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OS/2 is an operating system created by Microsoft and IBM and later developed by IBM exclusively. The name stands for "Operating System/2", because it was introduced as the preferred operating system for IBM's "Personal System/2 (PS/2)" line of second-generation Personal Computers.

OS/2 was intended as a protected mode successor of MS-DOS and Microsoft Windows. Notably, basic system calls were modeled after MS-DOS calls; their names even started with "Dos" and it was possible to link text mode applications in such a way that they could work on both systems ("bound" programs). Because of this heritage, in terms of look, feel and features, OS/2 is not unlike Windows in many ways; but it also shares similarities with Unix and XENIX.


Development history

Enthusiastic beginnings

IBM and Microsoft signed the Joint Development Agreement in August 1985.

OS/2 1.0 was announced in April 1987 and released in December, as a text mode-only OS. It however featured a rich API for controlling the video display (VIO) and getting keyboard and mouse events, a sort of a protected-mode BIOS. Not surprisingly, the video and keyboard APIs were also available to "bound" programs on MS-DOS. The promised GUI, Presentation Manager, was introduced with OS/2 1.1 in November 1988. Version 1.2 introduced the HPFS filesystem.

OS/2 and Windows-related books of the late 1980s acknowledged the existence of both systems and promoted OS/2 as the system for the future.


The collaboration between IBM and Microsoft unravelled in 1990, between the releases of Windows 3.0 and OS/2 1.3. Initially, at least publicly, Microsoft continued to insist the future belonged to OS/2. Steve Ballmer of Microsoft even took to calling OS/2 "Windows Plus". The public; however, decided it wanted Windows, not OS/2. The increasing popularity of Windows prompted Microsoft to shift its development focus from OS/2, and IBM grew concerned about delays in development of OS/2 2.0. Initially, the companies agreed that IBM would take over maintenance of OS/2 1.0 and development of OS/2 2.0, while Microsoft would continue development of OS/2 3.0, then known as "NT OS/2", which supposedly stood for New Technology. However, Microsoft decided to recast NT OS/2 as Windows NT, leaving all future OS/2 development to IBM. Windows NT's OS/2 heritage can be seen in its initial support for the HPFS filesystem (although write support was dropped in Windows NT 4.0 and read support was dropped in Windows 2000) and text mode OS/2 1.x applications (support dropped in Windows XP).

Competition with Windows

OS/2 2.0, released in 1992, was touted by IBM as "a better DOS than DOS and a better Windows than Windows". Indeed, because of the limitations of the Intel 80286 processor, OS/2 1.x could only run one "DOS box" at a time, and did this in an insecure way by ping-ponging the entire processor between real and protected modes, using the undocumented LOADALL machine instruction. A problem in DOS mode would crash the entire computer. In constrast, OS/2 2.0 could benefit from the virtual 8086 mode of the Intel 80386 processor in order to create a potentially totally secure virtual machine for DOS to run inside. However, such a secure VM incurrs a significant performance hit compared to original hardware, and unsecuring some parts of the VM (OS/2 was very configurable in this area) to make it more usable again allowed DOS programs to crash the computer. Just like Windows, OS/2 could not run protected mode DOS programs using the older VCPI interface; it only supported programs written according to DPMI.

Compatibility with Windows was achieved by adapting Windows GUI code to run inside a 16-bit OS/2 process, rather than above the original Windows kernel. For this reason, Windows could not run in the so-called "386 enhanced" mode (which did not matter very much, since OS/2 managed virtual memory and DOS virtual machines independently). Originally, Windows code was included in the distribution, but later red OS/2 versions reused whatever Windows version the user had installed previously, patching it on the fly.

This process containing Windows could either run full-screen, using its own set of video drivers, or "seamlessly", where Windows programs would appear directly on the OS/2 desktop. Again, this required specially enabled drivers. To achieve isolation between Windows programs, OS/2 needed to run several copies of Windows in parallel. This approach provided poor stability. For example, switching between the OS/2 desktop and the Windows desktop could easily hang the machine and required considerable system resources, especially memory. Microsoft used a simpler approach in Windows NT, translating Win16 system calls to Win32 ones by the means of a Windows-on-Windows adaptation layer.

OS/2 2.0 also provided a new GUI called the Workplace Shell and a 32-bit API for native programs, though the OS itself was a mixture of 16-bit and 32-bit code (as Windows 95 would be).

OS/2 Warp 3, released in 1994, was a fully 32-bit OS. It offered a host of benefits, notably broader hardware support, greater multimedia capabilities, Internet-compatible networking, and a basic office application suite. In 1996, Warp 4 added Java and voice recognition software. IBM also released server editions of Warp 3 and Warp 4. The UK-distributed free demo CD-Rom of OS/2 Warp essentially contained the entire OS and was easily, even accidentally cracked, meaning that even people who liked it didn't have to buy it.

