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PC-based IBM mainframe-compatible systems
PC-based IBM mainframe-compatible systems
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Since the rise of the personal computer in the 1980s, IBM and other vendors have created PC-based IBM mainframe-compatible systems 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).

Background

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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.

System/370

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IBM had demonstrated use of a mainframe instruction set in their first desktop computer—the IBM 5100, released in 1975. 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.

In 1980 rumors spread of a new IBM personal computer, perhaps a miniaturized version of the 370.[1] In 1981 the IBM Personal Computer appeared, but it was not based on the System 370 architecture. However, IBM did use their new PC platform to create combinations with additional hardware that could execute S/370 instructions locally.

Personal Computer XT/370

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In October 1983, IBM announced the IBM Personal Computer XT/370. This was essentially a three-in-one product. It could run PC DOS locally, it could also act as 3270 terminal, and finally—its most important distinguishing feature relative to an IBM 3270 PC—was that it could execute S/370 instructions locally.[2]

IBM XT/370 board and diagnostic diskette

The XT/370 was an IBM Personal Computer XT (System Unit 5160) with three custom 8-bit cards. The processor card (370PC-P),[3] contained two modified Motorola 68000 chips (which could emulate most S/370 fixed-point instructions and non-floating-point instructions), and an Intel 8087 coprocessor 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 512 KiB 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 256 KiB (motherboard) memory could be used to move data to the 512 KiB expansion card. The expansion memory was dual ported, and provided an additional 384 KiB to the XT Machine bringing the total RAM on the XT side to 640 KiB. The memory arbitrator could bank switch the second 128 KiB bank on the card to other banks, allowing the XT Intel 8088 processor to address all the RAM on the 370PC-M card.[4] Besides the 416 kB of usable RAM for S/370 applications, the XT/370 also supported up to 4 MB of virtual memory using the hard drive as its paging device.[5]

IBM claimed the XT/370 reached 0.1 MIPS (when the data fit in RAM). In 1984, the list price of an XT/370 in its typical configuration was approximately $12,000 so compared favorably with IBM's own mainframes on a $/MIPS basis; for example, an IBM 4341 delivered 1.2 MIPS for $500,000. While it theoretically reduced demand on customers' mainframes by offloading load onto the smaller computer, as customers purchased more XT/370s they likely increased the overall load on the mainframes, increasing IBM's mainframe sales.[5]

Similarly to the mainframe version of VM/CMS, the VM/PC also created the illusion of virtual disks, but on the PC version these were maintained as PC DOS files, either on floppy or hard disk. For example, the CMS virtual disk belonging to user FRED at device address 101 was stored as the DOS file FRED.101. The CMS IMPORT and EXPORT commands allowed extraction of files from these virtual drives as well as ASCII/EBCDIC conversion.[6]

The XT/370 came with an XT-style 83-key keyboard (10 function keys).[2] Newer revisions of the XT/370 dropped the PC3277-EM in favor of the IBM 3278/79 boards. The XT/370 was among the XT systems that could use a second hard drive mounted in the 5161 expansion chassis.[7]: 6–17 

BYTE in 1984 called the XT/370 "a qualified success". The magazine praised IBM for "fitting all of the 370's features into the XT", and hoped for technical improvements that "might result in an even better computer".[8]

The XT/370 was discontinued in April 1987.[9]

Personal Computer AT/370

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In 1984, IBM introduced the IBM Personal Computer AT/370[10] with similar cards as for the XT/370 and updated software, supporting 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.[4] The AT/370 used different, 16-bit interface co-processing cards than the XT, called PC/370-P2 and PC/370-M2. The latter card still had only 512 KB for memory, out of which 480 KB were usable for programs in S/370 mode, while 32 KB were reserved for microcode storage. For the terminal emulation function, the AT/370 came with the same 3278/79 Emulation Adapter as the late-series XT/370. The AT/370 motherboard itself was equipped with 512 KB of RAM.[7]: 9-26 to 9-28 

The AT/370 also ran VM/PC, but with PC DOS 3.0 instead of 2.10 that the XT version used.[11] VM/PC version 2, launched in November 1985, improved performance by up to 50%; it allowed add-on memory (in addition to the disk) to be used as a page cache for VM.[12]

