Reprogramming ENIAC involved a hike (part 1)

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(Once the army agreed to fund ENIAC, Mauchly and Eckert worked around the clock, seven days a week, hoping to complete the machine in time to contribute to the war. Their war-time effort was so intense that most days they ate all 3 meals in the company of the army Captain who was their liaison with their military sponsors. They were allowed a small staff but soon observed that they could hire only the most junior members of the University of Pennsylvania staff because the more experienced faculty members knew that their proposed machine would never work.
One of the most obvious problems was that the design would require 18,000 vacuum tubes to all work simultaneously. Vacuum tubes were so notoriously unreliable that even twenty years later many neighborhood drug stores provided a "tube tester" that allowed homeowners to bring in the vacuum tubes from their television sets and determine which one of the tubes was causing their TV to fail. And television sets only incorporated about 30 vacuum tubes. The device that used the largest number of vacuum tubes was an electronic organ: it incorporated 160 tubes. The idea that 18,000 tubes could function together was considered so unlikely that the dominant vacuum tube supplier of the day, RCA, refused to join the project (but did supply tubes in the interest of "wartime cooperation"). Eckert solved the tube reliability problem through extremely careful circuit design. He was so thorough that before he chose the type of wire cabling he would employ in ENIAC he first ran an experiment where he starved lab rats for a few days and then gave them samples of all the available types of cable to determine which they least liked to eat. Here's a look at a small number of the vacuum tubes in ENIAC:


The Zuse Z1 in its residential setting

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use's third machine, the Z3, built in 1941, was probably the first operational, general-purpose, programmable (that is, software controlled) digital computer. Without knowledge of any calculating machine inventors since Leibniz (who lived in the 1600's), Zuse reinvented Babbage's concept of programming and decided on his own to employ binary representation for numbers (Babbage had advocated decimal). The Z3 was destroyed by an Allied bombing raid. The Z1 and Z2 met the same fate and the Z4 survived only because Zuse hauled it in a wagon up into the mountains. Zuse's accomplishments are all the more incredible given the context of the material and manpower shortages in Germany during World War II. Zuse couldn't even obtain paper tape so he had to make his own by punching holes in discarded movie film. Because these machines were unknown outside Germany, they did not influence the path of computing in America. But their architecture is identical to that still in use today: an arithmetic unit to do the calculations, a memory for storing numbers, a control system to supervise operations, and input and output devices to connect to the external world. Zuse also invented what might be the first high-level computer language, "Plankalkul", though it too was unknown outside Germany.

The title of forefather of today's all-electronic digital computers is usually awarded to ENIAC, which stood for Electronic Numerical Integrator and Calculator. ENIAC was built at the University of Pennsylvania between 1943 and 1945 by two professors, John Mauchly and the 24 year old J. Presper Eckert, who got funding from the war department after promising they could build a machine that would replace all the "computers", meaning the women who were employed calculating the firing tables for the army's artillery guns. The day that Mauchly and Eckert saw the first small piece of ENIAC work, the persons they ran to bring to their lab to show off their progress were some of these female computers (one of whom remarked, "I was astounded that it took all this equipment to multiply 5 by 1000").

ENIAC filled a 20 by 40 foot room, weighed 30 tons, and used more than 18,000 vacuum tubes. Like the Mark I, ENIAC employed paper card readers obtained from IBM (these were a regular product for IBM, as they were a long established part of business accounting machines, IBM's forte). When operating, the ENIAC was silent but you knew it was on as the 18,000 vacuum tubes each generated waste heat like a light bulb and all this heat (174,000 watts of heat) meant that the computer could only be operated in a specially designed room with its own heavy duty air conditioning system. Only the left half of ENIAC is visible in the first picture, the right half was basically a mirror image of what's visible.

Two views of ENIAC: the "Electronic Numerical Integrator and Calculator" (note that it wasn't even given the name of computer since "computers" were people) [U.S. Army photo]

To reprogram the ENIAC you had to rearrange the patch cords that you can observe on the left in the prior photo, and the settings of 3000 switches that you can observe on the right. To program a modern computer, you type out a program with statements like:

    Circumference = 3.14 * diameter

To perform this computation on ENIAC you had to rearrange a large number of patch cords and then locate three particular knobs on that vast wall of knobs and set them to 3, 1, and 4.


Two views of the code-breaking Colossus of Great Britain

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The Harvard Mark I, the Atanasoff-Berry computer, and the British Colossus all made important contributions. American and British computer pioneers were still arguing over who was first to do what, when in 1965 the work of the German Konrad Zuse was published for the first time in English. Scooped! Zuse had built a sequence of general purpose computers in Nazi Germany. The first, the Z1, was built between 1936 and 1938 in the parlor of his parent's home.


The Atanasoff-Berry Computer

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Another candidate for granddaddy of the modern computer was Colossus, built during World War II by Britain for the purpose of breaking the cryptographic codes used by Germany. Britain led the world in designing and building electronic machines dedicated to code breaking, and was routinely able to read coded Germany radio transmissions. But Colossus was definitely not a general purpose, reprogrammable machine. Note the presence of pulleys in the two photos of Colossus below:


The DEC PDP-12

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Sure looks "mini", huh? But we're getting ahead of our story.

One of the earliest attempts to build an all-electronic (that is, no gears, cams, belts, shafts, etc.) digital computer occurred in 1937 by J. V. Atanasoff, a professor of physics and mathematics at Iowa State University. By 1941 he and his graduate student, Clifford Berry, had succeeded in building a machine that could solve 29 simultaneous equations with 29 unknowns. This machine was the first to store data as a charge on a capacitor, which is how today's computers store information in their main memory (DRAM or dynamic RAM). As far as its inventors were aware, it was also the first to employ binary arithmetic. However, the machine was not programmable, it lacked a conditional branch, its design was appropriate for only one type of mathematical problem, and it was not further pursued after World War II. It's inventors didn't even bother to preserve the machine and it was dismantled by those who moved into the room where it lay abandoned.


An integrated circuit

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The primary advantage of an integrated circuit is not that the transistors (switches) are miniscule (that's the secondary advantage), but rather that millions of transistors can be created and interconnected in a mass-production process. All the elements on the integrated circuit are fabricated simultaneously via a small number (maybe 12) of optical masks that define the geometry of each layer. This speeds up the process of fabricating the computer -- and hence reduces its cost -- just as Gutenberg's printing press sped up the fabrication of books and thereby made them affordable to all.

The IBM Stretch computer of 1959 needed its 33 foot length to hold the 150,000 transistors it contained. These transistors were tremendously smaller than the vacuum tubes they replaced, but they were still individual elements requiring individual assembly. By the early 1980s this many transistors could be simultaneously fabricated on an integrated circuit. Today's Pentium 4 microprocessor contains 42,000,000 transistors in this same thumbnail sized piece of silicon.

It's humorous to remember that in between the Stretch machine (which would be called a mainframe today) and the Apple I (a desktop computer) there was an entire industry segment referred to as mini-computers such as the following PDP-12 computer of 1969: