IBM Personal Computer

IBM Personal Computer

infobox computer
Name = IBM PC (model 5150)
Photo =
Type = Personal computer
Released = August 12, 1981
Discontinued = April 2, 1987
Processor = Intel 8088 @ 4.77 MHz
Memory = 16 kB ~ 256 kB
UCSD p-System

The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981. It was created by a team of engineers and designers under the direction of Don Estridge of the IBM Entry Systems Division in Boca Raton, Florida.

Alongside "microcomputer" and "home computer," the term "personal computer" was already in use before 1981. It was used as early as 1972 to characterize Xerox PARC's Alto. However, because of the success of the IBM Personal Computer, the term came to mean more specifically a microcomputer compatible with IBM's PC products.


The original line of PCs were part of an IBM strategy to get into the small computer market then dominated by the Commodore PET, Atari 8-bit family, Apple II and Tandy Corporation's TRS-80s, and various CP/M machines. ["Total share: 30 years of personal computer market share figures", Jeremy Reimer Dec.14, 2005] IBM's first desktop microcomputer was the IBM 5100, introduced in 1975. It was a complete system - with a built-in monitor, keyboard, and data storage. It was also very expensive - up to US$20,000. It was specifically designed for professional and scientific problem-solvers, not business users or hobbyists. [cite web|url=|title=Obsolete Technology Website|accessdate=2008-08-14] When the PC was introduced in 1981, it was originally designated as the IBM 5150, putting it in the "5100" series, though its architecture wasn't directly descended from the IBM 5100.

Rather than going through the usual IBM design process, a special team was assembled with authorization to bypass normal company restrictions and get something to market rapidly. This project was given the code name "Project Chess" at the IBM Entry Systems Division in Boca Raton, Florida. The team consisted of twelve people headed by Don Estridge and Chief Scientist Larry Potter. They developed the PC in about a year. To achieve this they first decided to build the machine with "off-the-shelf" parts from a variety of different original equipment manufacturers (OEMs) and countries. Previously IBM had always developed their own components. Secondly for scheduling and cost reasons, rather than developing unique IBM PC monitor and printer designs, project management decided to utilize an existing "off-the-shelf" IBM monitor developed earlier in IBM Japan as well as an existing Epson printer model. Consequently, the unique IBM PC industrial design elements were the system unit and keyboard. ″28th Annual Design Review″, "I.D. Magazine", Designers' Choice: IBM Personal Computer, Tom Hardy: Industrial Designer,1982.] They also decided on an open architecture, so that other manufacturers could produce and sell peripheral components and compatible software without purchasing licenses. IBM also sold an "IBM PC Technical Reference Manual" which included a listing of the ROM BIOS source code. [cite web|url=|title=The Virtual PC Museum|author=Charlie Anderson|publisher=Charlie Anderson|date=2003-11-13|accessdate=2006-12-16. Personal website with an image of the technical manual. "The official documentation came in cool three-ring binders, complete with slip covers. Completely typeset.... This book wasn't free, either—I think it cost $60. Supposedly, no Compaq BIOS programmer ever saw one of these. Yeah, right." cite web|url=|title=Jargon File 3.0.0—TechRef Jargon File, "TechRef: /tek'ref/ [MS-DOS] n. The original "IBM PC Technical Reference Manual", including the BIOS listing and complete schematics for the PC. The only PC documentation in the issue package that's considered serious by real hackers."]

At the time, Don Estridge and his team considered using the IBM 801 processor and its operating system that had been developed at the Thomas J. Watson Research Center in Yorktown Heights, New York (The 801 is an early RISC microprocessor designed by John Cocke and his team at Yorktown Heights.) The 801 was at least an order of magnitude more powerful than the Intel 8088, and the operating system many years more advanced than the DOS operating system from Microsoft, that was finally selected. Ruling out an in-house solution made the team’s job much easier and may have avoided a delay in the schedule, but the ultimate consequences of this decision for IBM were far-reaching. IBM had recently developed the Datamaster business microcomputer which used an Intel processor and peripheral ICs; familiarity with these chips and the availability of the Intel 8088 processor was a deciding factor in the choice of processor for the new product. Even the 62-pin expansion bus slots were designed to be similar to the Datamaster slots. Delays due to in-house development of the Datamaster software also influenced the design team to a fast track development process for the PC, with publicly-available technical information to encourage third-party developers. [ David J. Bradley, "The Creation of the IBM PC", BYTE Magazine Volume 15 No. 9 September 1990 pages 414-420 ]

Other manufacturers soon reverse engineered the BIOS to produce their own non-infringing functional copies. Columbia Data Products introduced the first IBM-PC compatible computer in June 1982. In November 1982, Compaq Computer Corporation announced the "Compaq Portable", the first portable IBM PC compatible. The first models were shipped in March 1983.