A November 1985 Computerworld article noted that the machine was "slow selling".[12] The AT/370 was discontinued alongside the XT/370 in April 1987.[9]

IBM 7437 VM/SP Technical Workstation

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In April 1988, IBM introduced a System/370 workstation that had been shipping to some customers since August 1987.[13] Officially called the IBM 7437 VM/SP Technical Workstation (and later also known as the Personal System/370), it was a freestanding tower that connected to a MCA card installed in a PS/2 Model 60, 70, or 80. The 7437 tower contained the processor and a 16 Mbytes main memory, and the PS/2 provided I/O and disk storage.[14] The 7437 ran the IBM VM/SP operating system, and one IBM representative described the 7437 "like a 9370 with a single terminal". It was intended for existing S/370 users and its November 1988 list price was $18,100 for a minimum 25-unit order.[15] One of its intended roles was to provide a single-user S/370-compatible computer that could run computer-aided design and engineering applications that originated on IBM mainframes such as CADAM and CATIA. Graphics support was provided by an IBM 5080 graphics system, a floor-standing tower. The 5080 was connected to the 7437 through the PS/2 via a cable and MCA adapter.[16]

Personal/370

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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.

Although a single-card product, the P/370 ran three times faster than the 7437, attaining 3.5 MIPS, on par with a low-end IBM 4381.[17] A subsequent book (by the same author) claims 4.1 MIPS for the P/370.[18]

The Personal/370 was available as early as November 1989 although on a "special bid basis".[19]

System/390

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In 1995 IBM introduced a card, the "Enhanced S/390 MicroProcessor Complex", which supported IBM ESA/390 architecture on a PC-based system.[20] IBM's PC-related products evolved to support that as well, employing the card (IBM part number 8640-PB0) in the "IBM PC Server 330 in 1998[21][22] and the IBM PC Server 500 models.[23]

S/390 Processor Card

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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

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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.

IBM PC Server 500. This server can contain a P/390 board.

P/390

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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 MB of RAM and were expandable to 256 MB. 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

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IBM S/390 Integrated Server

The S/390 Integrated Server (aka S/390 IS) is a mainframe housed in a comparatively 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 systems

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Fujitsu offers two based systems that make up the lower end of Fujitsu's S/390-based BS2000 mainframe product line.[24] The SQ100 is the slower configuration, using dual-core 2.93 GHz Intel Xeon E7220 processors, and is capable of up to 200RPF of performance.[25] The SQ200 was introduced more recently, uses six-core 2.66 GHz Xeon X7542 processors, and has performance of up to 700RPF.[26] All Intel 64-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.[27]

z/Architecture and today

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Since the late 1990s, PC processors have become fast enough to perform mainframe emulation without the need for a coprocessor card. There are currently several personal computer emulators available that support System/390 and z/Architecture.

  • FLEX-ES by Fundamental Software emulates both System/390 (ESA/390) and z/Architecture. Claimed to be one of the most popular PC-based IBM-compatible mainframe products (as of 2006). While FLEX-ES is capable of running on most PC hardware, the licensing agreement requires that 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.
  • Hercules, an open source emulator for the System/370, System/390, and z/Architecture instruction sets. It does however require a complete operating system in order to execute application programs. While IBM does not license its current operating systems to run on Hercules, earlier System/370 operating systems are in the public domain[citation needed] and can be legally run on Hercules.
  • zPDT (System/z Personal Development Tool), an IBM offering allowing IBM PartnerWorld Independent Software Developers (ISVs) to legally run z/OS 1.6 (or higher), DB2 V8 (or higher), z/VM, z/TPF, or z/VSE 4.1 (or higher) on PC-based machines that can be acquired based on a Linux emulation.
  • IBM ZD&T (Z Development and Test Environment), an IBM offering provides an x86-based environment that emulates Z hardware and runs genuine z/OS software, offering unmatched application portability and compatibility. IBM Z Development and Test Environment can be used for education, demonstration, and development and test of applications that include mainframe components.
  • The Z390 and zCOBOL is a portable macro assembler and COBOL compiler, linker, and emulator toolkit providing a way to develop, test, and deploy mainframe compatible assembler and COBOL programs using any computer that supports J2SE 1.6.0+ runtime.

