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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. The original advantage being that the PC-based mainframes had a much smaller footprint, required less power, and cost less money.

Contents 1 Background 2 System/370 3 System/390 3.1 S/390 Processor Card 3.2 R/390 3.3 P/390 3.4 S/390 Integrated Server 4 z/Architecture and Today 5 External links

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 three-phase electric power required by the machines.

The Personal/370 (aka P/370) is 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 MiB 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.

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 32 MiB of dedicated memory, with optional 32 MiB or 96 MiB daughter cards, for a combined total of 64 MiB or 128 MiB 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 MiB 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 MiB 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 GiB 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 MHz or 77 MHz POWER 2 processor and 32 MiB to 512 MiB 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.

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 32 MiB of RAM and were expandable to 256 MiB. The PC Server 500 featured eight MCA expansion slots while the PC Server 520 added two PCI expansion slots and removed two MCA slots.

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 MiB 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 18 GB HDs, with 2x HDs for redundancy). The supported OSs are OS/390, MVS/ESA, VM/ESA and VSE/ESA.

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.

• P/390 and R/390 with OS/390: An Introduction (IBM Redbook)
• P/390, R/390, S/390 Integrated Server: OS/390 New User's Cookbook (IBM Redbook)
• S/390 Integrated Server - Hardware Announcement; September 8, 1998
• Detail pictures of a PC Server 500, on the private website of Alfred Arnold
• Detail pictures of a S/390 IS (incl. screenshot of console), on the private website of Michael J. Ross