Fading out

Overall, OS/2 failed to catch on in the mass market and is today little used outside certain niches where IBM traditionally had a stronghold. For example, many banks, especially Automated Teller Machines, run OS/2 with a customized user interface; French SNCF national railways used OS/2 1.x in thousands of ticket selling machines. Nevertheless, OS/2 still maintains a small and dedicated community of followers. IBM, unlike Microsoft, charged ISVs for the OS/2 development kit, while Microsoft gave the Windows SDK away free.

Although IBM began indicating shortly after the release of Warp 4 that OS/2 would eventually be withdrawn, the company has only recently published a definite end-of-support date (December 2006). The latest IBM version is 4.52, which was released for both desktop and server systems in December 2001. A company called Serenity Systems has been reselling OS/2 since 2001, calling it eComStation. The latest version is 1.2, released in 2004.

IBM is still delivering fixes and updates on a regular basis. IBM urges customers to migrate their often highly complex applications to e-business technologies such as Java in a platform-neutral manner. Once application migration is completed, IBM recommends to migrate to a different operating system without giving any specific recommendations.

Security niche

OS/2 is virtually free of computer viruses, but this is not due to its architecture. Its design could make it as vulnerable as Windows, if not more so, because of its extensive scriptability, but its reduced mind and market share probably discourage virus writers. There are, however, OS/2-based antivirus programs dealing with DOS viruses and Windows viruses that could go through an OS/2 server.


There is a community of OS/2 users and developers, along with loyal company customers, hoping that IBM will release OS/2 or a significant part of it as open source. It is unlikely, though, that the entire OS will be open because it contains third-party code, much of it from Microsoft. Still, the community has suggested that, even if only the IBM portion of it is made open, the missing parts could be written by the same community to form a next-generation version of the OS. There is a onging petition to open parts of the OS. See Petition to open (http://www.os2world.com/petition/)

With the possibility of an open source future for OS/2, the OS may be given a new lease of life. IBM's current and heavy involvement with several open source projects indicate that opening parts of OS/2 will not be difficult for the company. But until then, OS/2's future remains in limbo.


Technically, OS/2 2.0 is often believed to be IBM's own work, a beta version, accompanied by an SDK, had already been released by Microsoft in the second half of 1990; OS/2 32-bit executable files have almost exactly the format of Windows 3.0 VxD device drivers (older 16-bit executables have the format of Windows executables). IBM also seems mostly responsible for the GUI part of OS/2 (notably, the Presentation Manager API did not change in 2.0), and probably for the divergence in syntax and semantics compared to Windows. This was an underlying cause for the breakup between IBM and Microsoft when Windows 3.0 became much more successful than OS/2. However, open source operating systems such as Linux have already profited from OS/2 indirectly through IBM's release of the JFS file system, which was ported from the OS/2 code base instead of the AIX original.

The graphic system has a layer named Presentation Manager that manages windows, fonts and icons. This is similar in functionality to a non-networked version of X11. On top of this lies the Workplace Shell (WPS), introduced in OS/2 2.0, which is an object-oriented shell allowing the user to access files and printers, and launch programs. WPS follows IBM's Common User Access user interface standards.

In recent years, IBM decided not to support the plethora of graphic cards, and licensed a reduced version of Scitech drivers.

WPS represents objects such as disks, folders, files, program objects, and printers using the System Object Model (SOM), which allows code to be shared among applications, possibly written in different programming languages. A distributed version called DSOM allowed objects on different computers to communicate. DSOM is based on CORBA. SOM is similar to, and a direct competitor to, Microsoft's Component Object Model. SOM and DSOM are no longer being developed.

OS/2 also includes a compound document technology called OpenDoc, which was developed with Apple. OpenDoc is also no longer being developed.

The multimedia capabilities of OS/2 are accessible through Media Control Interface commands. The last update (bundled with the IBM version of Netscape Navigator plugins) added support for MPEG files. Support for newer formats like PNG, progressive JPEG, DivX, OGG, MP3 comes from third parties. Sometimes it is integrated with the multimedia system, but in other offers it comes as standalone applications.

The TCP/IP stack is based on the open source BSD stack.

See also


"During the next 10 years, millions of programmers and users will utilise this system" Bill Gates, November 1988 (in the Foreword to the Inside OS/2 book by Gordon Letwin, Microsoft's architect for OS/2).

Bill Gates is supposed to have said that in a moment of absent-mindedness...

External links

ca:OS/2 da:OS/2 de:OS/2 es:OS/2 eo:OS/2 fr:OS/2 it:OS/2 hu:OS/2 nl:OS/2 no:OS/2 ja:OS/2 pl:OS/2 pt:OS/2 ru:OS/2 fi:OS/2 sv:OS/2 zh:OS/2


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