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By the time Atanasoff and Berry had built their machine, all of the fundamentals of the modern digital computer were in place. After that, it was just a matter of refinement. The next computers — Colossus, the Manchester SSEM, ENIAC, etc. — didn't introduce any new significant concepts. By their time, the elements of the modern digital computer had been invented. These were, as originally conceived:
During World War II, the Allies were attempting to break the German codes used for military communication. The lion's share of this activity was at Bletchley Park, near London. The Germans created their codes with machines that used patch panels and rotating switches. They assumed the machines created an unbreakable cipher.
The Enigma Cipher Machine
The British at Bletchley Park developed machines to help break the ciphers but these were inadequate for several reasons. They realized that they needed an electronic machine to do the work. One machine, code named "The Bombe", was designed specifically to break the Enigma Code. It used spinning switches to emulate the code wheels of the Enigma machine and electromechanical relays for Boolean functions. The Bombe was not a full-fledged computer. It emulated the Enigma machine and could do nothing else. To break the code of the more complex Lorenz cipher machine—that was used for the highest-level communications—they realized that they needed faster, more reliable switches and more flexibility. Using vacuum tubes for switches and photo multiplier tubes to read holes punched in paper tape, they built a deciphering machine code-named "Colossus". It became operational in December 1943.
A Reconstruction of the Bombe
Colossus was a digital computer, but was not a binary computer. Each vacuum tube was stable at five different voltage levels. Ten states could be held in a pair of tubes thus allowing decimal operations. This cut the number of tubes by about two-thirds and thus saved on electricity consumption (each Collossus machine consumed about a kilowatt of electricity).
Many historical sources describe Colossus as the first programmable general purpose computer. However, the people who built Colossus don't describe it that way[2]. Colossus was "programmable" through a panel of switches and patch cables that allowed the decoding of different ciphers. However, it was not able to step through a series of instructions like a modern computer does.
It is easy to assume that the people who built Colossus built upon the work of those before them. However, they were largely unaware of the advances of information machines and developed their techniques independently.
After the war, the British Prime Minister, Winston Churchill, ordered the complete destruction of all Colossus machines along with their documentation. However, copies of the design documents were recently found in the national archives of the U.S. With help from the original developers, a new Colossus Mark II was built in 2007.
A Reconstruction of Colossus
The Harvard Mark 1, which IBM called the Automatic Sequence Controlled Calculator (ASCC), was an electromechanical automatic digital calculator. It consisted of 78 adding machines and calculators that operated in parallel. It was programmed through punched tape and the output was printed on electric typewriters. It performed non-conditional loops by reading punched tape that was literally glued into a loop. The Mark I was a more like Babbage's design than it's contemporaries and didn't introduce any new technology to computer design. It was completed in early 1944.
The Mark I is most famous for a technician's joke. On the morning of September 9, 1947 a relay failed during a test. The relay module was replaced and work resumed. That afternoon a technician found a dead moth trapped in the relay. He carefully removed the moth with tweezers and taped it to the log book and wrote under it "First actual case of bug being found."
This log book, open to the page in question is now in display at the Smithsonian. This is, however, not the origin of the term "bug". Scientists and inventors used the term as early as the late 1800s. The deed is also often attributed to Grace Hopper. Although Hopper worked with the Mark 1, she said that she wasn't there at the time.
The Electronic Numerical Integrator And Computer (ENIAC) became operational in 1946. Until the mid 1980s ENIAC was considered to be the first general purpose digital computer. For many years, even long after the war, knowledge of Colossus was shrouded in secrecy and the work of Zuse and Atanasoff was virtually forgotten. As knowledge of these machines came to light, credit was given to the earlier machines.
ENIAC originally had to be custom wired for each new task (through banks of switches and patch panels). It is arguable that ENIAC was a general purpose computer since it could be rewired for virtually any task. However, the rewiring completely reconfigured the hardware and took days to accomplish. It didn't have the flexibility of a computer that could change its function by merely changing instructions stored in memory.
ENIAC. The wheeled cabinets with banks of switches are the data tables.
ENIAC was fully electronic, using between 17,000 and 19,000 vacuum tubes (sources disagree on the number). It also used portable data tables consisting of banks of switches in rolling cabinets (comparable to pluggable read-only memory modules used in early game consoles, but much larger). Temporary data storage was accomplished with vacuum tube flip-flops. These were used in units called accumulators that were comparable to the working registers of a modern microprocessor. Other units, called the "gates", performed the Boolean AND operation and yet other units, called the "buffers", performed the Boolean OR function. The accumulators could perform addition and subtraction. Other separate units could do multiplication, division and square root functions. ENIAC could also determine the sign of a number, compare numbers for equality or non-equality and perform a conditional branch.
ENIAC was eventually modified to step through a stored program entered into one of the data tables. Had this been done from the start, ENIAC would have been the first machine to combine all the elements of the modern general purpose computer.
After the war, Max Newman, who led the team that developed Colossus, was appointed to the Chair of Pure Mathematics at Victoria University of Manchester. He was influential in a project to build an experimental computer to test a memory device called the Williams Tube. This was a cathode ray tube that stored electrical charges in a way that could be used to store bits of data. Frederic C. Williams, Chair of Electrical Engineering led the team to build the Small-Scale Experimental Machine (SSEM) that was nick named "the Baby".
A Williams Tube Memory Unit
The SSEM was the first machine that brought all of the fundamentals of the general purpose digital computer into one system. It was fully electronic, using vacuum tubes for switches. It was also the first computer that could step through a stored program and was the first computer that could store instructions and data arbitrarily in any memory location. It could only perform two mathematical functions, subtraction and negation[3], but this is enough to perform any complex mathematical function (for example, negate then subtract becomes addition).
The SSEM was demonstrated in June of 1948, about three months before ENIAC was modified to operate on a stored program. It was never intended to be a useful computer. It was built strictly to test the concept of a stored program and, particularly, to test the Williams Tube memory. Once it was successfully demonstrated, funding became available to use its technology as the basis for the Manchester Mark 1.
On June 30, 1945, the then-classified ENIAC project released an incomplete document written by John von Neumann describing the general design of a computer. This paper describes a stored-program computer that uses the same memory bank to store programs and data. Until then, computers had separate systems to store programming and data. The new concept used in modern computers is now called von Neumann architecture. The old system is called Harvard architecture.
Early computers focused on mathematics. This is what gave rise to the notion that computers are calculating machines rather than information machines. Once computers became affordable to more agencies and mass storage became affordable, computers began to be used to store, search and present non-numeric data. Although a major function of computers was accounting, just as important was managing huge databases. Two major users of computers were banks and insurance companies. Government bureaucracies also became users of computers. The state departments of motor vehicles could store the massive amounts of data required to keep track of driver and automobile licensing. The Social Security Administration and U.S. Census Bureau had equally massive amounts of data to handle. Any organization with the financial resources would obtain computers to handle and distribute data. Numeric processing became only one of the uses of computers.
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| 1 | Mathematical functions are performed through Boolean functions. |
| 2 | Annals of the History of Computing, Volume 5, Number 3, July 1983 . p239 |
| 3 | Changing a number from positive to negative or vice-versa. |
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