Timeline

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Timeline of the IBM Personal Computer
IBM ThinkCentreIBM NetVistaIBM Palm Top PC 110IBM PC SeriesIBM AptivaIBM PS/ValuePointThinkPadEduQuestIBM PS/noteAmbra Computer CorporationIBM PCradioIBM PS/1IBM PS/55IBM PS/2IBM Personal Computer XT 286IBM PC ConvertibleIBM JXIBM Personal Computer AT/370IBM Personal Computer ATIBM Industrial ComputerIBM PCjrIBM Portable Personal ComputerIBM Personal Computer XT/370IBM 3270 PCIBM Personal Computer XTIBM 5550IBM Personal ComputerIBM System/23 DatamasterIBM 5120IBM 5110IBM 5100
Asterisk (*) denotes a model released in Japan only

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

PC-based IBM mainframe-compatible systems

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PC-based mainframe-compatible systems are desktop and server platforms primarily developed by , with implementations by other vendors such as , during the 1980s and 1990s that incorporate specialized hardware adapters or embedded processors to run operating systems and applications originally designed for 's System/370 and System/390 mainframe architectures, allowing for cost-effective mainframe emulation, development, testing, and limited production workloads on hardware. These systems emerged in response to the growing need for accessible mainframe resources amid the rise of personal computing, bridging the gap between high-end mainframes and affordable PCs by providing backward compatibility with established enterprise software while leveraging the modularity of the IBM PC architecture. The first notable example, the IBM Personal Computer XT/370, released in 1983, extended the standard IBM PC XT (model 5160) with three expansion cards—including a Motorola 68000-based co-processor modified to execute System/370 instructions—enabling it to run VM/PC, a virtual machine environment compatible with System/370 applications, and to emulate the IBM 3277 terminal for mainframe connectivity and file transfers. This setup supported up to 10 MB of fixed disk storage and facilitated scientific, technical, and business applications in environments like NASA, where performance evaluations highlighted its utility for development tasks despite limitations in processing speed compared to full mainframes. Building on this foundation, the , introduced in October 1984, upgraded the platform to the faster (model 5170) base, adding the AT/370 adapter consisting of a PC/370-P2 processor card and PC/370-M2 (providing 512 KB of memory addressable by the System/370 environment, of which 128 KB can also be used by the PC/AT when the 370 mode is inactive). It required at least 512 KB of system board memory and ran IBM DOS 3.00 alongside VM/PC for System/370 emulation, supporting optional peripherals like 20 MB fixed disks and high-capacity floppy drives to enhance mainframe integration for tasks such as operations and data exchange. These early models targeted developers, small businesses, and institutions needing mainframe-like capabilities without the expense of dedicated hardware, though they were constrained by the era's PC limitations in memory and I/O throughput. In the , advanced the concept with System/390-compatible systems, culminating in the IBM PC Server System/390 (also known as the P/390), which integrated a single-chip S/390 complex into PC Server 300 or 500 models starting in 1996. Featuring a 71 MHz clock speed delivering approximately 4.5 MIPS, up to 256 MB of for the S/390 environment, and Warp as the host OS, it supported full execution of , VM/ESA, and VSE/ESA without modification, emulating 3380/3390 DASD and 3480 tape devices via a Parallel Channel Adapter. Storage options reached up to 110 GB with RAID-5 configurations, and networking included Ethernet and , making it suitable for small to moderate workloads like application development, , and LAN-based . These systems collectively represented IBM's strategy to democratize mainframe , reducing costs for entry-level enterprise needs while maintaining architectural fidelity, but production ceased as and x86 servers with software supplanted them in the early 2000s.

Background and Context

Historical Development

The architecture, announced on June 30, 1970, established the foundational instruction set for mainframes, emphasizing backward compatibility with prior systems. was introduced in August 1972 to support complex, multi-user environments. This architecture became the baseline for subsequent efforts to extend mainframe capabilities beyond large-scale installations. The launch of the Personal Computer on August 12, 1981, ignited the personal computing revolution, shifting industry focus toward affordable, individual workstations and challenging the dominance of centralized mainframes. In response, initiated developments in the early to bridge mainframe and personal computing paradigms, aiming to leverage the growing PC for broader access to and . These initiatives emerged amid rising demand for , where minicomputers and PCs threatened to fragment the cohesive mainframe model had built. Key drivers for PC-based mainframe compatibility included substantial cost reductions, enabling System/370-level performance at desktop scales far below the expense of full mainframes, which often exceeded hundreds of thousands of dollars. Support for legacy software, such as the VM/SP operating system, preserved investments in existing applications by allowing them to run in virtualized environments on PC hardware, thus avoiding costly migrations. Additionally, these systems promoted and by providing accessible platforms for developers and smaller teams to learn and experiment with mainframe operations, fostering skills transfer without requiring dedicated large-scale .

