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The IBM 5100 was IBM's first attempt at producing a computer that was, kinda-sorta, "personal", a few years before the IBM PC. Released in 1975, it was almost the first "personal computer" from any manufacturer, but the MITS Altair beat it by a few months. Also, unlike the later PC, it was designed to somewhat resemble the older big-iron, big-corporate computers that IBM was used to, so it was rather bulky and expensive as far as personal computers went, though from the standpoint of IBM executives used to large mainframes, it was still a small and cheap computer by their standards. When the IBM PC was released, it was officially model 5150, so it was apparently regarded as being in "the same vein" as the 5100 in some way.
The 5100 (as well as a later version, the 5110) was available in models that supported the BASIC or APL programming languages, including a model with both languages built in and a switch to toggle between them. The Character Encoding was based on EBCDIC, with slightly different characters depending on whether BASIC or APL was selected. Only uppercase letters were displayed, but they were stored in the code positions of lowercase letters. The data storage medium was a tape cartridge, somewhat similar to an 8-Track tape.
The programming languages operated by translating commands into an underlying virtual-machine language which emulated IBM System/360 mainframes and System/34 minis.
Around 2000, somebody named John Titor showed up claiming to be a time traveler from the future, in a post-World-War-III world (that war is supposed to happen in 2015) where, to avoid yet another apocalypse to be triggered in 2038 by the Unix timestamp overflowing, his mission was to retrieve an IBM 5100, supposedly the only computer capable of fixing the future software because of an undocumented feature of the 5100 having to do with emulation of old IBM mainframe systems and transfer of programs and data between them and other platforms. There are some true technical facts in that story; Unix timestamps will indeed overflow in 2038 if the format is kept at 32-bit signed integers (the date range doubles if unsigned integers are used, and goes past the expected life of the universe if it's converted to a 64-bit integer), and this might cause problems with old software at that time (presumably newer software will have upgraded to one of the more capable formats). How this would relate to old IBM mainframes is unclear, as those systems didn't run any form of Unix. The 5100 did, apparently, actually have some undocumented features to do with mainframe emulation, stemming from the use of virtual machines modeled on earlier mainframes, so the machine might be of some use in dealing with archaic mainframe code still in use buried in future infrastructure. At any rate, we don't seem to be on that time traveler's timeline any more, since his future history included the cancellation of all Olympics after 2004, and we've in fact held two since then.