IBM System/4 Pi

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The IBM System/4 Pi is a family of radiation hardened 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 U.S. Navy used a similar variant in the carrier based A-6E/A-6E TRAM medium attack aircraft. The Shuttle was controlled by five AP-101s, 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 name of the system is derived from the fact that the angular measure of a complete sphere (solid angle) is 4π steradians, while the angular measure of a complete circle is 360 degrees; hence System/4 Pi and System/360. This implies that System/4 Pi is a version of the System/360 for the three-dimensional world of avionics.