Posts by kenshirriff@oldbytes.space
 (DIR) Post #B4ky6xjEjcoX6lODXU by kenshirriff@oldbytes.space
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       In 1967, IBM introduced the System/4 Pi line of aerospace computers, packing mainframe performance into a compact box. 4 Pi computers powered everything from military aircraft to the Space Shuttle to sonar systems on submarines. Thread...
       
 (DIR) Post #B4ky6yGYjjT8m6ypTk by kenshirriff@oldbytes.space
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       IBM's 4 Pi computers were built from simple TTL integrated circuits and used magnetic core memory for storage. "Flat-pack" integrated circuits on multi-layer circuit boards provided dense logic. The board below was round to fit inside a tactical missile, but most of the computers used standardized rectangular boards called "pages".
       
 (DIR) Post #B4ky6ywOECvgrqYEgC by kenshirriff@oldbytes.space
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       The Space Shuttle used a more advanced System/4 Pi computer called the AP-101B. A separate computer, the I/O Processor, provided 24 data buses to all parts of the Shuttle. These boxes still used individual TTL chips and magnetic core memory.
       
 (DIR) Post #B4ky6zRENXbEPUyrke by kenshirriff@oldbytes.space
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       In 1991, the Space Shuttle upgraded its computer to the AP-101S, replacing two 60-pound boxes with one box. Since there were five computers on the Shuttle, this change freed up 300 pounds for additional cargo. The CPU was implemented in three boards full of chips, one below. These boards were much larger than the earlier 4 Pi boards.
       
 (DIR) Post #B4ky6ztEhQ07oM5EP2 by kenshirriff@oldbytes.space
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       There were many more models of System/4 Pi computers, including refrigerator-sized computers used on submarines; see my article for details: https://www.righto.com/2026/03/ibm-4-pi-computer-history.html
       
 (DIR) Post #B4ky70TOWyvNcV06lM by kenshirriff@oldbytes.space
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       I forgot to explain the 4 Pi name. IBM's System/360 mainframes got their name because they supported a full 360º circle of applications, from business to scientific. The 4 Pi aerospace computers, however, were designed for the 3-D world, a "sphere" of applications. While a circle has 360º, a sphere has 4π steradians of solid angle.
       
 (DIR) Post #B6pYx96CeMUBNSw38q by kenshirriff@oldbytes.space
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       In the 1980s, if you wanted your IBM PC to be faster for spreadsheets or CAD, you could buy the Intel 8087 floating-point chip. Internally, the chip runs special code called microcode. We opened the chip, extracted the microcode, and are figuring out how it works. 1/n
       
 (DIR) Post #B6pYxAF6OlCMvM6feS by kenshirriff@oldbytes.space
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       A microscope reveals the complex circuitry of the 8087 chip. The microcode ROM in the center holds the micro-instructions. It executes these micro-instructions to add, subtract, compute square roots, arctangents, and so forth. 2/n
       
 (DIR) Post #B6pYxAjaZPaKRuN1Ae by kenshirriff@oldbytes.space
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       A close-up of the microcode ROM shows the individual bits. Unlike most ROMs, the 8087 uses four transistor sizes, letting it store two bits per transistor. This complex, semi-analog approach was necessary to fit all the data into the chip; the 8087 pushed the limits of 1980 technology. 3/n
       
 (DIR) Post #B6pYxBJ2RbwQDqxKQS by kenshirriff@oldbytes.space
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       We are reverse-engineering the meaning of the 8087's microcode. For more details, see my latest article:https://www.righto.com/2026/05/microcode-inside-intel-8087-floating.html
       
 (DIR) Post #B6pYxBq0T2JRs6NeoS by kenshirriff@oldbytes.space
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       I did this work with the Opcode Collective, especially Smartest Blob and Gloriouscow.The full ROM image: https://8087.martypc.net/#dzi=tiles%2Ftiles.dziA description of the 8087 microcode: https://github.com/a-mcego/granite/blob/main/tools/8087mc/bin/8087.md