PC-based IBM-compatible mainframes

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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 computers. They are also referred to as plug-compatible mainframes, a term used for the original System/360 and 370 compatible clones, although "plug-compatible" is not often strictly accurate since frequently such systems do not support direct attachment of mainframe peripherals. For a period of time PC-based mainframe-compatible systems had a lower price and did not require as much electricity or floor space. However, they sacrificed performance and were not as dependable as mainframe-class hardware. These products have been popular with mainframe developers, in education and training settings, for very small companies with non-critical processing, and in certain disaster relief roles (such as field insurance adjustment systems for hurricane relief).

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[edit] Background

Up until the mid 1990's, 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. Modern mainframes are now physically comparatively small and require little or no special building arrangements.

[edit] System/370

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.

[edit] System/390

As IBM's mainframes became much more powerful and had bigger instruction sets, IBM's PC-related products evolved as well.

[edit] S/390 Processor Card

An important goal in the design of the S/390 Processor Card was complete compatibility with existing mainframe operating systems and software. The processor implements all of the ESA/390 and XA instructions which prevents the need for instruction translation. There are three generations of the card:

  • 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 MIPS. It was built to plug into a MicroChannel host system.
  • The second version was built for a 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 PCI host system. There was an extremely rare (possibly only ever released as pre-production samples) 1 GB memory version of the P/390E card.

[edit] R/390

R/390 was the designation used for the expansion card used in an IBM RS/6000 server. The original R/390 featured a 67 MHz or 77 MHz POWER 2 processor and 32MB to 512MB of RAM, depending on the configuration. The MCA P/390 expansion card can be installed in any MCA RS/6000 system, while the PCI P/390 card can be installed in a number of early PCI RS/6000s; all such configurations are referred to as an R/390. R/390 servers require AIX version 4 to be run as the host operating system.

[edit] P/390

P/390 was the designation used for the expansion card used in an IBM PC Server and was less expensive than the R/390. The original P/390 server was housed in an IBM PC Server 500 and featured a 90 MHz Intel Pentium processor for running OS/2. The model was revised in mid-1996 and rebranded as the PC Server 520, which featured a 133 MHz Intel Pentium processor. Both models came standard with 32MB of RAM and were expandable to 256MB. The PC Server 500 featured eight MCA expansion slots while the PC Server 520 added two PCI expansion slots and removed two MCA slots.

[edit] S/390 Integrated Server

The S/390 Integrated Server (aka S/390 IS) is a mainframe housed in a comparably small case (HxWxD are 82 x 52 x 111 cm). It became available from November 1998. It is intended for customers who do not require the I/O bandwidth and performance of the S/390 Multiprise 3000 (which has the same size). Only 256 MB of ECC Memory and a single CMOS main processor (performance about 8 MIPS) are used. A Pentium II is used as IOSP (I/O Service Processor). It supports four ESCON and to four parallel channels. Standard PCI and ISA slots are present. A maximum of 255 GB internal harddisks are supported (16x 18GB HDs, with 2x HDs for redundancy). The supported OSs are OS/390, MVS/ESA, VM/ESA and VSE/ESA.

[edit] z/Architecture and Today

Since the late 1990s, PC processors have become fast enough to perform mainframe emulation without the need for a peripheral card. One of the most popular PC-based IBM-compatible mainframe products as of 2006 is Fundamental Software's FLEX-ES product. FLEX-ES emulates both System/390 (ESA/390) and z/Architecture. While FLEX-ES is capable of running on most PC hardware, the licensing agreement prohibits it. FLEX-ES must run on the machine with which it was sold; in the past, this included Compaq Proliant and HP servers, but today this is nearly always an approved IBM xSeries server or a ThinkPad laptop.

Another popular emulator is the open source Hercules emulator, which has been in development since 1999 and emulates the System/370, System/390, and z/Architecture instruction sets. While Hercules cannot legally run modern IBM operating systems, earlier System/370 operating systems are in the public domain and can be legally run on Hercules.

As of December, 2007, there are no authorized PC-based 64-bit mainframe-compatible systems. Thus there is no legal way to run z/OS 1.6 (or higher), DB2 V8 (or higher), z/TPF, or z/VSE 4.1 (or higher) on PC-based machines.

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