Alternate History

IBM 5100 (Caroline Era)

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The IBM 5100, an influential early Microprocessor-based computer introduced in 1975.

The IBM 5100 was an influential early small computer released by IBM in 1975.


This was the first computer to use IBM's seminal and widely-used Microprocessor board. So called because of its use of microcode to reduce the amount of circuitry required, this was one of the first, and certainly the most popular implementations of the idea of implementing an entire CPU on a single board. The CPU was six-bit and shared some op-codes with the IBM 360 instruction set, though it was also optimised for FORTRAN. There was space inside the case for up to 4 Kw of RAM, the ROM being mapped onto a different memory area. There was a 5" CRT-based monitor built in with the option of using an external monitor, which could be switched between black on green and green on black, and a tape drive using IBM's seven track system.


There were three lots of system software, all in ROM: APL, FORTRAN and a terminal emulator. The FORTRAN was a scaled-down version of the mainframe dialects without a number of features and the APL was somewhat hampered by the relatively small address space. The terminal emulator was used to access mainframes and was the beginning of the use of microcomputers to use online services. It could also be used to prepare code for compilation on mainframes and as a text editor. The operating system was a direct copy of DTSS.


Although the 5100 was not particularly successful commercially, it introduced the idea of a small user-friendly computer to a wider public outside the realm of information technology. When IBM offered its Microprocessor chip set for general sale in 1979, a number of manufacturers began to produce their own microcomputers. The Sinclair Ambassador line of domestic computers is typical in sharing various features with the 5100, though these are more obvious in the Ambassador I than later products: it has a built-in CRT monitor and tape drive, a socket for an external monitor, runs DTSS and FORTRAN and can be used as a text editor and terminal. It also uses the Microprocessor chips, though they are only part of the single board which carries the digital hardware.


Sometime in 2002, a man was found attempting to steal a IBM 5100 from IBM's museum in Armonk, NY at 3 am. He was arrested and interviewed by police, and made the claim that he was from a parallel timeline in which information technology was more advanced. Although under normal circumstances this claim would have been regarded as a fanciful story, a device was found on his person who appeared to use advanced digital technology completely beyond what is possible even today. Various theories have been propounded as to the nature of the device and why someone allegedly from a more advanced technology would bother to attempt to obtain a computer which to him would be very primitive. This incident was a state secret and was revealed accidentally in 2010.

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