IBM Personal System/2

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Personal System 2 Series of Computers.png
IBM Personal System/2 series of computers
Developer International Business Machines Corporation (IBM)
Type Professional Computer
Release date April 1987; 30 years ago (1987-04)
Predecessor IBM Personal Computer/AT
Successor IBM PC Series

The Personal System/2 or PS/2 was IBM's third generation of personal computers. Released in 1987, it officially replaced the IBM PC, XT, AT, and PC Convertible in IBM's lineup.

The PS/2 line was created by IBM in an attempt to recapture control of the PC market by introducing an advanced yet proprietary architecture. IBM believed its considerable market presence plus the reliability of the PS/2 ensured that the systems would sell in relatively large numbers, especially to large businesses.

The other major manufacturers balked at IBM's licensing terms to develop and sell compatible hardware, particularly as IBM demanded royalties on a per machine basis, and so in response to the PS/2, they developed the Extended Industry Standard Architecture (EISA). Also the evolving Wintel architecture was seeing a period of dramatic reductions in price, and so these developments prevented the PS/2 from returning control of the PC market to IBM.

However, many of the PS/2's innovations, such as the 16550 UART (serial port), 1.44 MB 3.5-inch floppy disk format, 72-pin SIMMs, the PS/2 keyboard and mouse ports, and the VGA video standard, went on to become standards in the broader PC market.[1][2]

The OS/2 operating system was announced at the same time as the PS/2 line and was intended to be the primary operating system for models with Intel 286 or later processors. However, at the time of the first shipments, only PC DOS was available. OS/2 1.0 (text-mode only) and Microsoft's Windows 2.0 became available several months later. IBM also released AIX PS/2, a UNIX operating system for PS/2 models with Intel 386 or later processors.

IBM Personal System/2 Model 30 286. Power-on self-test, bootstrapping, power-off


For years before IBM released the PS/2, rumors spread about IBM's plans for successors to its IBM PC, XT, and AT personal computers. Among the rumors that did not come true:[3]

  • The company would use proprietary, hard-to-copy versions of the Intel 80286 and 80386 processors.
  • The company would release a version of its VM mainframe operating system for them.
  • The company would design the new computers to make third-party communications products more difficult to design.

IBM's PS/2 was designed to remain software compatible with their PC/AT/XT line of computers upon which the large PC clone market was built, but the hardware was quite different. PS/2 had two BIOSes; one was named ABIOS (Advanced BIOS) which provided a new protected mode interface and was used by OS/2, and the other was named CBIOS (Compatible BIOS) which was included in order for the PS/2 to be software compatible with the PC/AT/XT. CBIOS was so compatible that it even included Cassette BASIC. While IBM did not publish the BIOS source code, it did promise to publish BIOS entry points.[3]

Micro Channel Architecture[edit]

With the IBM PS/2 line, Micro Channel Architecture (MCA) was also introduced.[3] MCA was conceptually similar to the channel architecture of the IBM System/360 mainframes. MCA was technically superior to ISA and allowed for higher speed communications within the system. MCA featured many advances not seen in other standards until several years later. Transfer speeds were on par with the much later PCI standard. MCA allowed one-to-one, card to card, and multi-card to processor simultaneous transaction management which is a feature of the PCI-X bus format.

Bus mastering capability, bus arbitration, and a primitive form of plug-and-play BIOS management of hardware were all benefits of MCA. (One book from the year 2000 writes: "MCA used an early (and user-hostile) version of what we know now as “Plug-N′-Play”, requiring a special setup disk for each machine and each card."[4] MCA never gained wide acceptance outside of the PS/2 line due to IBM's anti-clone practices[citation needed] and incompatibilities with ISA. IBM offered to sell an MCA license to anyone who could afford the royalty. However, royalties were required for every MCA-compatible machine sold and a payment for every IBM-compatible machine the particular maker had made in the past.[citation needed] There was nothing unique in IBM insisting on payment of royalties on the use of its patents applied to Micro Channel based machines. However, up until that time, some companies had failed to pay IBM for the use of its patents on the earlier generation of Personal Computer.



The PS/2 IBM Model M keyboard used the same 101-key layout of the previous IBM PC/AT Extended keyboard, itself derived from the original IBM PC keyboard.[3] European variants had 102 keys with the addition of an extra key to the right of the left Shift key. The Model M, using a buckling spring mechanism, is still being manufactured by Unicomp.


The original IBM PS/2 mouse.
The PS/2 connection ports (later colored purple for keyboard and green for mouse, according to PC 97) were once commonly used for connecting input devices.

PS/2 systems introduced a new specification for the keyboard and mouse interfaces, which are still in use today and are thus called "PS/2" interfaces. The PS/2 keyboard interface was electronically identical to the long-established AT interface, but the cable connector was changed from the 5-pin DIN connector to the smaller 6-pin mini-DIN interface. The same connector and a similar synchronous serial interface was used for the PS/2 mouse port.

