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The PC Future That Briefly Was: IBM PS/2 and OS/2

A look back at the second generation IBM personal computer range and operating system which boasted numerous features which were advanced for the time.

While the MITS Altair 8800 is generally considered to be the first ever personal computer — or at least successful personal computer — the IBM PC served as the catalyst which made personal computing mainstream and the enormous success that it went on to become. And while if you’d previously never heard of the original IBM Model 5150 which launched in 1981, you will likely at least be vaguely aware of those which followed: the PC/XT and then the Intel 80286 processor-based PC/AT. All three of which proved immensely popular and kickstarted an entire industry of competing clones and compatible peripherals. A lineage that is still very much alive and well today.

However, with IBM’s subsequent second generation of personal computers came a divergence in a key area, bringing with it technical superiority — albeit that, much like the technically superior Betamax video format which competed with VHS, ultimately didn’t win out.

Personal System/2

The Personal System/2 — or PS/2 for short — range of personal computers launched in 1987 and entry level machines all featured the same bus as used by previous generation PC/XT/AT computers, which went on to be known as Industry Standard Architecture (ISA).

ISA played a key role in the runaway success of the PC, thanks to IBM publishing comprehensive specifications and memory map for the initial PC/XT versions of the bus. This meant that anyone could make compatible hardware and software, which lead to a rapidly growing ecosystem.

Higher end PS/2 models however sported a new expansion bus that was much more advanced than ISA.

Micro Channel Architecture

IBM MCA bus XGA graphics adapter, Wikimedia/Drahtlos (CC BY-SA 4.0).


The ISA bus is fairly simple and pretty much a buffered extension of the processor address and data buses, together with power, clocks and interrupts etc. Starting out as an 8-bit bus, ISA was upgraded to 16-bit for use with the 80286. Users had to manually configure things such as IRQ lines and I/O addresses, which were set using physical jumpers and software, with misconfiguration and IRQ conflicts not uncommon and resulting in a machine that may crash or simply not boot.

Things then got worse when IBM chose not to release the timings for the AT bus and as processor speeds increased, the only way to maintain reliability and backward compatibility with older cards, was to keep the bus running at slower speeds. Attempts were made outside of IBM to define a standard 32-bit bus for the Intel 80386 processor, but ultimately failed.

IBM saw an opportunity and this time decided that their new bus standard would need to be licensed by third party manufacturers, thereby providing another source of income.

Mainframe inspired

Micro Channel Architecture (MCA) took its inspiration and name from the channel subsystem employed by IBM mainframes, which utilised dedicated processors in an architecture that relieved the main CPU from more mundane tasks, such as disk and network I/O. This and numerous other advanced architectural features are what gave the mainframe significantly higher performance than a micro or mini computer with a similar clock speed and RAM.

Key characteristics of the MCA bus include:

  • 32 address and data lines (ISA had 24 and 16 respectively)
  • SRAM cache to minimise processor wait states
  • More advanced interrupts
  • Improved DMA
    • Burst mode (10MB/s instead of just 1MB/s)
    • Support for use with more devices
  • Bus mastering and support for fast peer-to-peer (card-to-card) comms
  • Reliable support for multiprocessor designs
  • Video lines which can pass-through integrated graphics
  • Audio lines
  • Software readable card IDs and software (“plug and play”) configuration
  • Better mechanical design and grounding
  • Properly documented specification

It may come as no surprise to learn that the advanced for its time MCA bus went on to find use not only in desktop PCs, but also in IBM RS/6000 (UNIX) and AS/400 mini computers, plus the IBM 9370 “baby mainframe”.

MCA bus cards were not cheap and could cost twice that of a similar ISA card, but offered many advantages, not least of which being that their plug-and-play nature made support easier — although a special floppy disk holding configuration did need to be kept with each machine.

Outside of IBM, the MCA bus enjoyed relatively minimal success, with seemingly only NCR Corporation making a significant investment across numerous systems and components. Ultimately PC manufacturers did not want to pay royalties to IBM and so created their own alternative, the Extended ISA (EISA) bus, which itself enjoyed very limited success. Then a few years later PCI came along and this incorporated a number of MCA’s advanced features, but not all.

Other novel features

IBM PS/2 486 Processor Complex,

Higher end PS/2 machines also typically featured a “processor complex” card which plugged into the “planar” — a simpler backplane style “motherboard” — meaning that performance upgrades could be offered which went beyond simply swapping out a socketed processor chip.

Each MCA bus PS/2 shipped with a Reference Disk (floppy) that included programs which were used to configure the system. While today this may sound rather inconvenient, it was a great improvement over having to pull apart your system and move jumpers around or flick DIP switches in order to configure peripherals. Over time this disk would grow to include the Adapter Description Files (ADF) that were shipped with additional MCA cards installed. The diskette would also provide diagnostics and utilities to format hard drives.

PS/2 also introduced the 15-pin D-sub VGA, which was used regardless of the particular graphics configuration — e.g. MCGA, IBM 8514 or VGA — and went on to become an industry standard. It similarly introduced 72-pin SIMM memory and perhaps most famously, PS/2 keyboard and mouse connections, all of which also went on to become de facto industry standards.

