Editing IBM PS/2 (section)

==Technology==
{| class="wikitable" style="float:right;clear:both;"
|+Predecessors to the PS/2
|-
! Name !! Year
|-
| [[IBM Personal Computer]] || 1981
|-
| [[IBM Personal Computer XT]] || 1983
|-
| [[IBM Portable Personal Computer]] || 1984
|-
| [[IBM PCjr]] || 1984
|-
| [[IBM Personal Computer/AT]] || 1984
|-
| [[IBM PC Convertible]] || 1986
|-
| [[IBM Personal Computer XT#IBM XT 286|IBM Personal Computer XT 286]] || 1986
|}
IBM's PS/2 was designed to remain software compatible with their PC/AT/XT line of computers upon which the large [[IBM PC compatible|PC clone]] market was built, but the hardware was quite different. PS/2 had two [[BIOS]]es: one named ABIOS (Advanced BIOS) which provided a new [[protected mode]] interface and was used by OS/2, and CBIOS (Compatible BIOS) which was included to be software compatible with the PC/XT/AT. 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 point]]s.{{r|byte198706}}

===Micro Channel architecture===
{{main|Micro Channel architecture}}
With certain models to the IBM PS/2 line, [[Micro Channel architecture|Micro Channel Architecture]] (MCA) was also introduced.{{r|byte198706}} MCA was conceptually similar to the channel architecture of the [[IBM System/360]] mainframes. MCA was technically superior to [[Industry Standard Architecture|ISA]] and allowed for higher-speed communications within the system. The majority of MCA's features would be seen in later buses with the exception of: streaming-data procedure, channel-check reporting, error logging<ref>{{cite web|url=http://www.mcamafia.de/pdf/server_hmm_s30h2501_01.pdf|access-date=30 April 2023|title=Hardware Maintenance Manual|date=March 1996|website=mcamafia.de|archive-date=25 April 2023|archive-url=https://web.archive.org/web/20230425135058/http://www.mcamafia.de/pdf/server_hmm_s30h2501_01.pdf|url-status=live}}</ref> and internal bus-level video pass-through for devices like the [[IBM 8514]]. Transfer speeds were on par with the much later [[Peripheral Component Interconnect|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|plug-and-play]] management of hardware were all benefits of MCA. Gilbert Held in his 2000 book ''Server Management'' observes: "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."<ref name="Held2000">{{cite book|author=Gilbert Held|title=Server Management|url=https://books.google.com/books?id=Y9q-i_HGtAAC&pg=PA199|year=2000|publisher=CRC Press|isbn=978-1-4200-3106-5|page=199}}</ref> MCA never gained wide acceptance outside of the PS/2.

When setting up the card with its disk, all choices for interrupts and other changes were accomplished automatically by the PC reading the old configuration from the floppy disk. This made necessary changes, then recorded the new configuration to the floppy disk. This meant that the user must keep that same floppy disk matched to that particular PC. For a small organization with a few PCs, this was annoying, but less expensive and time-consuming than bringing in a PC technician to do installation. But for large organizations with hundreds or even thousands of PCs, permanently matching each PC with its own floppy disk was a logistical nightmare. Without the original, (and correctly updated) floppy disk, no changes could be made to the PC's cards.

===Keyboard/mouse===

====Layout====
The PS/2 IBM [[Model M keyboard]] used the same 101-key layout of the previous [[IBM PC/AT]] Extended keyboard.{{r|byte198706}}  European variants had 102 keys with the addition of an extra key to the right of the left Shift key.

====Interface====
{{further|PS/2 port}}
[[Image:IBM PS2 hiiri.jpg|upright|thumb|left|The original IBM PS/2 mouse]]
[[Image:ps-2-ports.jpg|thumb|right|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 [[Computer keyboard|keyboard]] and [[computer mouse|mouse]] interfaces, which are still in use today{{Citation needed|date=March 2025|reason=This might have been true a decade ago, but practically no new peripherals uses these interfaces, and very little (newly manufactured) computer hardware even has the physical connectors.}} (though increasingly supplanted by [[USB]] devices) and are thus called "PS/2" interfaces. The PS/2 keyboard interface, inspired by Apple's ADB interface, was electronically identical to the long-established [[AT Keyboard|AT]] interface, but the cable connector was changed from the 5-pin [[DIN connector]] to the smaller 6-pin [[Mini-DIN connector|mini-DIN]] interface. The same connector and a similar synchronous serial interface was used for the PS/2 mouse port.

