIAC, McNulty, Jennings, Snyder, Wescoff, Bilas, and Lichterman have since been recognized for their contributions to computing.

Following the initial six programmers, an expanded team of a hundred scientists was recruited to continue work on the ENIAC. Among these were several women, including Gloria Ruth Gordon. Adele Goldstine wrote the original technical description of the ENIAC.


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Role in the Hydrogen Bomb

Although the Ballistic Research Laboratory was the sponsor of ENIAC, one year into this three-year project John von Neumann, a mathematician working on the hydrogen bomb at Los Alamos National Laboratory, became aware of this computer. Los Alamos subsequently became so involved with ENIAC that the first test problem run consisted of computations for the hydrogen bomb, not artillery tables. The input/output for this test was one million cards.



ENIAC Inventors: John W. Mauchly (left), and J. Presper Eckert (right).

ENIAC Inventors: John W. Mauchly (left), and J. Presper Eckert (right).


Role in development of the Monte Carlo Methods

Related to ENIAC’s role in the hydrogen bomb was its role in the Monte Carlo method becoming popular. Scientists involved in the original nuclear bomb development used massive groups of people doing huge numbers of calculations (“computers” in the terminology of the time) to investigate the distance that neutrons would likely travel through various materials. John von Neumann and Stanislaw Ulam realized the speed of ENIAC would allow these calculations to be done much more quickly. The success of this project showed the value of Monte Carlo methods in science.

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