PC-based IBM mainframe-compatible systems

IBM PC-based mainframe systems
Industry	Computer hardware
Predecessor	IBM mainframes
Founded	1983
Products	IBM Personal Computer XT/370, IBM Personal Computer AT/370, IBM 7437 VM/SP Technical Workstation, Personal/370, S/390 Processor Card, R/390, P/390, S/390 Integrated Server
Owner	IBM

Since the advent of the personal computer in the 1980s, IBM and other manufacturers created PC-based systems compatible with IBM mainframes designed to work alongside larger IBM mainframe computers. These alternatives provided cost savings, lower energy use, and a reduced physical footprint compared to conventional mainframes, though they compromised on speed and dependability. Their primary users included mainframe software developers, educational institutions, small enterprises with non-critical tasks, and emergency response teams.

Background

Prior to the mid-1990s, mainframes were sizable machines necessitating dedicated spaces equipped with specialized power supplies and cooling systems. Contemporary mainframes have evolved to be more compact and demand far less specialized infrastructure.

System/370

IBM initially showcased a mainframe instruction set in its 1975 desktop model, the IBM 5100, which utilized microcode to execute a modified IBM APL interpreter grounded in the System/370 architecture.

In 1981, IBM unveiled the IBM Personal Computer, which was not based on System/370 architecture but was later enhanced with hardware capable of local execution of S/370 instructions.

Personal Computer XT/370

October 1983 saw IBM introduce the IBM Personal Computer XT/370, a hybrid machine able to run PC DOS, function as a 3270 terminal, and locally process S/370 instructions. This model was an IBM Personal Computer XT (System Unit 5160) outfitted with three proprietary 8-bit expansion cards: a processor card containing modified Motorola 68000 processors along with an Intel 8087 coprocessor that emulated S/370 instructions; a memory card featuring 512 KiB of RAM; and a 3270 terminal emulator card enabling software downloads from host mainframes.

The system booted into DOS and operated the VM/PC Control Program. Its memory configuration supported up to 640 KiB RAM on the XT side plus virtual memory extending up to 4 MB via the hard disk. IBM reported that the XT/370 could reach 0.1 MIPS when data resided entirely in RAM. With a price point near in 1984, it offered an attractive cost-to-performance ratio compared to IBM’s traditional mainframes like the IBM 4341, which delivered 1.2 MIPS at .

VM/PC software managed virtual disks as PC DOS files, simplifying file handling and enabling conversions between ASCII and EBCDIC character sets. The XT/370 came with an 83-key keyboard, and later versions replaced the terminal emulator card with IBM 3278/79 interface boards. Additional hard drives were supported through the IBM 5161 expansion chassis.

In its 1984 review, BYTE magazine termed the XT/370 "a qualified success," commending IBM for merging System/370 capabilities into the XT platform while expecting further refinements.

Production of the XT/370 ceased in April 1987.

Personal Computer AT/370

In 1984, IBM released the IBM Personal Computer AT/370, which featured upgraded cards similar to those in the XT/370 but with improved software supporting larger hard drives and direct memory access (DMA) transfers. This system delivered nearly a 60% performance increase over the XT/370. It incorporated 16-bit interface co-processor cards named PC/370-P2 and PC/370-M2; the latter contained 512 KB of memory (480 KB allocated for S/370 applications and 32 KB for microcode). The AT/370 also included the 3278/79 Emulation Adapter from the XT/370 line and came equipped with 512 KB of onboard RAM.

Operating VM/PC alongside PC DOS version 3.0—an upgrade from DOS 2.10 used on the XT model—the AT/370 benefited from VM/PC version 2 released in November 1985, which enhanced performance by as much as 50% and allowed additional memory to function as a page cache for VM. Despite these advancements, market acceptance was limited, leading to its discontinuation together with the XT/370 in April 1987.

IBM 7437 VM/SP Technical Workstation

In April 1988, IBM launched the IBM 7437 VM/SP Technical Workstation, also referred to as the Personal System/370. This standalone tower connected to PS/2 Models 60, 70, or 80 through an MCA card. The tower contained the CPU and 16 MB of main memory, while the PS/2 managed input/output operations and disk storage. Operating on IBM's VM/SP system, the 7437 was positioned as a single-terminal equivalent to the IBM 9370, aimed at users familiar with S/370 environments.

By November 1988, it was priced at $18,100 for orders of at least 25 units. Its purpose was to provide a single-user S/370-compatible platform capable of running engineering and CAD software originally developed for mainframes, such as CADAM and CATIA. The IBM 5080 graphics system—a separate floor-standing unit—offered graphics support by connecting via cable and MCA adapter through the PS/2.

Personal/370

The Personal/370 (P/370) was introduced by IBM as a single-slot 32-bit MCA expansion card designed for PS/2 or RS/6000 machines. It enabled execution of System/370 operating systems like MUSIC/SP, VM, and VSE alongside host OSs such as OS/2 or AIX, supporting multiple simultaneous users. The card fully implemented the S/370 CPU, including an integrated FPU co-processor and contained 16 MB of memory. Host OS and hardware managed I/O channels and system control. An optional 370 channel card allowed connection to mainframe-specific peripherals like 3270 local control units and tape drives.

