SWAC (computer)

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The SWAC (Standards Western Automatic Computer) was an early electronic digital computer built in 1950 by the U.S. National Bureau of Standards (NBS). It was designed by Harry Huskey. Like the SEAC, built about the same time, the SWAC was a small-scale interim computer designed to be built quickly and put into operation while the NBS waited for more powerful computers to be completed (in particular, the RAYDAC by Raytheon).

The machine used 2300 vacuum tubes. It had 256 words of memory (using Williams tubes), with each word being 37 bits. It had only seven basic operations: add, subtract, and multiply (single precision and double precision versions); comparison, data extraction, input, and output.

When the SWAC was completed in July 1950, it was the fastest computer in the world. It continued to hold that status until the IAS computer was completed a year later. It could add two numbers and store the result in 64 microseconds. A similar multiplication took 384 microseconds. It was used by the NBS until 1954, and then by UCLA until 1967 (with modifications).

In 1952, Raphael M. Robinson used the SWAC to discover five Mersenne primes—the largest prime numbers known at the time, with 157, 183, 386, 664, and 687 digits.