Once the "IBM PC" became a commercial success, the product came back under the more usual tight IBM management control. IBM's tradition of "rationalizing" their product lines, deliberately restricting the performance of lower-priced models in order to prevent them from "cannibalizing" profits from higher-priced models, worked against them.

Third-party distribution

ComputerLand and Sears Roebuck partnered with IBM from the beginning of development. IBM's head of sales and marketing, H.L. ('Sparky') Sparks, relied on these retail partners for important knowledge of the marketplace.

As a natural progression, Computerland and Sears became the main outlets for the new product. More than 190 Computerland stores already existed, while Sears was in the process of creating a handful of in-store computer centers for sale of the new product. This guaranteed IBM widespread distribution across the United States.

Targeting the new PC at the home market, Sears Roebuck sales failed to live up to expectations. This unfavourable outcome revealed that the original strategy - targeting the office market - was the key to higher sales.


"The IBM PC range"
Model nameModel #IntroducedCPUFeatures
PC5150Aug 19818088Floppy disk or cassette [IBM did not offer own brand cassette recorders, but the 5150 had a cassette player jack, and IBM anticipated that entry level home users would connect their own cassette recorders for data storage instead of using the more expensive floppy drives (and use their existing TV sets as monitors); to this end, IBM initially offered the 5150 in a basic configuration without any floppy drives or monitor at the price of $1,565, whereas they offered a system with a monitor and single floppy drive for an initial $3,005. Few if any users however bought IBM 5150 PCs without floppy drives.] system (external hard drive an optional extra [One or two 10 MB hard drives could be installed into an optionally available expansion unit that had a separate 130 Watt power supply; a hard drive could not normally be installed into the main system unit, because its 63.5 Watt power supply was not sufficient for that (cf. [] ). Few of these expansion units were ever sold, but users found out that they could replace the 63.5 Watt power supply in the system unit with a 130 Watt power supply, thus allowing them to retrofit internal hard drives into their IBM 5150 PCs.)] )
XT5160Mar 19838088First IBM PC to come with an internal hard drive as standard.
XT/3705160/588Oct 19838088System/370 mainframe emulation
3270 PC5271Oct 19838088With 3270 terminal emulation
PCjr4860Nov 19838088Floppy-based home computer
PC Portable5155Feb 19848088Floppy-based portable
AT5170Aug 198480286Medium-speed hard disk
Convertible5140Apr 19868088Microfloppy laptop portable
XT 2865162Sep 198680286Slow hard disk, but zero wait state memory on the motherboard. This 6 MHz machine was actually faster than the 8 MHz ATs (when using planar memory) because of the zero wait states

All IBM personal computers are software compatible with each other in general, but not every program will work in every machine. Some programs are time sensitive to a particular speed class. Older programs will not take advantage of newer higher-resolution display standards.


The original PC had a version of Microsoft BASICIBM Cassette BASIC — in ROM. The CGA (Color Graphics Adapter) video card could use a standard television set or an RGBI monitor for display; IBM's RGBI monitor was their display model 5153. The other option that was offered by IBM was an MDA (Monochrome Display Adapter) and their monochrome display model 5151. It was possible to install both an MDA and a CGA card and use both monitors concurrently, [ [ Dual-Head operation on vintage PCs ] ] if supported by the application program. For example, AutoCAD allowed use of a CGA card for graphics and a separate monochrome board for text menus. Some model 5150 PCs with CGA monitors and a printer port also included the MDA adapter by default, because IBM provided the MDA port and printer port on the same adapter card; it was in fact an MDA/printer port combo card.