Purpose and Technical Foundations

PC-based IBM mainframe-compatible systems emerged to reduce the financial and logistical barriers associated with traditional mainframe computing, enabling software developers, testers, and small organizations to leverage mainframe environments on cost-effective personal hardware. By supporting VM/PC, a limited version of the VM/370 operating system, these systems allowed unaltered mainframe applications—particularly those under the Conversational Monitor System (CMS)—to run locally, facilitating development, , and deployment without requiring access to expensive centralized mainframes. This approach preserved investments in legacy System/370 and System/390 software while promoting during the 1980s PC proliferation. At their core, these systems rely on emulation of the (ISA), executed on dedicated co-processor hardware, such as a modified in early models, through interpreters and hardware-assisted execution. Common commercial instructions are handled directly by dedicated hardware components for efficiency, while less frequent or complex instructions are emulated in software, ensuring binary compatibility with mainframe code. This hybrid emulation strategy translates the 32-bit System/370 instructions into the PC's native 16-bit environment, maintaining the semantic behavior of the original . Key enabling concepts include support, which provides isolated execution environments akin to those in VM/SP, allowing multiple mainframe sessions to coexist on a single PC; I/O channel emulation, where PC bus interfaces simulate the block-multiplexed channel architecture for attaching peripherals like disks and tapes; and memory management units that implement dynamic address translation to support up to 4 megabytes of virtual storage in 4-kilobyte pages, aligning with mainframe addressing modes such as real and virtual storage. These elements ensure seamless integration of mainframe workloads with PC resources, including bridging between CMS and PC DOS via import/export mechanisms. Addressing inherent challenges, such as the vast performance gaps between PC clock speeds (typically 4-8 MHz) and mainframe processors (up to 10 MIPS), involved optimized for instruction dispatch and add-on hardware accelerators to minimize emulation overhead, achieving usable throughput for interactive and batch applications. Fixed-disk storage supplemented limited RAM to handle paging demands, while compatibility testing verified adherence to System/370 principles under VM/SP HPO environments.

System/370 Era Systems

PC XT/370 and AT/370

The IBM Personal Computer XT/370, announced in October 1983, was an add-on option for the standard IBM PC XT that provided System/370 compatibility through emulation hardware. It consisted of three custom expansion cards installed in the XT's slots, transforming the base machine—powered by an Intel 8088 processor running at 4.77 MHz—into a hybrid system capable of executing unmodified System/370 instructions alongside PC DOS applications. This design targeted users needing localized mainframe functionality without full-scale hardware, particularly for development environments. Hardware for the XT/370 included a processor card featuring two modified microprocessors, one reprogrammed with custom to emulate the System/370 instruction set and the other handling control functions, paired with a modified coprocessor for floating-point operations. A dedicated provided 512 KB of main memory for System/370 mode (with up to 8 MB support), dual-ported and bank-switched to share resources with the PC's up to 640 KB RAM. The third card served as a 3270 and included a block channel emulator supporting IBM 3705-compatible front-end processors and peripherals such as DASD and tape drives. In System/370 mode, the XT/370 ran the VM/PC control program, a DOS-hosted variant of VM/SP that supported up to three concurrent virtual machines under CMS, enabling and development of mainframe applications like batch jobs and interactive sessions. It could also operate as a standalone 3270 terminal connected to a host via , facilitating file transfers and remote access to full VM/SP environments on larger systems. This setup allowed developers to prototype and debug code locally, reducing dependency on central mainframes for routine tasks. The IBM Personal Computer AT/370, introduced in October 1984 as an upgrade for the AT platform, built on this foundation with enhanced performance for the 80286-based host system running at 6 MHz. It retained the three-card architecture but improved I/O throughput and integrated better with the AT's expanded bus, supporting VM/PC version 1.1 for CMS operations and up to three sessions, including local processing and host emulation. Hardware specifications mirrored the XT/370's emulation approach, with three specialized microprocessors for fixed-point, floating-point, and auxiliary System/370 functions, 640 KB PC-mode RAM (480 KB addressable in VM/PC mode), a 20 MB fixed disk, and the same channel emulator for DASD/tape attachments, though virtual memory was capped at 8 MB. Performance for the XT/370 was limited to approximately 0.1 MIPS in commercial workloads when data resided in RAM, roughly equivalent to 20-40% of an IBM 4331 processor's speed, making it suitable only for light development rather than production use. The AT/370 improved this to 40-60% of the 4331's capability in commercial tasks (about 0.3-0.45 MIPS) and 1.25-2.5 times in scientific computing, with typical CMS response times under 1 second for editing operations. These systems were priced at around $12,000 in typical configurations, reflecting the added emulation hardware and software licensing. Both the XT/370 and AT/370 faced inherent limitations, including slow emulation overhead from the 68000-based design, restricted scalability beyond 8 MB real memory, and incompatibility with certain System/370 peripherals requiring full channel adapters. High costs and modest performance hindered adoption outside niche development roles, leading to discontinue support by in favor of more integrated and faster alternatives like dedicated workstations.