Additionally, the PS/2 introduced a new software data area known as the Extended BIOS Data Area (EBDA). Its primary use was to add a new buffer area for the dedicated mouse port. This also required making a change to the "traditional" BIOS Data Area (BDA) which was then required to point to the base address of the EBDA.


Most of the initial range of PS/2 models were equipped with a new frame buffer known as the Video Graphics Array, or VGA for short. This effectively replaced the previous EGA standard.[3] VGA increased graphics memory to 256 KB and provided for resolutions of 640×480 with 16 colors, and 320×200 with 256 colors. VGA also provided a palette of 262,144 colors (as opposed to the EGA palette of 64 colors). The IBM 8514 and later XGA computer display standards were also introduced on the PS/2 line.



Although the design of these adapters did not become an industry standard as VGA did, their 1024×768 pixel resolution was subsequently widely adopted as a standard by other manufacturers, and "XGA" became a synonym for this screen resolution. The lone exception were the bottom-rung Model 25 and 30, which had a cut-down version of VGA referred to as MCGA. This supported CGA graphics modes, VGA 320x200x256 and 640x480x2 mode, but not EGA or color 640x480.

MCA IBM XGA-2 Graphics Card

VGA video connector[edit]

All of the new PS/2 graphics systems (whether MCGA, VGA, 8514, or later XGA) used a 15-pin D-sub connector for video out. This used analog RGB signals, rather than four or six digital color signals as on previous CGA and EGA monitors. The digital signals limited the color gamut to a fixed 16 or 64 color palette with no room for expansion. In contrast, any color depth (bits per primary) can be encoded into the analog RGB signals so the color gamut can be increased arbitrarily by using wider (more bits per sample) DACs and a more sensitive monitor. The connector was also compatible with analog grayscale displays. Unlike earlier systems (like MDA and Hercules) this was transparent to software, so all programs supporting the new standards could run unmodified whichever type of display was attached. (On the other hand, whether the display was color or monochrome was undetectable to software, so selection between application displays optimized for color or monochrome, in applications that supported both, required user intervention.) These grayscale displays were relatively inexpensive during the first few years the PS/2 was available, and they were very commonly purchased with lower-end models.

The VGA connector became the de facto standard for connecting monitors and projectors on both PC and non-PC hardware over the course of the early 1990s, replacing a variety of earlier connectors.


Some PS/2 models used a quick-attachment socket on the back of the floppy drive. This connector is incompatible with a standard 5.25" floppy connector because IBM also supplied power to the drive via the connector, which was not done for the 5.25" drive. Also shown on the right is the special IBM-only hard drive which incorporates power and data into a single connector. Power supplies of certain PS/2 models did not have the additional spare 4-pin power connectors for use with internal storage.
Close-up view of unusual 72-pin MCA internal hard drive connector.

Apple had first popularized the 3.5" floppy on the Macintosh line and IBM brought them to the PC in 1986 with the PC Convertible. In addition, they could be had as an optional feature on the XT and AT. The PS/2 line used entirely 3.5" drives which assisted in their quick adoption by the industry, although the lack of 5.25" drive bays in the computers created problems later on in the 90s as they could not accommodate internal CD-ROM drives. In addition, the lack of 5.25" floppy drives meant that PS/2 users could not easily run the large body of existing IBM compatible software.[6]

While the 3.5" disk format itself was standard, the data cable on the PS/2 also carried the power for the drives, meaning that most 3.5" floppy drives could not be used. In the initial line-up, IBM used 720 KB double density (DD) capacity drives on the 8086-based models and 1.44 MB high density (HD) on the 80286-based and higher models. By the end of the PS/2 line they had moved to a somewhat standardized capacity of 2.88 MB.

The PS/2 floppy drives were notorious for not having a capacity detector. 1.44 MB floppies had a hole so that drives could identify them from 720 KB floppies, preventing users from formatting the smaller capacity disks to the higher capacity (doing so would work, but with a higher tendency of data loss). Clone manufacturers implemented the hole detection, but IBM did not. As a result of this a 720 KB floppy could be formatted to 1.44 MB in a PS/2, but the resulting floppy would only be readable by a PS/2 machine.[7]

The PS/2 used several different types of internal hard drives. Early models used ESDI drives. Some desktop models provided a single connector which incorporated power and data, as shown in the photo above. Later models used ATA/IDE or Parallel SCSI. Typically, desktop PS/2 models only permitted use of one hard drive inside the computer case. Additional storage could be attached externally, using the optional SCSI interface.


