PC-based IBM-compatible mainframes

PC-based IBM-compatible mainframes

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. 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).



Up until the mid-1990s, 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.


IBM had demonstrated use of a mainframe instruction set in a desktop computer with the IBM 5100. This product used microcode to execute many of the System/370's processor instructions, so that it could run a slightly modified version of IBM's APL mainframe program interpreter.


"The VM/PC Software traces it's origins to an activity initiated at the IBM Cambridge Scientific Center to build a prototype CMS workstation, This prototype was called CnS(Cambridge nano-System).

[citation needed]

Personal Computer XT/370

In October, 1983, IBM announced the IBM Personal Computer XT/370. This was an XT with three custom 8-bit cards. The processor card (370PC-P)[1], contained two modified Motorola 68000 chips,and an 8087 modified to emulate the S/370 floating point instructions. The second card (370PC-M), which connected to the first with a unique card back connector contained 512Kib of memory. The third card (PC3277-EM), was a 3270 terminal emulator required to download system software from the host mainframe. The XT/370 computer booted into DOS, then ran the VM/PC Control Program. The card's memory space added additional system memory, so the first 256Kib (motherboard) memory could be used to move data to the 512kb expansion card. The expansion memory was dual ported, and provided an additional 384Kb to the XT Machine bringing the total ram on the XT side to 640 kb. The memory arbitrator could bank switch the second 128 kb bank on the card to other banks, allowing the XT 8088 processor to address all the RAM on the 370PC-M card. [2]

Personal Computer AT/370

On April 2, 1986, IBM announced the IBM Personal Computer AT/370, with similar cards as for the XT/370 and updated software, to support both larger hard disks, and DMA transfers from the 3277 card to the AT/370 Processor card. The system was almost 60% faster than the XT/370. [2]

IBM 7437

As of November 1988, IBM was shipping a workstation version of the System/370 hardware intended to run IBM's VM/SP operating system. It was a freestanding tower, that connected to a MCA card installed in a PS/2 Model 60, 70, or 80.[3]


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 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.


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

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.


R/390 was the designation used for the expansion card used in an IBM RS/6000 server. The original R/390 featured a 67 or 77 MHz POWER2 processor and 32 to 512 MB 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 need to run AIX version 4 as the host operating system.


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.

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; the S/390 CPU used in the Integrated Server is in fact the P/390 E-card. 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.

Fujitsu PC-based mainframes

Fujitsu offers two PC-compatible systems that make up the lower end of Fujitsu's S/390-based[4] BS2000 mainframe product line. The SQ100 is the slower configuration, running on dual-core Xeon E7220 processors, and is capable of up to 200RPF of performance.[5] The SQ200 was introduced more recently, uses 6-core Xeon X7542 processors, and has performance of up to 700RPF.[6] All x86-based BS2000 mainframes can run Linux or Windows in separate partitions. Fujitsu also continues to make custom S/390-native processors and mainframe hardware for the high end of its BS2000 line.[7]

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. According to Tom Lehmann, cofounder of TurboHercules, a Nehalem EX equipped PC should be able to give a performance of 3.200 MIPS by software emulation.

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 except for IBM PartnerWOrld Independent Software Devloeprs (ISVs) who can use a feature called [System z Personal Development Tool zPDT based on a Linux emulation. IBM Rational application developers can obtain zPDT through the licensing of Rational Developer for System z Unit Test (RDz UT).

See also


  1. ^ http://priorartdatabase.com/IPCOM/000059679#
  2. ^ a b Scott Mueller Upgrading and Repairing PCs, Second Edition, Que Books, 1992, ISBN 0-88022-856-3 pages 73-75, page 94
  3. ^ Fisher, Sharon; LaPlante, Alice (1988-11-07), "IBM's VM/SP Device Cuts Mainframe Load", InfoWorld 10 (45): 113, http://books.google.com/books?id=RzsEAAAAMBAJ&lpg=PT112&ots=pHbpnmiw5r&dq=ibm%207437&pg=PT112#v=onepage&q&f=false 
  4. ^ [1]
  5. ^ [2]
  6. ^ [3]
  7. ^ [4]

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