IBM 7437 VM/SP Workstation and Personal/370

The 7437 VM/SP Technical Workstation, introduced in April 1988, represented an advanced standalone System/370-compatible system designed primarily for VM/SP development and testing. This PS/2-based (using ) incorporated a dedicated 370 emulation card and supported up to 32 MB of RAM, enabling efficient of mainframe environments on a desktop scale. It also featured graphical interfaces to facilitate visual mainframe , allowing developers to interact with virtual terminals and resources in a more intuitive manner than earlier add-on solutions. Building on earlier XT/AT emulation techniques, the 7437 provided a more integrated experience for tasks. In 1993, released the Personal/370, a single-slot (MCA) adapter card that could be installed in a PS/2 or compatible PC to provide System/370 processing, offering enhanced portability for individual users. This model supported up to 32 MB of RAM, Ethernet connectivity for networked operations, and complete compatibility with and VM operating systems, making it ideal for personal development and small-scale testing by programmers. Marketed specifically to developers seeking affordable access to mainframe capabilities without large-scale hardware, the Personal/370 emphasized ease of use in non-enterprise settings. Key innovations in these systems included performance improvements reaching up to 2 MIPS, which allowed for faster execution of mainframe workloads compared to prior PC-based emulators. Enhanced peripheral emulation, such as support for 3270 terminals, enabled seamless integration with legacy mainframe peripherals, while bundled software packages facilitated training and rapid onboarding for VM/SP and MVS environments. These features addressed limitations in earlier designs by providing more robust I/O handling and virtual machine management. The 7437 and Personal/370 found adoption in educational institutions for teaching mainframe programming and in small businesses for cost-effective development and prototyping. However, as the System/390 architecture gained prominence in the mid-1990s with superior scalability and performance, IBM phased out these System/370-based workstations, shifting focus to more advanced compatible systems.

