IBM arguably introduced PC-based mainframe-compatible systems even prior to the company's famous 1981 introduction of the IBM Personal Computer. At the very least, IBM borrowed heavily from System/370 processor architecture to build some early PC-like systems. One example is the IBM 5100, which used microcode to execute many of the System/370's processor instructions in order to run a slightly modified version of IBM's APL.SV mainframe program interpreter.
In October, 1983, barely two years after the introduction of the IBM PC, IBM announced the IBM Personal Computer XT/370. Not to be confused with the IBM 5271 or 5281 product line (the IBM 3270 PCs), the XT/370 actually had a small 370-class processor installed in one of the machine's card slots. The processor card contained three main integrated circuits: a custom System/370 subset processor, a Motorola MC68000R support processor, and a floating point processor. Another card installed in a second slot and attached via ribbon cable provided the memory (512 KB) for the processor card, separate memory from the PC's own. This machine was able to run CMS, specifically a single user version of CMS called VM/PC.[1]
Later, IBM introduced the Personal/370 (aka P/370), a single slot 32-bit MCA card that can be added to a PS/2 or RS/6000 computer to run System/370 OSs (like MUSIC/SP, VM, VSE) parallel to DOS or OS/2 (in PS/2) or AIX (in RS/6000) supporting multiple concurrent users. It is a complete implementation of the S/370 Processor including a FPU co-processor and 16 MB memory. Management and standard I/O channels are provided via the host OS/hardware. An additional 370 channel card can be added to provide mainframe-specific I/O such as 3270 local control units, 3400/3480 tape drives or 7171 protocol converters.
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