The initial desktop ''[[IBM PS/2 Model 50|Model 50]]'' and ''[[IBM PS/2 Model 60|Model 60]]'' also featured a new cableless internal design, based on use of interposer circuit boards to link the internal drives to the planar (motherboard). Additionally, these machines could be largely disassembled and reassembled for service without tools.

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.

Another new PS/2 innovation was the introduction of bidirectional [[parallel port]]s which, in addition to their traditional use for connecting a printer, could now function as a high-speed data transfer interface. This allowed the use of new hardware such as parallel port [[Image scanner|scanner]]s, CD-ROM drives, and also enhanced the capabilities of printers by allowing them to communicate with the host PC and send back signals instead of simply being a passive output device.

===Graphics===
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 [[Enhanced Graphics Adapter|EGA]] standard.{{r|byte198706}} VGA increased graphics memory to 256&nbsp;KB and provided for resolutions of 640×480 with 16 colors, and {{resx|320×200}} with 256 colors. VGA also provided a palette of [[List of monochrome and RGB color formats#18-bit RGB|262,144 colors]] (as opposed to the EGA palette of 64 colors). The [[IBM 8514]] and later [[Extended Graphics Array|XGA]] [[computer display standard]]s were also introduced on the PS/2 line.

Key monitors and their maximum resolutions:
*8504: 12-inch, {{resx|640×480}}, 60&nbsp;Hz [[non-interlaced]], 1991, monochrome
*8507: 19-inch, {{resx|1024×768}}, 43.5&nbsp;Hz [[interlaced]], 1988, monochrome
*8511: 14-inch, {{resx|640×480}}, 60&nbsp;Hz [[non-interlaced]], 1987
*8512: 14-inch, {{resx|640×480}}, 60&nbsp;Hz [[non-interlaced]], 1987
*8513: 12-inch, {{resx|640×480}}, 60&nbsp;Hz [[non-interlaced]], 1987
*8514: 16-inch, {{resx|1024×768}}, 43.5&nbsp;Hz [[interlaced]], 1987
*8515: 14-inch, {{resx|1024×768}}, 43.5&nbsp;Hz [[interlaced]], 1991
*8516: 14-inch, {{resx|1024×768}}, 43.5&nbsp;Hz [[interlaced]], 1991
*8518: 14-inch, {{resx|640×480}}, 75&nbsp;Hz [[non-interlaced]], 1992
*9515: 14-inch, {{resx|1024×768}}, 43.5&nbsp;Hz [[interlaced]], 1992
*9517: 16-inch, {{resx|1280×1024}}, 53&nbsp;Hz [[interlaced]], 1991
*9518: 14-inch, {{resx|640×480}}, [[non-interlaced]], 1992
*38F4737: 10-inch, {{resx|640×480}}, [[non-interlaced]], 1989, amber monochrome plasma screen; this display was exclusive to models P70 and P75

In truth, all XGA {{resx|1024×768}} monitors are multimode, as XGA works as an add-on card to a built-in VGA and transparently passes the VGA signal through when not operating in a high-resolution mode. All of the listed 85xx displays can therefore sync 640×480 at 60&nbsp;Hz (or {{resx|720×400}} at 70&nbsp;Hz) in addition to any higher mode they may also be capable of. This however is not true of the 95xx models (and some unlisted 85xx's), which are specialist workstation displays designed for use with the XGA-2 or Image Adapter/A cards, and whose fixed frequencies all exceed that of basic VGA – the lowest of their commonly available modes instead being {{resx|640×480}} at 75&nbsp;Hz, if not something much higher still. It is also worth noting that these were still merely dual- or "multiple-frequency" monitors, not variable-frequency (also known as multisync); in particular, despite running happily at {{resx|640×480}}, {{resx|720×400}} and {{resx|1024×768}}, an (e.g.) 8514 cannot sync the otherwise common intermediate {{resx|800×600}} SVGA resolution, even at the relatively low 50 to 56&nbsp;Hz refresh rates initially used.

Although the design of these adapters did not become an industry standard as VGA did, their {{resx|1024×768}} pixel resolution was subsequently widely adopted as a standard by other manufacturers, and XGA became a synonym for this screen resolution. The only exceptions were the bottom-rung 8086-based Model 25 and 30, which had a cut-down version of VGA referred to as [[Multi-Color Graphics Array|MCGA]]; the 286 models came with VGA. This supported CGA graphics modes, VGA {{resx|320×200}} 256 color and {{resx|640×480}} monochrome mode, but not EGA or color {{resx|640×480}}.

[[File:MCA IBM XGA-2.jpg|thumb|MCA IBM XGA-2 graphics card]]

====VGA video connector====
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) [[digital-to-analog converter|DACs]] and a more sensitive monitor. The connector was also compatible with analog grayscale displays. Unlike earlier systems such as [[IBM Monochrome Display Adapter|MDA]] and [[Hercules Graphics Card|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.

