ENIAC

By the end of its operation in 1956, ENIAC contained 20,000 vacuum tubes; 7,200 crystal diodes; 1,500 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 ft × 3 ft × 98 ft) in size, occupied 167 m² (1,800 sq ft) and consumed 150 kW of electricity.[19][20] This power requirement led to the rumor that whenever the computer was switched on, lights in Philadelphia dimmed.[21] 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.[22] In 1953, a 100-word magnetic-core memory built by the Burroughs Corporation was added to ENIAC.[23]

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 5,000 simple addition or subtraction operations between any of them and a source (e.g., another accumulator or a constant transmitter) per 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.

Cpl. Irwin Goldstein (foreground) sets the switches on one of ENIAC's function tables at the Moore School of Electrical Engineering. (U.S. Army photo)[24]

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

The other nine 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 three function tables.[25][26]

The references by Rojas and Hashagen (or Wilkes)[16] 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 2,800 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[16] states that a division with a 10 digit quotient required 6 milliseconds.) If the result had fewer than ten digits, it was obtained faster.

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.[27] 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 an interview in 1989 with Eckert, "We had a tube fail about every two days and we could locate the problem within 15 minutes."[28] In 1954, the longest continuous period of operation without a failure was 116 hours—close to five days.

A function table from ENIAC on display at Aberdeen Proving Ground museum.

Kay McNulty, Betty Jennings, Betty Snyder, Marlyn Meltzer, Fran Bilas, and Ruth Lichterman were the first programmers of the ENIAC. They were not, as computer scientist and historian Kathryn Kleiman was once told, "refrigerator ladies", i.e., models posing in front of the machine for press photography.[37] Nevertheless, some of the women did not receive recognition for their work on the ENIAC in their lifetimes.[18]

These early programmers were drawn from a group of about two hundred women employed as computers at the Moore School of Electrical Engineering at the University of Pennsylvania. The job of computers was to produce the numeric result of mathematical formulas needed for a scientific study, or an engineering project. They usually did so with a mechanical calculator. This was one of the few technical job categories available to women at that time.[38] Betty Holberton (née Snyder) continued on to help write the first generative programming system (SORT/MERGE) and help design the first commercial electronic computers, the UNIVAC and the BINAC, alongside Jean Jennings.[39] McNulty developed the use of subroutines in order to help increase ENIAC's computational capability.[40]

Herman Goldstine selected the programmers, whom he called operators, from the computers who had been calculating ballistics tables with mechanical desk calculators, and a differential analyzer prior to and during the development of ENIAC.[18] Under Herman and Adele Goldstine's direction, the computers studied ENIAC's blueprints and physical structure to determine how to manipulate its switches and cables, as programming languages did not yet exist. Though contemporaries considered programming a clerical task and did not publicly recognize the programmers' effect on the successful operation and announcement of ENIAC,[18] McNulty, Jennings, Snyder, Wescoff, Bilas, and Lichterman have since been recognized for their contributions to computing.[41][42][43]

The "programmer" and "operator" job titles were not originally considered professions suitable for women. The labor shortage created by World War II helped enable the entry of women into the field.[18] However, the field was not viewed as prestigious, and bringing in women was viewed as a way to free men up for more skilled labor. For example, the National Advisory Committee for Aeronautics said in 1942, "It is felt that enough greater return is obtained by freeing the engineers from calculating detail to overcome any increased expenses in the computers' salaries. The engineers admit themselves that the girl computers do the work more rapidly and accurately than they would. This is due in large measure to the feeling among the engineers that their college and industrial experience is being wasted and thwarted by mere repetitive calculation".[18]

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.[44] Adele Goldstine wrote the original technical description of the ENIAC.[45]

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.[46] Los Alamos subsequently became so involved with ENIAC that the first test problem run consisted of computations for the hydrogen bomb, not artillery tables.[7] The input/output for this test was one million cards.[47]

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.[48] The success of this project showed the value of Monte Carlo methods in science.[49]

A few months after ENIAC's unveiling in the summer of 1946, as part of "an extraordinary effort to jump-start research in the field",[53] the Pentagon invited "the top people in electronics and mathematics from the United States and Great Britain"[53] to a series of forty-eight lectures given in Philadelphia, Pennsylvania; all together called The Theory and Techniques for Design of Digital Computers—more often named the Moore School Lectures.[53] Half of these lectures were given by the inventors of ENIAC.[54]

ENIAC was a one-of-a-kind design and was never repeated. The freeze on design in 1943 meant that the computer design would lack some innovations that soon became well-developed, notably the ability to store a program. Eckert and Mauchly started work on a new design, to be later called the EDVAC, which would be both simpler and more powerful. In particular, in 1944 Eckert wrote his description of a memory unit (the mercury delay line) which would hold both the data and the program. John von Neumann, who was consulting for the Moore School on the EDVAC, sat in on the Moore School meetings at which the stored program concept was elaborated. Von Neumann wrote up an incomplete set of notes (First Draft of a Report on the EDVAC) which were intended to be used as an internal memorandum—describing, elaborating, and couching in formal logical language the ideas developed in the meetings. ENIAC administrator and security officer Herman Goldstine distributed copies of this First Draft to a number of government and educational institutions, spurring widespread interest in the construction of a new generation of electronic computing machines, including Electronic Delay Storage Automatic Calculator (EDSAC) at Cambridge University, England and SEAC at the U.S. Bureau of Standards.[55]

A number of improvements were made to ENIAC after 1947, including a primitive read-only stored programming mechanism using the function tables as program ROM,[55][56][57] after which programming was done by setting the switches.[58] The idea have been worked out in several variants by Richard Clippinger and his group, on the one hand, and the Goldstines, on the other,[59] and it was included in the ENIAC patent.[60] Clippinger consulted with von Neumann on what instruction set to implement.[55][61][62] Clippinger had thought of a three-address architecture while von Neumann proposed a one-address architecture because it was simpler to implement. Three digits of one accumulator (#6) were used as the program counter, another accumulator (#15) was used as the main accumulator, a third accumulator (#8) was used as the address pointer for reading data from the function tables, and most of the other accumulators (1–5, 7, 9–14, 17–19) were used for data memory.