The PS/2 introduced the 72-pin SIMM[citation needed] which became the de facto standard for RAM modules by the mid-90s in mid-to-late 486 and early Pentium desktop systems. 72-pin SIMMs were 32/36 bits wide and replaced the old 30-pin SIMM (8/9-bit) standard. The older SIMMs were much less convenient because they had to be installed in sets of two or four to match the width of the CPU's 16-bit (Intel 80286 and 80386SX) or 32-bit (80386 and 80486) bus. 72-pin SIMMs were also made with greater capacities.


At launch, the PS/2 family comprised the Model 30, 50, 60 and 80;[3] the Model 25 was launched a few months later.

The PS/2 Models 25 and 30 (IBM 8525 and 8530 respectively) were the lowest-end models in the lineup and meant to replace the IBM PC and XT. Model 25s came with either an 8086 CPU running at 8Mhz, 512k of RAM, and 720k floppy disks, or 80286 CPU. The 8086s had no expansion slots and a built-in monitor, which could be either color or monochrome, while the 80286 models came with VGA monitor and ISA expansion slots. A cut-down Model M with no numeric keypad was standard, with the normal keyboard being an extra-cost option. There was a very rare later model called the PS/2 Model 25-SX which sported either a 16 mHz or 20 mHz 386 CPU, up to 12 MB of memory, IDE hard drive, VGA Monitor and 16 bit ISA slots making it the highest available model 25 available denoted by model number 8525-L41.

Case badge on model 25-SX 386-20 8525-L41

The Model 30 had either an 8086 or 286 CPU and sported the full 101-key keyboard and standalone monitor along with five ISA expansion slots. 8086 models had 720k floppies while 286 models had 1.44MB ones. Both the Model 25 and 30 could have an optional 20MB ST-506 hard disk (which in the Model 25 took the place of the second floppy drive if so equipped and used a proprietary 3.5" form factor). 286-based Model 30s are otherwise a full AT-class machine and support up to 4MB of RAM.

IBM Personal System/2 Model 25

Later ISA PS/2 models comprised the Model 30-286 (a Model 30 with an Intel 286 CPU), Model 35 (IBM 8535) and Model 40 (IBM 8540) with Intel 386SX or IBM 386SLC processors.

The higher-numbered models were equipped with the Micro Channel bus and mostly ESDI or SCSI hard drives (models 60-041 and 80-041 had MFM hard drives). PS/2 Models 50 (IBM 8550) and 60 (IBM 8560) used the Intel 286 processor, the PS/2 Models 70 (IBM 8570) and 80 used the 386DX, while the mid-range PS/2 Model 55SX (IBM 8555-081) used the 16/32-bit 386SX processor. Later Model 70 and 80 variants (B-xx) also used 25 MHz Intel 486 processors, in a complex called the Power Platform.

The externally very similar Models 60 and 80 next to each other
IBM Model 70 (case open over case closed)

The PS/2 Models 90 (IBM 8590/9590) and 95 (IBM 8595/9595/9595A) used Processor Complex daughterboards holding the CPU, memory controller, MCA interface, and other system components. The available Processor Complex options ranged from the 20 MHz Intel 486 to the 90 MHz Pentium and were fully interchangeable. The IBM PC Server 500, which has a motherboard identical to the 9595A, also uses Processor Complexes.

Other later Micro Channel PS/2 models included the Model 65SX with a 16 MHz 386SX; various Model 53 (IBM 9553), 56 (IBM 8556) and 57 (IBM 8557) variants with 386SX, 386SLC or 486SLC2 processors; the Models 76 and 77 (IBM 9576/9577) with 486SX or 486DX2 processors respectively; and the 486-based Model 85 (IBM 9585).

The IBM PS/2E (IBM 9533) was the first Energy Star compliant personal computer. It had a 50 MHz IBM 486SLC processor, an ISA bus, four PC card slots, and an IDE hard drive interface. The environmentally friendly PC borrowed many components from the ThinkPad line and was composed of recycled plastics, designed to be easily recycled at the end of its life, and used very little power.

IBM also produced several portable and laptop PS/2s, including the Model L40 (ISA-bus 386SX), N33 (IBM's first notebook-format computer from year 1991, Model 8533, 386SX), N51 (386SX/SLC), P70 (386DX) and P75 (486DX2).

The IBM PS/2 Server 195 and 295 (IBM 8600) were 486-based dual-bus MCA network servers supporting asymmetric multiprocessing, designed by Parallan Computer Inc.

The IBM PC Server 720 (IBM 8642) was the largest MCA-based server made by IBM, although it was not, strictly speaking, a PS/2 model. It could be fitted with up to six Intel Pentium processors interconnected by the Corollary C-bus and up to eighteen SCSI hard disks. This model was equipped with seven combination MCA/PCI slots.

The IBM ThinkPad 700C, aside from being labeled "700C PS/2" on the case, featured MCA and a 486SLC CPU.