System/390 Era Systems

S/390 Processor Cards and Integrated Servers

The S/390 Processor Card, introduced in the mid-1990s as the P/390 Complex, was a Micro Channel-based add-in card designed for integration into PC servers such as the PC Server 500, providing compatibility with the ESA/390 while evolving from earlier System/370 emulation approaches. This card featured a single-chip processor running at 71 MHz with approximately 4.5 MIPS performance, supporting up to 128 MB of ECC RAM through a base 32 MB configuration expandable via optional 32 MB or 96 MB daughter cards. It enabled full execution of standard S/390 operating systems, including , VM/ESA, and VSE/ESA, without software modifications, and handled I/O through device managers emulating S/370 channel interfaces for peripherals like tape and communication controllers. Although limited in —lacking support for Parallel Sysplex clustering or multiprocessor configurations—the card facilitated small-scale deployments for legacy application hosting and development in distributed PC environments. Building on the processor card technology, the S/390 Integrated Server Model 3006-B01, announced in 1998, offered a standalone, PC-like solution as an extension of the Multiprise series, incorporating enhanced P/390 logic for fixed-installation mainframe operations. This system featured 256 MB of standard ECC S/390 on the processor card, paired with 128 MB of I/O and Service Processor (IOSP) emulating expanded storage, and delivered supporting up to 90 TSO users or 340 VM/CMS users in typical workloads. Key technical capabilities included ESA/390 instruction set compatibility, logical partitions (LPARs) for to isolate workloads, and I/O options such as up to four ESCON or parallel channel adapters (with fiber channel support), 16 PCI/ISA slots, and SSA RAID-5 arrays scalable to 255 GB across 16 hot-swap drives for . It ran unmodified , VM/ESA, and VSE/ESA, alongside for service functions on an integrated Pentium-based . These integrated servers targeted cost-effective, small-scale mainframe needs, such as computing for and hosting legacy applications like or IMS, often replacing older S/370 systems like the 4341 or 9375. With a base price around $50,000, including the S/390 daughter card component at $18,855 ($3,000 per MIPS), the solution emphasized ease of integration into PC networks via Ethernet, , or interfaces, enabling hybrid environments for development and entry-level production without the footprint of full-scale mainframes.

R/390 and P/390

The R/390, introduced in April 1996, was a rack-mountable hybrid system combining an RS/6000 RISC platform with an integrated S/390 microprocessor complex, enabling System/390 compatibility in a compact form factor suitable for data centers with space constraints. It utilized a 67 MHz POWER2 processor in its base model (7012-390), with configurations supporting up to 512 MB of RAM, integrated SCSI-2 dual-port fast/wide interfaces, and up to four Micro Channel slots for expansion. Designed to facilitate the transition from older S/370 systems like the 43xx series to S/390 technology, the R/390 supported , /ESA, VM/ESA, and VSE/ESA, while running AIX as the host operating system. Key features included CMOS-based S/390 processing for power efficiency, emulated DASD support for devices such as 3390 and 3380 volumes via drives, and remote management through TCP/IP-based tools, targeting small and medium-sized organizations for distributed production and disaster recovery applications. Building on S/390 processor card technology, the P/390 offered enhanced deployment flexibility as a PC Server-based System/390 solution introduced in mid-1995 and enhanced in subsequent versions. Hosted on an IBM PC Server 520 with a 133 MHz Pentium processor, it incorporated an S/390 microprocessor adapter card providing up to 256 MB of dedicated RAM in its enhanced configuration, alongside support for up to 38 GB of internal SCSI disk storage across 18 bays. The system delivered approximately 4.5 MIPS of S/390 processing performance in the base configuration and approximately 7 MIPS in the enhanced configuration, and ran OS/390, VSE, MVS (any version), and VM, with OS/2 serving as the host environment. Its design emphasized portability for development and field testing, featuring CMOS technology for low power consumption, SCSI interfaces for tape and disk emulation (including up to four 3490E-compatible drives), channel emulators for S/370 compatibility, and remote management via AWS2703 commands and LAN connectivity such as token-ring or Ethernet. Targeted at mobile developers, distributed environments, and disaster recovery setups, the P/390 enabled mainframe applications to operate concurrently with PC workloads, though its adoption was constrained by the era's hardware limitations in size and thermal management. Both systems were discontinued in the early 2000s as consolidated focus on larger-scale zSeries platforms, marking the end of these entry-level, flexible System/390 offerings.

Fujitsu PC-Based Implementations

entered into a licensing agreement with in the early following a dispute over mainframe operating , allowing to implement hardware and software compatible with 's System/370 and System/390 architectures. This partnership, formalized after in 1983 and further resolved in 1988, enabled to develop compatible systems tailored for the Japanese and Asian markets, where demand for cost-effective mainframe alternatives was high. Under the agreement, paid substantial fees for access to technical information, supporting the creation of enterprise-grade solutions integrated with local peripherals and 's proprietary mainframes. In the late , introduced the SQ series as S/390-compatible PC-based servers, providing entry-level mainframe functionality for business applications under the BS2000 operating system. The SQ100, for example, supported up to 1 GB of memory and was designed for small-scale workloads. These systems featured customized I/O interfaces for Japanese peripherals, enhanced reliability for enterprise use, and seamless integration with 's larger mainframe ecosystem. 's PC-based implementations served as affordable alternatives to IBM's offerings, finding adoption in and sectors across for tasks requiring mainframe compatibility without full-scale hardware. By the , these evolved into hybrid systems combining PC form factors with advanced BS2000 OS support on S/390-compatible platforms like the SQ series.