The most commonly used storage medium was the floppy disk, though cassette tape was originally envisoned by IBM as a low-budget alternative. Accordingly, the IBM 5150 PC was available with one or two floppy drives or without any drives or storage medium; in the latter case IBM intended that users connect their own existing cassette recorders via the 5150's cassette jack. [The cassette tape port was mechanically identical to, and located next to, the keyboard port on the 5150's mainboard.] A hard disk could not be installed into the 5150's system unit without retrofitting a stronger power supply, but an "Expansion Unit", aka the "IBM 5161 Expansion Chassis" was available, which came with one 10MB hard disk and also allowed the installation of a second hard disk. The system unit had five expansion slots; the expansion unit had eight; however, one of the system unit's slots and one of the expansion unit's slots had to be occupied by the Extender Card and Receiver Card, respectively, which were needed to connect the expansion unit to the system unit and make the expansion unit's other slots available, for a total of 11 slots, some of which however had to already be occupied by display, disk, and I/O adapters, etc. as none of these were available on-board with the 5150; the only on-board connectors were the keyboard and cassette ports. The original PC's maximum memory using IBM parts was 256 kB, 64 kB on the main board and three 64 kB expansion cards. The processor was an Intel 8088 (early 1978 version, later were 1978/81/2 versions of intel chip; second-sourced AMDs were used after 1983) running at 4.77 MHz (4/3 the standard NTSC color burst frequency of 3.579545 MHz), which could be replaced with a NEC V20 for a slight increase in processing speed. An Intel 8087 co-processor could also be added for hardware floating-point arithmetic. IBM sold it in configurations with 16 kB or 64 kB of RAM preinstalled using either nine or thirty-six 16-kbit DRAM chips. (As was common at the time, an extra bit was used for parity checking of memory.)

Although the TV-compatible video board, cassette port and FCC Class B certification were all aimed at making it a home computer [ David J. Bradley "The Creation of the IBM PC", "BYTE",ISSN 0360-5280/09,Volume 15, Number 9, September 1990 pp. 414-420 ] the original PC proved too expensive for the home market. At introduction a PC with 64 kB of RAM and a single 5 1/4 inch floppy drive and monitor sold for US $3,005, while the cheapest configuration ($1,565) that had no floppy drives, only 16KB RAM, and no monitor (again, the expectation was that users would connect their existing TV sets and cassette recorders) proved too unattractive and low-spec, even for its time (cf. footnotes to the above IBM PC range table). [ Whence Came the IBM PC Test and Measurement World retrieved 2008 03 02 ] [ Gene Smart and Andrew Reinhardt, "15 years of Bits, Bytes and Other Great Moments", "BYTE Magazine", September 1990 pg. 382 ] While the 5150 did not become a top selling home computer, its floppy-based configuration became an unexpectedly large success with businesses.


The "IBM Personal Computer XT", IBM's model 5160, was an enhanced machine that was designed for business use. It had 8 expansion slots and a 10 megabyte hard disk (later versions 20MB). Unlike the model 5150 PC, the model 5160 XT no longer had a cassette jack. The XT could take 256 kB of memory on the main board (using 64 kbit DRAM); later models were expandable to 640 kB. (The 384 kB of BIOS ROM, video RAM, and adapter ROM space filled the rest of the one megabyte address space of the 8088 CPU.) It was usually sold with a Monochrome Display Adapter (MDA) video card. The processor was a 4.77 MHz Intel 8088 and the expansion bus 8-bit Industry Standard Architecture (ISA) with standards.


The "IBM Personal Computer/AT", announced August 1984, uses an Intel 80286 processor, originally at 6 MHz. It has a 16-bit ISA bus and 20 MB (20 million bytes) hard drive. A faster model, running at 8 MHz, was introduced in 1986. IBM made some attempt at marketing it as a multi-user machine, but it sold mainly as a faster PC for power users. Early PC/ATs were plagued with reliability problems, in part because of some software and hardware incompatibilities, but mostly related to the internal 20 MB hard disk. While some people blamed IBM's hard disk controller card and others blamed the hard disk manufacturer Computer Memories Inc. (CMI), the IBM controller card worked fine with other drives, including CMI's 33-megabyte model. The problems introduced doubt about the computer and, for a while, even about the 286 architecture in general, but after IBM replaced the 20 MB CMI drives, the PC/AT proved reliable and became a lasting industry standard.