An advanced machine such as the PS/2 called for a suitably advanced operating system.

Operating System/2

Many of us who had to use Windows 3.1 and Windows for Workgroups still suffer related trauma 30 years on. Those earlier versions of the ubiquitous O/S were little more than a graphical shell running on top of MS DOS and it wasn’t really until the advent of XP, that most Windows users got to benefit from the features and stability that is taken for granted today; freezes and crashes were common — usually while downloading large files over a dial-up connection! — configuration could be a nightmare and Windows was all too often an infuriating experience.

Meanwhile the version of OS/2 released around the same time as Windows 3.1 provided preemptive multitasking with full memory protection. Not only this, but in addition to running OS/2 programs, it could also run Windows programs alongside these, with DDE between the two and OLE between Windows programs.

Interestingly, OS/2 started out as a joint development between IBM and Microsoft, with the first text-mode only release being made in December 1987, followed by the first GUI release around a year later. Early versions used a hotkey combination for task switching, with the Presentation Manager user interface being introduced in October 1988. Other features introduced with v1.1 included an Extended Edition which featured IBM comms (SNA/X.25/APPC/LU 6.2) and Lan Manager support, plus IBM Db2 Query Manager and SQL. Extended Edition v1.2 then went on to add support for Ethernet and TCP/IP.

Sadly, the collaboration fell apart in 1990 and planned OS/2 and Windows API compatibility never happened. At the same time Microsoft enjoyed significant success with Windows 3.0, thanks to this being bundled with new PCs, while in contrast OS/2 was sold as a packaged add-on. This put OS/2 at a major disadvantage. Furthermore, Microsoft started development on NT in late 1989, to address growing issues with Windows — and also putting it in direct competition with OS/2.

IBM continued with OS/2 development and April 1992 saw the introduction of the 32-bit v2.0 release, which also featured DOS compatibility and by extension, Windows compatibility by running Windows user mode components inside a virtual DOS machine (VDM). With an almost complete version of Windows 3.0 being included with v2.0, and Windows 3.1 with OS/2 v2.1. By default Windows applications would run side-by-side — under a single adapted Windows instance — but Windows apps could also be made to run truly isolated from each other, by essentially running multiple instances of Windows, each in its own VDM.

OS/2 version 3.0 “Warp” was released in 1994 and Warp 4 in 1996. New features included more hardware support, multimedia and Internet features, Java support, and speech recognition. Communications support was also expanded upon, notably via feature rich add-on software for communicating with IBM mini and mainframe computer systems. Which was of course supported by MCA bus adapters for IBM Token Ring networks, plus 3270 mainframe comms, for example.

OS/2 enjoyed reasonable success in certain key markets, such as banking, where it would nicely fit into a larger IBM architecture, which central banking operations running on IBM mainframe. However, the final OS/2 Warp version, 4.52, was released in December 2001, with sales stopping four years later and IBM continuing support up until December 2006.

A replacement for OS/2 had also been planned, named Workplace OS and borrowing from both OS/2 and IBM AIX (UNIX), while building on top of a new microkernel code base. This was to run on POWER systems, rather than PCs, and the plan was that it would eventually replace IBM OS/400 also. Development started in 1991 and it was released in 1995, but despite enormous investment, progress was slow and adoption limited — with development soon cancelled.


OS/2 had a dedicated following and although now somewhat diminished, still does to this day. Numerous vendors approached IBM to take on and continue development of OS/2. eComStation is one OS/2 derivative, which was developed by a series of companies, with the first release made in 2001 and the latest in 2011. The other is ArcaOS, developed by Arca Noae under license from IBM, with a first release in 2017 and the most recent in December 2021.

Building on the final IBM release, Warp 4.52, ArcaOS supports SMP with up to 64x processor cores and is ACPI compliant. However, it is only a 32-bit O/S and has a number of limitations when compared with contemporary modern operating systems. Support for recent network adapters and its audio subsystem come from FreeBSD and Linux respectively, although ArcaOS itself is closed source. Software bundled with the O/S includes ports of Firefox, Thunderbird and OpenOffice.

Final words

It is all too often the case that the best or most advanced technology does not win out and this alone is not sufficient to guarantee success; other factors, such as distribution models, marketing and of course, implementation, have a vital role to play.

The MCA bus never truly gained a foothold due to its licensing model, which was clearly IBM’s response to all the clones which competed with their first generation PCs. However, the licensing costs involved made the bus standard unattractive to potential licensees, the market went with compromise instead and, unfortunately, adoption is the principle measure of success for standards.

OS/2 meanwhile got off to a rocky start with the Microsoft collaboration breaking down. Following which the go-to-market strategy was eclipsed by Windows, as this was being volume licensed to clone manufacturers for bundling with new PCs. Furthermore, the otherwise technically superior O/S suffered some stability issues, due to one particular poor architectural decision. Hence it’s easy to see how OS/2 never came to enjoy the success that it deserved. Another example of a very exciting for the time technology, which is now largely consigned to history.

Andrew Back

Open source (hardware and software!) advocate, Treasurer and Director of the Free and Open Source Silicon Foundation, organiser of Wuthering Bytes technology festival and founder of the Open Source Hardware User Group.
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