z/Architecture and Modern Evolution

Transition to zSeries Compatibility

In 2000, IBM introduced as a 64-bit extension to the Enterprise Systems Architecture/390 (ESA/390), announced in October of that year to enhance , , and support for modern workloads. This architecture emphasized integration with distributions and advanced through the Processor Resource/System Manager (PR/SM), enabling efficient logical partitioning (LPARs) for mixed environments running multiple operating systems simultaneously. Building briefly on S/390-era hardware like the Multiprise 3000, maintained full with System/370 and System/390 software via emulated modes, allowing seamless migration of legacy applications. PC-based adaptations emerged in the mid-2000s through IBM's development of compact zSeries servers incorporating processors, such as the zSeries 800 (introduced in 2002) and later z890 models, which represented entry-level mainframes in smaller form factors compared to traditional large-scale systems. These systems supported and operating systems in configurations suitable for departmental use, with the zSeries 800 offering up to 32 GB of RAM using technology and scalability in 8 GB increments. Although not in standard desktop PC chassis, their reduced size—approximately 72 inches high by 28 inches wide—facilitated deployment in constrained spaces, bridging the gap between full mainframes and . Key developments included enhanced I/O capabilities with FICON (Fibre Connection) channels, providing high-speed fibre optic connectivity at up to 100 MB/s per channel and supporting up to 32 channels for improved data access in virtualized setups. These features ensured for ESA/390 peripherals while introducing support for modern networking like OSA-Express for . However, challenges arose from increased power consumption—up to 3.2 kVA per unit—and substantial cooling requirements (10,400 to 16,000 BTUs), alongside high acquisition costs, which limited widespread adoption beyond specialized roles. This led to hybrid PC-zSeries configurations, where compact zSeries units integrated with x86-based PCs for development and testing environments, leveraging for virtualized workloads and HiperSockets for internal high-speed communication between LPARs.

Contemporary PC-Based Solutions

In the post-2010 era, PC-based solutions for compatibility have shifted toward software emulation and hybrid integrations rather than dedicated hardware, enabling development, testing, and limited production workloads on x86 platforms. IBM's zEnterprise EC12 (zEC12), announced in , introduced hybrid through the BladeCenter Extension (zBX), which integrates x86-based blades alongside mainframe processors for unified management of z/OS and distributed workloads, supporting up to 5.5 GHz processing and enhanced I/O capabilities in a rack-integrated form. This approach laid the groundwork for later z13 (2015) and z14 (2017) systems, which further optimized hybrid environments with scalable memory up to 32 TB per system, though full mainframe functionality remains tied to proprietary chips rather than pure x86. PC-server hybrids incorporating zEC12-derived elements, such as in compact blade configurations like the zBC12, allow z/OS operation in smaller footprints with up to 489 GB RAM, facilitating compatibility for and migration testing on x86-augmented setups. Software emulation has become a cornerstone for PC-based compatibility, with the open-source Hercules emulator providing a robust platform to run System/370, ESA/390, and z/Architecture software on standard x86 PCs or servers. Hercules, actively maintained into the 2020s with version 4.9 updates enhancing zVector support and instruction performance, enables non-production use of z/OS, z/VM, and legacy applications for education, debugging, and verification without hardware costs. IBM's ZD&T (z Development and Test Environment), evolved from earlier tools like zPDT, offers licensed emulation of z/OS on x86 Linux hosts, supporting up to multiple virtual processors and integration with tools like Docker for containerized testing; as of 2023 updates, it accommodates z/OS V2.5 on platforms including Ubuntu and RHEL, with deployment on bare-metal PCs or VMs. While ZD&T enforces production restrictions per IBM licensing, it supports host configurations with substantial RAM (typically 64 GB+ recommended for multi-LPAR setups), enabling realistic simulation of mainframe environments on commodity hardware. Contemporary trends emphasize integration and cost-effective migration, with PC-based emulators bridging legacy mainframe code to hybrid setups. IBM Z workloads on AWS, launched in 2021, leverage ZD&T for dev/test deployments on x86 instances, allowing emulation within EC2 environments for seamless and application modernization, reducing infrastructure costs by up to 50% for non-critical tasks compared to physical mainframes. Appliances like ZD&T streamline legacy migration by providing isolated test beds on PCs or , where developers can refactor or code using integrated IDEs, achieving faster cycles and lower TCO through . As of 2025, the landscape prioritizes hybrid -PC configurations over standalone PC mainframes, with no new dedicated x86 hardware emulators emerging; instead, compatibility relies on layers like 's hybrid mesh, integrating emulated environments with public clouds for secure, scalable access to 70% of enterprise data originating from mainframes. This evolution supports AI-driven operations and sustainability goals, as seen in z17 integrations, while phasing out older tools like zPDT by 2026 in favor of -native alternatives.