The main circuit board in an IBM PC is called the motherboard (IBM terminogy calls it a "planar"). This carries the CPU and memory, and has a bus with slots for expansion cards.

The bus used in the original PC became very popular, and was subsequently named ISA. It is in use to this day in computers for industrial use. Later, requirements for higher speed and more capacity forced the development of new versions. IBM introduced the MCA bus with the PS/2 line. The VESA Local Bus allowed for up to three, much faster 32-bit cards, and the EISA architecture was developed as a backward compatible standard including 32-bit card slots, but it only sold well in high-end server systems. The lower-cost and more general PCI bus was introduced in 1994 and has now become ubiquitous.

The motherboard is connected by cables to internal storage devices such as hard disks, floppy disks and CD-ROM drives. These tend to be made in standard sizes, such as 3.5" (90 mm) and 5.25" (133.4 mm) widths, with standard fixing holes. The case also contains a standard power supply unit (PSU) which is either an AT or ATX standard size.

Intel 8086 and 8088-based PCs require expanded memory (EMS) boards to work with more than one megabyte of memory. The original IBM PC AT used an Intel 80286 processor which can access up to 16 megabytes of memory (though standard DOS applications cannot use more than one megabyte without using additional APIs.) Intel 80286-based computers running under OS/2 can work with the maximum memory.


The original 1981 IBM PC's keyboard at the time was an extremely reliable and high quality electronic keyboard originally developed in North Carolina for the Datamaster system [ David Bradley, BYTE Sept. 1990 ] . Each key was rated to be reliable to over 100 million keystrokes. For the IBM PC, a separate keyboard housing was designed with a novel usability feature that allowed users to adjust the keyboard angle for personal comfort. Compared with the keyboards of other small computers at the time, the IBM PC keyboard was far superior and played a significant role in establishing a high quality impression. For example, the industrial design of the keyboard, together with the system unit, was recognized with a major design award. "Byte" magazine in the fall of 1981 went so far as to state that the keyboard was 50 percent of the reason to buy an IBM PC. The importance of the keyboard was definitely established when the 1983 IBM PCjr flopped, in very large part for having a much different and mediocre Chiclet keyboard that made a poor impression on customers. Oddly enough, the same thing almost happened to the original IBM PC when in early 1981 management seriously considered substituting a cheaper but lower quality keyboard. This mistake was narrowly avoided by the advice of one of the original development engineers.

However, the original 1981 IBM PC's keyboard was severely criticized by typists for its non-standard placement of the Return and left Shift keys. In 1984, IBM corrected this on its AT keyboard, but shortened the 'backspace' key, making it harder to reach. In 1987, it introduced the enhanced keyboard, which relocated all the function keys and the Ctrl keys. The Esc key was also relocated to the opposite side of the keyboard.

Another criticism of the original keyboard was the relatively loud "clack" sound each key made when pressed. Since typewriter users were accustomed to keeping their eyes on the hardcopy they were typing from and had come to rely on the mechanical sound that was made as each character was typed onto the paper to ensure that they had pressed the key hard enough (and only once), the PC keyboard electronic "clack" feature was intended to provide that same reassurance. However, it proved to be very noisy and annoying, especially if many PCs were in use in the same room, and later keyboards were significantly quieter.

An "IBM PC compatible" may have a keyboard that does not recognize every key combination a true IBM PC does, such as shifted cursor keys. In addition, the "compatible" vendors sometimes used proprietary keyboard interfaces, preventing the keyboard from being replaced.

Although the PC/XT and AT used the same style of keyboard connector, the low-level protocol for reading the keyboard was different between these two series. An AT keyboard could not be used in an XT, nor the reverse. Third-party keyboard manufacturers provided a switch to select either AT-style or XT-style protocol for the keyboard.

erial port addresses and interrupts

The serial port is an 8250 or a derivatve (such as the 16450 or 16550), mapped to eight consecutive IO addresses and one interrupt request line.

Only COM1: and COM2: addresses were defined by the original PC. Attempts to share IRQ 3 and IRQ4 to use additional ports require special measures in hardware and software, since shared IRQs were not defined in the original PC design.

