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IBM PCs (IBM Personal Computers) and compatible models from other vendors are the most widely used computer systems in the world. They are typically single user personal computers, although they have been adapted into multi-user models for special applications.

Table of contents

Models There are hundreds of models of IBM compatible computers. The models of IBM's first-generation Personal Computer (PC) series have names:

  • The original PC was an IBM attempt to get into the home computer market then dominated by the Apple II. It had a version of BASIC in ROM. The CGA (Colour Graphics Adapter) video card could use a standard TV for display. The standard storage device was cassette tape. Floppy disk was an optional extra; no hard disk was available. It had only five expansion slots; maximum memory using IBM parts was 256 K, 64 on the main board and three 64 K expansion cards. The processor was an Intel 8088 running at 4.77 MHz.
  • The original PC failed miserably in the home market, but was widely used in business. The PC XT was an enhanced machine designed for business use. It had 8 expansion slots and a 10 megabyte hard disk. It could take 256 K of memory on the main board. It was usually sold with an MDA (Monochrome Display Adapter). The processor was still a 4.77 MHz Intel 8088 and the expansion bus still 8-bit ISA with XT bus architecture.
  • The PC AT used an Intel 80286 processor, originally at 6 MHz and later 8. It had a 16-bit ISA bus and 20 meg harddrive. IBM made some attempt at marketing it as a multi-user machine, but it sold mainly as a faster PC for power users.
  • IBM Convertible[?]
  • IBM Portable[?]
  • IBM PC Junior[?].

The models of its second generation, the Personal System/2 (PS/2), are known by model number: Model 25[?], Model 30[?]. Within each series, the models are also commonly referenced by their CPU clock rate.

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 main circuit board in an IBM PC is called the motherboard. 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. The EISA was developed as a backward compatible standard, but due to high complexity and medium performance it dod not really catch on. Instead, the more specialized PCI or AGP busses are now used for expansion cards.

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" (88.9 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 EMS[?] (expanded memory) 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 MS-DOS applications cannot use more than one megabyte without EMS). Intel 80286-based computers running under OS/2 can work with the maximum memory.


The original 1981 IBM PC's keyboard was severely criticised 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 its enhanced keyboard[?], which relocated all the function keys and placed the control key in an awkward location for touch typists. The escape key was relocated to the opposite side of the keyboard. By relocating the function keys, IBM made it impossible for software vendors to use them intelligently. What's easy to reach on one keyboard is difficult on the other, and vice versa. To the touch typist, these deficiencies are maddening.

An "IBM PC compatible" may have a keyboard which does not recognize every key combination a true IBM PC does, e.g. shifted cursor keys. In addition, the "compatible" vendors sometimes use proprietary keyboard interfaces, preventing you from replacing the keyboard.

See also: Keyboard layout


Character set

The original IBM PC used the 7 bit ASCII alphabet as the basis, but in addition this was extended to am 8 bit somewhat haphazardly collected set of characters unique for the IBM PC. This set was not really suitable for international use, 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, being replaced by more well thought out schemes for character coding, like the ISO 8859-1 and Unicode.

Storage mediums

Technically, the standard storage medium for the original IBM PC model 5150 was a cassette port. Being pretty much obsolete even by 1981 standards, very few, if any, IBM PC probably left the factory without a floppy disk drive installed. The 1981 PC had one or two 360 kilobyte 5 1/4 inch single sided double density floppy disk drives.

In 1984, IBM introduced the 1.2 megabyte dual sided floppy disk along with its AT model. Although often used as backup storage, the high density floppy was not often used for interchangeability. In 1986, IBM introduced the 720 kilobyte 3.5" microfloppy disk on its Convertible laptop computer. It introduced the 1.44 megabyte double density version with the PS/2 line. These disk drives could be added to existing older model PCs.

The first IBM PC that included a fixed, non-removable, hard disks was the XT. Hard disks for IBM compatibles are now available with very large storage capacities. If a hard disk is added that is not compatible with the existing disk controller, a new controller board must be plugged in. However, one disk's internal standard does not conflict with another, since all programs and data must be copied onto it to begin with.


All IBM PCs includes a relatively small piece of software stored in ROM and used mainly for bootstraping, called a BIOS. In addition, the original IBM PC came with BASIC in ROM. Later, Basic and BasicA were distributed on floppy but ran and referenced routines in ROM.

IBM PC and PS/2 models

PC range
Model nameIntroducedCPUFeatures
PCAug 19818088Floppy disk system
XTMar 19838088Slow hard drive
XT/370Oct 19838088IBM 370 mainframe emulation
3270 PCOct 19838088With 3270 terminal emulation
PCjrNov 19838088Floppy-based home computer
PC PortableFeb 19848088Floppy-based portable
ATAug 1984286Medium-speed hard disk
ConvertibleApr 19868088Microfloppy laptop portable
XT 286Sep 1986286Slow hard disk

PS/2 range
25August 19878086PC bus (limited expansion)
30April 19878086PC bus
30August 1987286PC bus
50April 1987286Micro Channel Architecture bus
50ZJune 1988286Faster Model 50
55 SXMay 1989386SXMCA bus
60April 1987286MCA bus
70June 1988386Desktop, MCA bus
P70May 1989386Portable, MCA bus
80April 1987386Tower, MCA bus

IBM PC compatible specifications
width (bits)
width (bits)
disk drive
Hard drive
80884.77-9.51681 (1)5.25, 360K
3.5, 720K
3.5, 1.44M
2866-251-8 (1)5.25, 360K
5.25, 1.2MB
20-300DOS, OS/2
38616-3332321-16 (2)3.5, 720K
3.5, 1.44MB

  1. Under DOS, RAM is expanded beyond 1M with EMS memory boards
  2. Under DOS, RAM is expanded beyond 1M with normal "extended" memory and a memory management program.

Article based on IBM PC (http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?query=ibm+pc) at FOLDOC (http://www.foldoc.org).

All Wikipedia text is available under the terms of the GNU Free Documentation License

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