In March 1948 the converter unit was installed,[63] which made possible programming through the reader from standard IBM cards.[64][65] The "first production run" of the new coding techniques on the Monte Carlo problem followed in April.[63][66] After ENIAC's move to Aberdeen, a register panel for memory was also constructed, but it did not work. A small master control unit to turn the machine on and off was also added.[67]

The programming of the stored program for ENIAC was done by Betty Jennings, Clippinger, Adele Goldstine and others.[68][56][55] It was first demonstrated as a stored-program computer in April 1948,[69] running a program by Adele Goldstine for John von Neumann. This modification reduced the speed of ENIAC by a factor of 6 and eliminated the ability of parallel computation, but as it also reduced the reprogramming time[62][55] to hours instead of days, it was considered well worth the loss of performance. Also analysis had shown that due to differences between the electronic speed of computation and the electromechanical speed of input/output, almost any real-world problem was completely I/O bound, even without making use of the original machine's parallelism. Most computations would still be I/O bound, even after the speed reduction imposed by this modification.

Early in 1952, a high-speed shifter was added, which improved the speed for shifting by a factor of five. In July 1953, a 100-word expansion core memory was added to the system, using binary-coded decimal, excess-3 number representation. To support this expansion memory, ENIAC was equipped with a new Function Table selector, a memory address selector, pulse-shaping circuits, and three new orders were added to the programming mechanism.[55]

The Z3 and Colossus were developed independently of each other, and of the ABC and ENIAC during World War II. Work on the ABC at Iowa State University was stopped in 1942 after John Atanasoff was called to Washington, D.C., to do physics research for the U.S. Navy, and it was subsequently dismantled.[74] The Z3 was destroyed by the Allied bombing raids of Berlin in 1943. As the ten Colossus machines were part of the UK's war effort their existence remained secret until the late 1970s, although knowledge of their capabilities remained among their UK staff and invited Americans. ENIAC, by contrast, was put through its paces for the press in 1946, "and captured the world's imagination". Older histories of computing may therefore not be comprehensive in their coverage and analysis of this period. All but two of the Colossus machine were dismantled in 1945; the remaining two were used to decrypt Soviet messages by GCHQ until the 1960s.[75][76] The public demonstration for ENIAC was developed by Snyder and Jennings who created a demo that would calculate the trajectory of a missile in 15 seconds, a task that would have taken several weeks for a human computer.[40]

For a variety of reasons (including Mauchly's June 1941 examination of the Atanasoff–Berry Computer, prototyped in 1939 by John Atanasoff and Clifford Berry), U.S. Patent 3,120,606 for ENIAC, applied for in 1947 and granted in 1964, was voided by the 1973 decision of the landmark federal court case Honeywell v. Sperry Rand, putting the invention of the electronic digital computer in the public domain and providing legal recognition to Atanasoff as the inventor of the first electronic digital computer.

Detail of the back of a section of ENIAC, showing vacuum tubes

Pieces of ENIAC are held by the following institutions:

• The School of Engineering and Applied Science at the University of Pennsylvania has four of the original forty panels (Accumulator #18, Constant Transmitter Panel 2, Master Programmer Panel 2, and the Cycling Unit) and one of the three function tables (Function Table B) of ENIAC (on loan from the Smithsonian).[77]
• The Smithsonian has five panels (Accumulators 2, 19, and 20; Constant Transmitter panels 1 and 3; Divider and Square Rooter; Function Table 2 panel 1; Function Table 3 panel 2; High-speed Multiplier panels 1 and 2; Printer panel 1; Initiating Unit)[77] in the National Museum of American History in Washington, D.C.[18] (but apparently not currently on display).
• The Science Museum in London has a receiver unit on display.
• The Computer History Museum in Mountain View, California has three panels (Accumulator #12, Function Table 2 panel 2, and Printer Panel 3) and portable function table C on display (on loan from the Smithsonian Institution).[77]
• The University of Michigan in Ann Arbor has four panels (two accumulators, High-speed Multiplier panel 3, and Master Programmer panel 2),[77] salvaged by Arthur Burks.
• The United States Army Ordnance Museum at Aberdeen Proving Ground, Maryland, where ENIAC was used, has Portable Function Table A.
• The U.S. Army Field Artillery Museum in Fort Sill, as of October 2014, had obtained seven panels of ENIAC that were previously housed by The Perot Group in Plano, Texas.[78] There are accumulators #7, #8, #11, and #17;[note 1] panel #1 and #2 that connected to function table #1,[77] and the back of a panel showing its tubes. A module of tubes is also on display.
• The United States Military Academy at West Point, New York, has one of the data entry terminals from the ENIAC.
• The Heinz Nixdorf MuseumsForum in Paderborn, Germany, has three panels (Printer panel 2 and High-speed Function Table)[77] (on loan from the Smithsonian Institution). In 2014 the museum decided to rebuild one of the accumulator panels - reconstructed part has the look and feel of a simplified counterpart from the original machine.[79]