===Storage===
[[File:IBM PS2 MCA Model 70, riser for floppy and hard drive.jpg|thumb|Some PS/2 models used a quick-attachment socket on the back of the floppy drive which is incompatible with a standard 5.25-inch floppy connector.]]
[[Image:IBM PS2 MCA Model 55 SX, hard drive bus closeup.jpg|thumb|left|Close-up of unusual 72-pin MCA internal hard drive connector]]

Apple had first popularized the 3.5-inch 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-inch drives which assisted in their quick adoption by the industry, although the lack of 5.25-inch drive bays in the computers created problems later on in the 1990s as they could not accommodate internal CD-ROM drives. In addition, the lack of built-in 5.25-inch floppy drives meant that PS/2 users could not immediately run the large body of existing IBM-compatible software.<ref>{{cite web |url=http://www.disktrend.com/pdf/portrpkg.pdf |archive-url=https://web.archive.org/web/20120328044723/http://www.disktrend.com/pdf/portrpkg.pdf |title=100th Anniversary Conference: Magnetic Recording and Information Storage |date=1998-12-14 |access-date=2014-03-24 |archive-date=2012-03-28 |author=Jim Porter |publisher=disktrend.com}}</ref> However IBM made available optional external 5.25-inch drives, with internal adapters for the early PS/2 models, to enable data transfer.
[[File:IBM 3.5-inch DD and HD diskettes.jpg|thumb|3.5-inch DD and HD floppies]]
In the initial lineup, IBM used 720&nbsp;KB double density (DD) capacity drives on the 8086-based models and 1440&nbsp;KB 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 2880&nbsp;KB.

The PS/2 floppy drives lacked a capacity detector. 1440&nbsp;KB floppies had a hole so that drives could distinguish them from 720&nbsp;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 1440&nbsp;KB in a PS/2, but the resulting floppy would only be readable by a PS/2 machine.<ref>{{cite web|url=http://ohlandl.ipv7.net/floppy/floppy.html#Format_720K_On_144MB|title=floppy|first=Louis|last=Ohland|website=ohlandl.ipv7.net|access-date=2011-02-19|archive-date=2011-07-23|archive-url=https://web.archive.org/web/20110723160004/http://ohlandl.ipv7.net/floppy/floppy.html#Format_720K_On_144MB|url-status=live}}</ref>

PS/2s primarily used [[Mitsubishi]] floppy drives and did not use a separate Molex power connector; the data cable also contained the power supply lines. As the hardware aged the drives often malfunctioned due to bad quality [[capacitor]]s.{{Citation needed|date=February 2019}}

The PS/2 used several different types of internal hard drives. Early models used [[Modified Frequency Modulation|MFM]] or [[Enhanced Small Disk Interface|ESDI]] drives. Some desktop models used combo power/data cables similar to the floppy drives. Later models used [[DBA ESDI]] 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.

===Memory===
Later PS/2 models introduced the [[SIMM#72-pin SIMMs|72-pin SIMM]]<ref>{{Cite news|url=https://www.pcworld.com/article/258989/the_ibm_ps_2_25_years_of_pc_history.html|title=The IBM PS/2: 25 Years of PC History|work=PCWorld|access-date=2018-08-28|language=en|archive-date=2018-10-04|archive-url=https://web.archive.org/web/20181004213847/https://www.pcworld.com/article/258989/the_ibm_ps_2_25_years_of_pc_history.html|url-status=live}}</ref> which became the [[de facto]] standard for [[Random-access memory|RAM]] modules by the mid-1990s in mid-to-late [[i486|486]] and nearly all [[Pentium (original)|Pentium]] desktop systems. The 72-pin SIMMs were 32/36 bits wide and replaced the old [[SIMM#30-pin SIMMs|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 [[i386#The 80386SX variant|80386SX]]) or 32-bit (80386 and [[i486|80486]]) data bus, and would have been extremely inconvenient to use in Pentium systems (which featured a 64-bit memory bus). The 72-pin SIMMs were also made with greater capacities (starting at 1 MB and ultimately reaching 128 MB, instead of 256 KB to 16 MB (and usually no more than 4 MB) for 30-pin) and in a more finely graduated range (powers of 2, instead of powers of 4).

Many PS/2 models also used proprietary IBM SIMMs and could not be fitted with commonly available types. However industry standard SIMMs could be modified to work in PS/2 machines if the SIMM-presence and SIMM-type detection bridges, or associated contacts, were correctly rewired.{{Citation needed|date=November 2021}}