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The IBM System/4 Pi is a family of avionics computers used, in various versions, on the B-52 bomber, the F-15 fighter, NASA's Skylab space station, and the Space Shuttle, as well as other aircraft. It descends from the System/360 mainframe family of computers.
The top-of-the-line 4 Pi is the AP-101, used in the F-15 and B-52. The Shuttle was controlled by five AP-101's, four of which were arranged in a redundant configuration, with the fifth as backup. Skylab employed the model TC-1, which had a 16-bit word length, in contrast to the AP-101's 32 bits.
The origin of the name is interesting: The angular measure of a complete circle is 360 degrees. The angular measure of a complete sphere (solid angle) is 4π steradians. Hence the System/4 Pi is a version of the System/360 for the three-dimensional world of avionics.
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