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Since the rise of the [[personal computer]] in the 1980s, [[IBM]] and other vendors have created '''PC-based IBM-compatible mainframes''' which are compatible with the larger IBM [[mainframe computer]]s. They are also referred to as ''plug-compatible mainframes'', a term used for the original [[System/360]] and [[System/370|370]] compatible clones.
==Background==
Up until the late 1980s, mainframes were very large machines that often occupied entire rooms. The rooms were often air conditioned and had special power arrangements to accommodate the [[three-phase electric power]] required by the machines.
==System/370==
The Personal/370 (aka P/370) is a single slot 32-bit [[Micro Channel architecture|MCA]] card that can be added to a [[IBM Personal System/2|PS/2]] or [[RS/6000]] computer to run System/370 OSs (like [[MUSIC/SP]], [[VM (Operating system)|VM]], [[VSE]]) parallel to [[DOS]] or [[OS/2]] (in PS/2) or [[AIX operating system|AIX]] (in RS/6000) supporting multiple concurrent users. It is a complete implementation of the S/370 Processor including a [[Floating point unit|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.▼
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 PC]]s), 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.[http://www.xt370.net]
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==System/390==
As IBM's mainframes became much more powerful and had bigger instruction sets, IBM's PC-related products evolved as well.
===S/390 Processor Card===
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* The original S/390 Processor Card incorporated 32MB of dedicated memory, with optional 32MB or 96MB daughter cards, for a combined total of 64MB or 128MB of RAM. The processor was officially rated at 4.5 [[Million instructions per second|MIPS]]. It was built to plug into a [[Micro Channel architecture|MicroChannel]] host system.
* The second version was built for a [[Peripheral Component Interconnect|PCI]] host system. It included 128 MB of dedicated memory as standard, and was still rated at 4.5 MIPS.
* The third version, referred to as a P/390E card (for Enhanced), included 256 MB of dedicated memory and was rated at 7 MIPS. It, too, was built for a [[Peripheral Component Interconnect|PCI]] host system. There was an extremely rare (possibly only ever released as pre-production samples)
===R/390===
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