Although confined to a single expansion card, the P/370 delivered performance three times greater than the IBM 7437, achieving speeds around 3.5 MIPS—comparable to entry-level IBM 4381 systems. Some later reports suggest performance reaching up to 4.1 MIPS.

The Personal/370 became available starting November 1989 through a "special bid" procurement process.

System/390

In 1995, IBM unveiled the "Enhanced S/390 MicroProcessor Complex," a processor card supporting the IBM ESA/390 architecture for PC-based platforms. This innovation was incorporated into IBM’s PC product range, including models such as the "IBM PC Server 330" in 1998 and various IBM PC Server 500 configurations.

S/390 Processor Card

Engineered for full compatibility with existing mainframe operating systems and applications, the S/390 Processor Card executed all ESA/390 and XA instructions natively without requiring translation. It was released in three iterations:

The first generation featured dedicated memory of 32MB, expandable via daughter cards to either 64MB or 128MB, delivered a performance rating of 4.5 MIPS, and was intended for MicroChannel-based hosts.
The second generation supported PCI hosts, came standard with 128MB of memory, and maintained a performance level of 4.5 MIPS.
The third generation, known as P/390E (Enhanced), offered increased dedicated memory of 256MB and boosted performance to approximately 7 MIPS on PCI platforms. A rare pre-production variant with up to 1 GB of memory was also produced.

R/390

The R/390 designation applies to an expansion card designed for IBM RS/6000 servers. The initial R/390 model incorporated either a 67 MHz or 77 MHz POWER2 processor with memory configurations ranging from 32 MB up to 512 MB depending on setup. The MCA-based P/390 card could be installed in any MCA-equipped RS/6000 system, whereas the PCI version fit several early PCI RS/6000 models; all such configurations fall under the R/390 category. These servers required AIX version 4 as their host operating system.

P/390

The P/390 served as an expansion card option for the IBM PC Server, aimed at providing a more affordable choice compared to the R/390. Initially, the P/390 was integrated into an IBM PC Server 500 equipped with a 90 MHz Intel Pentium CPU operating under OS/2. By mid-1996, this model was refreshed and renamed the PC Server 520, which featured a faster 133 MHz Pentium processor. Both versions started with 32 MB of RAM and allowed upgrades up to 256 MB. The PC Server 500 offered eight MCA slots, whereas the PC Server 520 introduced two additional PCI slots but reduced the number of MCA slots by two.

S/390 Integrated Server

Released in November 1998, the S/390 Integrated Server (S/390 IS) is a compact mainframe system measuring 82 x 52 x 111 cm, tailored for users requiring less I/O throughput than the larger Multiprise 3000. It comes equipped with 256 MB of ECC memory and utilizes a single CMOS main processor delivering approximately 8 MIPS, powered by the P/390 E-card CPU. A Pentium II processor functions as the I/O Service Processor (IOSP). The system supports four ESCON channels along with up to four parallel channels and includes standard PCI and ISA expansion slots. Internal storage capacity reaches up to 255 GB via sixteen 18GB drives configured with redundancy. Supported operating systems encompass OS/390, MVS/ESA, VM/ESA, and VSE/ESA.

Fujitsu PC-based systems

Fujitsu's entry-level S/390-based BS2000 mainframe product line features two systems: the SQ100 and SQ200 models. The SQ100 is powered by dual-core 2.93 GHz Intel Xeon E7220 processors, delivering performance levels up to 200 RPF. Meanwhile, the more advanced SQ200 utilizes six-core 2.66 GHz Xeon X7542 processors, achieving performance figures as high as 700 RPF. All Intel 64-bit BS2000 mainframes from Fujitsu support partitioning for Linux or Windows environments. Additionally, Fujitsu manufactures proprietary S/390-compatible processors and mainframe hardware tailored for its higher-end BS2000 series.

z/Architecture and today

Beginning in the late 1990s, advancements in PC processor capabilities have enabled mainframe emulation without relying on dedicated coprocessor cards. Presently, multiple PC-based emulators accommodate System/390 and z/Architecture environments:

FLEX-ES, developed by Fundamental Software, emulates both System/390 (ESA/390) and z/Architecture. As of 2006, it stands as one of the leading IBM-compatible mainframe solutions on PCs. FLEX-ES operates on most PC platforms; however, its licensing restricts usage to the originally sold hardware—typically an approved IBM xSeries server or a ThinkPad laptop.

Hercules is an open-source emulator supporting instruction sets from System/370, System/390, and z/Architecture. It requires a full operating system to execute applications. While IBM does not license current OS releases for Hercules, earlier System/370 operating systems available in the public domain can legally be employed.

zPDT (System/z Personal Development Tool) is an IBM offering that allows IBM PartnerWorld Independent Software Developers to run versions of z/OS starting from 1.6+, DB2 V8+, z/TPF, or z/VSE from version 4.1+ through Linux-based PC emulations.

IBM ZD&T (Z Development and Test Environment) delivers an x86-hosted platform that simulates Z hardware for executing authentic z/OS software. It facilitates education, demonstrations, development, and testing of mainframe applications while ensuring high compatibility and portability.

The Z390 and zCOBOL toolkits provide portable macro assembler environments alongside COBOL compiler/linker/emulator solutions designed for creating and running mainframe-compatible applications on any J2SE 1.6.0+ platform.

Additional resources