IBM's SSEC, the First Computer that Can Modify a Stored Program


In January 1948 IBM announced its first large-scale digital calculating machine, the Selective Sequence Electronic Calculator (SSEC). The SSEC was the first computer that could modify a stored program. It featured 12,000 vacuum tubes and 21,000 electromechanical relays.

“IBM's Selective Sequence Electronic Calculator (SSEC), built at IBM's Endicott facility under the direction of Columbia Professor Wallace Eckert and his Watson Scientific Computing Laboratory staff in 1946-47, . . . was moved to the new IBM Headquarters Building at 590 Madison Avenue in Manhattan, where it occupied the periphery of a room 60 feet long and 30 feet wide. . . . [Estimates of the] dimensions of its "U" shape [were] at 60 + 40 + 80 feet, 180 feet in all, (about half a football field!)”

 "Designed, built, and placed in operation in only two years, the SSEC contained 21,400 relays and 12,500 vacuum tubes. It could operate indefinitely under control of its modifiable program. On the average, it performed 14-by-14 decimal multiplication in one-fiftieth of a second, division in one-thirtieth of a second, and addition or subtraction on nineteen-digit numbers in one-thirty-five-hundredth of second... For more than four years, the SSEC fulfilled the wish Watson had expressed at its dedication: that it would serve humanity by solving important problems of science. It enabled Wallace Eckert to publish a lunar ephemeris ... of greater accuracy than previously available... the source of data used in man's first landing on the moon". "For each position of the moon, the operations required for calculating and checking results totaled 11,000 additions and subtractions, 9,000 multiplications, and 2,000 table look-ups. Each equation to be solved required the evaluation of about 1,600 terms — altogether an impressive amount of arithmetic which the SSEC could polish off in seven minutes for the benefit of the spectators" (, accessed 03-24-2010).

The SSEC remained sufficiently influential in the popular view of mainframes that it was the subject of a cartoon by Charles Addams published on the cover of The New Yorker magazine in February 11, 1961, in which the massive machine produced a Valentine's Day card for its elderly woman operator!

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