Character set

The original IBM PC used the 7-bit ASCII alphabet as its basis, but extended it to 8 bits with nonstandard character codes. This character set was not suitable for some international applications, and soon a veritable cottage industry emerged providing variants of the original character set in various national variants. In IBM tradition, these variants were called code pages. These codings are now obsolete, having been replaced by more systematic and standardized forms of character coding, such as ISO 8859-1, Windows-1251 and Unicode. The original character set is known as code page 437.

torage media

Cassette tape

As mentioned above, IBM equipped the model 5150 with a cassette port for connecting a cassette drive, and originally intended compact cassettes to become the 5150's most common storage medium. However, adoption of the floppy- and monitor-less configuration was low; few (if any) IBM PCs left the factory without a floppy disk drive installed. Also, DOS was not available on cassette tape, only on floppy disks (hence "Disk Operating System"). 5150s with just external cassette recorders for storage could only use the built-in ROM BASIC as their operating system. As DOS saw increasing adoption, the incompatibility of DOS programs with PCs that used only cassettes for storage made this configuration even less attractive.

Floppy diskettes

Most or all 5150 PCs had one or two 5¼ inch floppy disk drives. These floppy drives were either single-sided double-density drives (SS/DD, aka SSDD), or double-sided double-density drives (DS/DD, aka DSDD). The IBM PC never used single density floppy drives. The drives and disks were commonly referred to by capacity, e.g. "160KB floppy disk" or "360KB floppy drive", but because this is not entirely unambiguous, they are here referred to using the less commonly used but more accurate SSDD and DSDD terminology. DSDD drives were backwards compatible; they could read and write SSDD floppies. The same type of physical diskette could be used for both drives, [Even if SSDD disks were not advertised as/rated for DSDD use, they usually could be DSDD formatted all the same. DSDD-rated floppy disks could always be used as SSDD floppies. This just meant that one side was not written on. ] however to convert a 5¼" SSDD disk to a DSDD disk, it needed to be reformatted, at which point SSDD drives could no longer read it.

The disks were Modified Frequency Modulation (MFM) coded in 512-byte sectors, and were soft-sectored. [cite book | last = IBM | title = Technical Reference: Personal Computer Hardware Reference Library | publisher = IBM Corp.| date = July 1982 | edition = Revised Edition | pages = page 2-93 | id =6025008] They contained 40 tracks per side at the 48 track per inch (TPI) density, [Sometimes the tracks were also referred as cylinders, which is technically correct and analogous to hard drive cylinders. One floppy disk track equaled one cylinder, however with double-sided floppies, only the first side's cylinder numbers were identical to the track numbers; on the second side, the cylinders 1-40 corresponded to tracks 41-80 of the formatted floppy. ] and initially were formatted to contain 8 sectors per track. This meant that SSDD disks initially had a formatted capacity of 160 KB, [163,840 bytes, i.e. 512 bytes x 8 sectors x 40 tracks on the one side used] while DSDD disks had a capacity of 320 KB. [327,680 bytes, i.e. 512 bytes x 8 sectors x 40 tracks x 2 sides] However, the DOS operating system was later updated to allow formatting the disks with 9 sectors per track. This yielded a formatted capacity of 180 KB with SSDD disks/drives, [184,320 bytes, i.e. 512 bytes x 9 sectors x 40 tracks on the one side used] and 360 KB with DSDD disks/drives. [368,640 bytes, i.e. 512 bytes x 9 sectors x 40 tracks x 2 sides] The "unformatted" capacity of the floppy disks was advertised as 250KB (SSDD) and 500KB (DSDD), however these "raw" 250/500KB were not the same thing as the usable formatted capacity; under DOS, the maximum capacity for SSDD and DSDD disks was 180KB and 360KB, respectively. Regardless of type, the file system of all floppy disks was FAT12.

While the SSDD drives initially were the only floppy drives available for the model 5150 PC, IBM later switched to DSDD drives, and the majority of 5150 PCs sold eventually shipped with one or two DSDD drives. The 5150's successor, the model 5160 IBM XT, never shipped with SSDD drives; it generally had one double-sided 360 kB drive (next to its internal hard disk). While it was technically possible to retrofit more advanced floppy drives such as the high-density drive (released in 1984) into the original IBM PC, this was not an option offered by IBM for the 5150 model, and the move to high-density 5.25" floppies in particular was notoriously fraught with compatibility problems.

