Emulator in Microcode
The 5100 was based on IBM's innovative concept that, using an emulator written in microcode, a small and relatively cheap computer could run programs already written for much larger, and much more expensive, existing computers, without the time and expense of writing and debugging new programs.
Two such programs were included: a slightly modified version of APL.SV, IBM's APL interpreter for its System/370 mainframes, and the BASIC interpreter used on IBM's System/3 minicomputer. Consequently, the 5100's microcode was written to emulate most of the functionality of both a System/370 and a System/3.
IBM later used the same approach for its 1983 introduction of the XT/370 model of the IBM PC, which was a standard IBM PC XT with the addition of a System/370 emulator card.
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