The PS/2's controversial hardware design was tied to a marketing strategy that was similarly unsuccessful. During the 1980s, IBM's advertising of the original PC and its other product lines had frequently used the likeness of Charlie Chaplin. For the PS/2, however, IBM augmented this character with a notorious jingle that seemed more suitable for a low-end consumer product than a business-class computing platform:

“How ya' gonna' do it?
PS/2 It!
It's as easy as I.B.M.”

Another campaign featured actors from the television show M*A*S*H playing the staff of a contemporary (i.e. late-1980s) business in roles reminiscent of their characters' roles from the series. Harry Morgan, Larry Linville, William Christopher, Wayne Rogers, Gary Burghoff, Jamie Farr, and Loretta Swit were in from the beginning, whereas Alan Alda joined the campaign later.[8]

The profound lack of success of these advertising campaigns led, in part, to IBM's termination of its relationships with its global advertising agencies; these accounts were reported by Wired magazine to have been worth over $500 million a year, and the largest such account review in the history of business.[9]

Overall, the PS/2 line was largely unsuccessful with the consumer market, even though the PC based Models 30 and 25 were an attempt to address it. With what was widely seen as a technically competent but cynical attempt to gain undisputed control of the market, IBM unleashed an industry and consumer backlash. In large part, IBM failed to establish a link in the consumer's mind between the PS/2 MicroChannel architecture and the new OS/2 operating system.[10] The firm suffered massive financial losses for the remainder of the decade, forfeited its previously unquestioned position as the industry leader, and eventually lost its status as the largest single manufacturer of personal computers, first to Compaq and then to Dell. After the failure of the PS/2 line to establish a new standard, IBM was forced to revert to building ISA PCs—following the industry it had once led—with the PS/1 line and later the Aptiva and PS/ValuePoint lines.

Still, the PS/2 platform experienced success in the business sector where the reliability, ease of maintenance and strong support from IBM offset the rather daunting cost of the machines. Also, many people still lived with the motto "Nobody ever got fired for buying an IBM." The model 55SX and later 56SX were the leading sellers for almost their entire lifetimes. Many models of PS/2 systems saw a production life span that took them well into the late 1990s.

See also[edit]


  1. ^ IBM Personal System/2 Hardware Interface Technical Reference (PDF). IBM. May 1988. 68X2330. Retrieved 2016-11-26. 
  2. ^ "PS/2 Reference Manuals". MCA Mafia. 2006-03-04. Retrieved 2016-11-26. 
  3. ^ a b c d e f BYTE editorial staff (June 1987). "The IBM PS/2 Computers". BYTE. p. 100. Retrieved 5 November 2013. 
  4. ^ Gilbert Held (2000). Server Management. CRC Press. p. 199. ISBN 978-1-4200-3106-5. 
  5. ^
  6. ^ Jim Porter (1998-12-14). "100th Anniversary Conference: Magnetic Recording and Information Storage" (PDF). Archived from the original (PDF) on 2012-03-28. Retrieved 2014-03-24. 
  7. ^ Ohland, Louis. "floppy". 
  8. ^ "M*A*S*H Cast Commercials - IMB PS/2". Retrieved 13 April 2014. 
  9. ^ Wired, Issue 3.08, August 1995
  10. ^ McCracken, Harry. "25 Years of IBM’s OS/2: The Strange Days and Surprising Afterlife of a Legendary Operating System" – via 

Further reading[edit]

  • Burton, Greg. IBM PC and PS/2 pocket reference. NDD (the old dealer channel), 1991.
  • Byers, T.J. IBM PS/2: A Reference Guide. Intertext Publications, 1989. ISBN 0-07-009525-6.
  • Dalton, Richard and Mueller, Scott. IBM PS/2 Handbook . Que Publications, 1989. ISBN 0-88022-334-0.
  • Held, Gilbert. IBM PS/2: User's Reference Manual. John Wiley & Sons Inc., 1989. ISBN 0-471-62150-1.
  • Hoskins, Jim. IBM PS/2. John Wiley & Sons Inc., fifth revised edition, 1992. ISBN 0-471-55195-3.
  • Leghart, Paul M. The IBM PS/2 in-depth report. Pachogue, NY: Computer Technology Research Corporation, 1988.
  • Newcom, Kerry. A Closer Look at IBM PS/2 Microchannel Architecture. New York: McGraw-Hill, 1988.
  • Norton, Peter. Inside the IBM PC and PS/2. Brady Publishing, fourth edition 1991. ISBN 0-13-465634-2.
  • Outside the IBM PC and PS/2: Access to New Technology. Brady Publishing, 1992. ISBN 0-13-643586-6.
  • Shanley, Tom. IBM PS/2 from the Inside Out. Addison-Wesley, 1991. ISBN 0-201-57056-4.

External links[edit]