IBM's original floppy disk controller card also included an external 37-pin D-shell connector. This allowed users to connect additional external floppy drives by third party vendors.IBM themselves did not offer external floppy drives. [However, IBM later offered the 5161 Expansion Unit, which could allow the installation of additional floppies, though this was not a typical configuration as the Expansion Unit shipped with one or two hard drives occupying the available drive bays.]

Fixed disks

As mentioned elsewhere in this article, the 5150 could not itself power hard drives without retrofitting a stronger power supply, but IBM later offered the 5161 Expansion Unit, which not only provided more expansion slots, but also included a 10MB (later 20MB) hard drive powered by the 5161's own separate 130-watt power supply.

The first IBM PC that shipped with an internal, fixed, non-removable hard disk was IBM's model 5160, the XT. However, as other IBM-compatible PCs started to appear, hard disks with larger storage capacities than the 5160's and 5161's initial 10MB (later 20MB) also became available, and could — space permitting — be installed into either the IBM PC's Expansion Unit or into PSU-upgraded model 5150 IBM PCs (or into XTs). Adding a third-party hard disk sometimes required plugging in a new controller board, because some of these hard drives were not compatible with the existing disk controller. Some third party hard disks for IBM PCs even sold as kits including a controller card and replacement power supply. Finally, some hard disks were integrated with their controller in a single expansion board, commonly called a "Hard Card".

OS support

The IBM PC's ROM BASIC supported cassette tape storage. DOS itself did not support cassette tape storage. PC-DOS version 1.00 supported only 160KB SSDD floppies, but version 1.1, which was released 9 months after the PC's introduction, supported 160KB SSDD and 320KB DSDD floppies. Support for the slightly larger 9 sector per track 180KB and 360KB formats arrived 10 further months later in March 1983.

Original software

All IBM PCs include a relatively small piece of software stored in ROM. The original IBM PC 40 KB ROM included 8 KB for power-on self-test (POST) and basic input/output system (BIOS) functions plus 32 KB BASIC in ROM (Cassette BASIC). The ROM BASIC interpreter was the default user interface if no DOS boot disk was present. BASICA was distributed on floppy disk and provided a way to run the ROM BASIC under PC-DOS control.

In addition to PC-DOS, buyers could choose either CP/M-86 or UCSD p-System as operating systems. Due to their higher prices, they never became very popular and PC-DOS or MS-DOS came to be the dominant operating system.


While the IBM PC technology is largely obsolete by today's standards, many are still in service. As of June 2006, IBM PC and XT models are still in use at the majority of U.S. National Weather Service upper-air observing sites. The computers are used to process data as it is returned from the ascending radiosonde, attached to a weather balloon. They are being phased out over a several year period, to be replaced by the Radiosonde Replacement System Fact|date=August 2008.

ee also

* Influence of the IBM-PC on the PC market
* Conventional memory
* IBM token ring networks
* Aptiva
* List of IBM products
* ThinkPad
* ThinkCentre
* 8088 Corruption
* Input/Output Base Address
* IBM 5120
* IBM 5160
* IBM 5155
* IBM 4860



* Norton, Peter (1986). "Inside the IBM PC. Revised and enlarged". New York. Brady. ISBN 0-89303-583-1.
* August 12, 1981 [ press release announcing the IBM PC] (PDF format).
* Mueller, Scott (1992). "Upgrading and Repairing PCs, Second Edition", Que Books, ISBN 0-88022-856-3
* IBM (1983). "Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT". IBM Part Number 6936831.
* IBM (1984). "Personal Computer Hardware Reference Library: Guide to Operations, Portable Personal Computer". IBM Part Numbers 6936571 and 1502332.
* IBM (1986). "Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT Model 286". IBM Part Number 68X2523.

External links

* [ Vintage IBM]
* [ IBM PC history and technical information]
* [ What a legacy! The IBM PC's 25 year legacy]
* [ - IBM PC turns 25]
* [ IBM-5150] and collection of old digital and analog computers at
* [ IBM PC images and information]
* [ A brochure from November, 1982 advertising the IBM PC]

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