Chronological Overview

1980s Milestones

The launch of the Personal Computer on August 12, 1981, represented a pivotal moment in computing history by introducing affordable desktop systems capable of supporting business applications, thereby inspiring subsequent efforts to emulate architectures on PC hardware. In October 1983, IBM announced the , the first commercially available hybrid system that extended the with three expansion cards to execute System/370 instructions, allowing users to run select mainframe software directly on a desktop machine while maintaining PC compatibility. This innovation was followed in 1984 by the introduction of the , which integrated similar System/370 processing capabilities into the faster platform, thereby improving performance and broadening accessibility for mainframe emulation in personal computing environments. In April 1988, IBM debuted the 7437 VM/SP Technical Workstation, a specialized PC-based system that supported VM/SP Release 5 applications for System/370 environments, enabling networked development and testing without requiring full-scale mainframes. These developments in the drove a transition toward hybrid setups, where PCs augmented traditional mainframe operations by providing cost-effective tools for , terminal emulation, and localized processing.

1990s Advancements

In the early 1990s, continued to advance PC-based mainframe compatibility by introducing the Personal/370 system, a (MCA) card that enabled System/370 instruction execution on PS/2 personal computers, providing developers with affordable, local access to VM/SP, /SP, and VSE/SP environments without requiring full mainframe resources. By mid-1995, shifted focus to System/390 compatibility with the announcement of the P/390, an enhanced S/390 complex designed as a PCI co-processor card for IBM PC Server systems, allowing unmodified execution of , VM/ESA, and VSE/ESA operating systems on standard PC hardware for development, testing, and small-scale production workloads. Complementing this, the R/390 was introduced the same year as a rack-mounted variant, offering a compact, standalone S/390 server for data centers with up to 1 GB of central storage and support for multiple operating systems, reducing space and power requirements compared to traditional mainframes. In 1999, further emphasized portability and integration with the S/390 Integrated Server (Model 3006-B01), a desk-side unit based on the P/390 technology that combined S/390 processing with PC I/O capabilities, enabling mobile deployment for remote testing and integration while supporting up to 4 GB of memory and high-speed networking. These advancements coincided with broader industry trends, including rapid declines in PC hardware costs—driven by scaling of processors and memory prices dropping over 90% from 1990 to 1999—which democratized access to mainframe-like computing for educational institutions and small businesses.

2000s and Beyond

Following the S/390-era hardware, IBM ceased development of new PC-based mainframe-compatible systems around 2000, transitioning to software-based emulation solutions that allowed z/Architecture compatibility on standard x86 PCs. The open-source Hercules emulator, first released in 1999, received ongoing updates to support z/Architecture features, enabling developers to run legacy mainframe software on personal computers without specialized hardware. From 2021 onward, PC-based mainframe compatibility shifted toward software emulation and services, with the rise of Development and Test Environment (ZD&T) enabling testing on x86 PCs for developer workflows. In 2022, IBM launched Wazi as a Service, providing -native dev/test access via VPC, reducing reliance on local hardware. By 2025, IBM discontinued the ZD&T Personal Edition for PCs, marking the end of pure hardware-emulated PC mainframes in favor of -based solutions like as-a-Service, while open-source emulators such as continued updates to support features up to the z16 processor. This evolution highlighted a transition to virtualized, development-focused uses, enhancing accessibility for hybrid environments without dedicated PC hardware.

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