IBM SSEC

The IBM Selective Sequence Electronic Calculator (SSEC) was an electromechanical computer built by IBM. Its design was started in late 1944 and it operated from January 1948 to August 1952. It had many of the features of a stored-program computer, and was the first operational machine able to treat its instructions as data, but it was not fully electronic. Although the SSEC proved useful for several high-profile applications, it soon became obsolete. As the last large electromechanical computer ever built, its greatest success was the publicity it provided for IBM.

During World War II, International Business Machines Corporation (IBM) funded and built an Automatic Sequence Controlled Calculator (ASCC) for Howard H. Aiken at Harvard University. The machine, formally dedicated in August 1944, was widely known as the Harvard Mark I. The President of IBM, Thomas J. Watson Sr., did not like Aiken's press release that gave no credit to IBM for its funding and engineering effort. Watson and Aiken decided to go their separate ways, and IBM began work on a project to build their own larger and more visible machine.

Astronomer Wallace John Eckert of Columbia University provided specifications for the new machine; the project budget of almost $1 million was an immense amount for the time. Francis "Frank" E. Hamilton (1898–1972) supervised the construction of both the ASCC and the SSEC. Robert Rex Seeber Jr. was also hired away from the Harvard group, and became known as the chief architect of the new machine. Modules were manufactured in IBM's facility at Endicott, New York, under Director of Engineering John McPherson after the basic design was ready in December 1945.

The February 1946 announcement of the fully electronic ENIAC energized the project. The new machine, called the IBM Selective Sequence Electronic Calculator (SSEC), was ready to be installed by August 1947. Watson called such machines "calculators" because "computer" then referred to humans employed to perform calculations and he "wanted to convey the message that IBM's machines were not designed to replace people. Rather they were designed to help people, by relieving them of drudgery."

The SSEC was installed on three sides of a room on the ground floor of a building near IBM's headquarters at 590 Madison Avenue in New York City, behind a large window where it was visible to people passing by on the busy street. The space had formerly been occupied by a women's shoe store. The noisy SSEC was sometimes called Poppa by the viewing pedestrians. It was dedicated and first demonstrated to the public on January 27, 1948. A. Wayne Brooke served as the chief electronic engineer for the machine's operation starting in 1950. Herb Grosch, the second person with a Ph.D. hired by IBM, was one of its first programmers. Other early programmers included Edgar "Ted" Codd and John Backus. Elizabeth "Betsy" Stewart was chief operator, and often appeared in publicity photos.

The SSEC was an unusual hybrid of vacuum tubes and electromechanical relays. Approximately 12,500 vacuum tubes were used in the arithmetic unit, control, and its eight (relatively high-speed) registers, which had an access time of less than one millisecond. About 21,400 relays were used for control and 150 lower-speed registers, with an access time of 20 milliseconds. The relay technology was similar to the ASCC, based on technology invented by Clair D. Lake (1888–1958). The arithmetic logic unit of the SSEC was a modified IBM 603 electronic multiplier, which had been designed by James W. Bryce. The bulky tubes were military surplus radar technology, which filled one entire wall. The memory was organized as signed 19-digit decimal numbers. Multiplication was computed with 14 digits in each factor. Most of the quoted 400,000 digit capacity was in the form of reels of punched paper tape.

Addition took 285 microseconds and multiplication 20 milliseconds, making arithmetic operations much faster than the Harvard Mark I. Data that had to be retrieved quickly was held in electronic circuits; the remainder was stored in relays and as holes in three continuous card-stock tapes that filled another wall. A chain hoist was needed to lift the heavy reels of paper into place. The machine read instructions or data from 30 paper tape readers connected to three punches, and another a table look-up unit consisted of another 36 paper tape readers. A punched card reader was used to load data, and results were produced on punched cards or high-speed printers. The 19-digit word was stored on the card stock tape or registers in binary-coded decimal, resulting in 76 bits, with two extra bits for indicating positive or negative sign and parity, while the two side rows were used for sprockets. The familiar 80 columns of IBM punched card technology were recorded sideways as one column of the tape.

Using well-tested technology, the SSEC's calculations were accurate and precise for its time, but one early programmer, John Backus, said "you had to be there the entire time the program was running, because it would stop every three minutes, and only the people who had programmed it could see how to get it running again”. ENIAC co-designer J. Presper Eckert (no relation to the IBM Eckert) called it "some big monstrosity over there that I don't think ever worked right".