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IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5.0
IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5.0

The IBM PC (Personal Computer), is a trade mark of IBM. The predecessor of the current personal computers, it was introduced in August 1981. The original model was designated the IBM 5150.

The phrase "personal computer" was common currency before 1981, and was used as early as 1972 to characterize the Xerox PARC Alto. However, due to the success of the IBM PC, what had been a generic term came to mean specifically a microcomputer compatible with IBM's specification (see IBM PC compatible). (The term "personal computer" is still occasionally used to the wider generic sense).

  • For a discussion of generic "Personal Computers", see personal computer.
  • For details of the first generation of microcomputers that largely died out with the Personal Computer revolution, see home computers.

The IBM PC Concept

The original PC was an IBM attempt to get into the home computer market then dominated by the Apple II.

Rather than going through the usual IBM design process, which had already failed to design an affordable microcomputer (for example the failed IBM 5100), a special team were assembled to bypass normal company restrictions and get something to market rapidly. The project was given the code name Project Chess.

The team consisted of just 12 people headed by William Lowe . They succeeded — development of the PC took 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 (OEM)s and countries. Previously IBM had developed their own components. Second they decided on an open architecture so that other manufacturers could produce and sell compatible machines — the IBM PC compatibles, so the specification of the ROM BIOS was published. IBM hoped to maintain their position in the market by royalties from licensing the BIOS, and by keeping ahead of the competition.

Unfortunately for IBM, other manufacturers rapidly reverse engineered the BIOS to produce their own royalty-free versions. (Compaq Computer Corporation manufactured the first cloned IBM PC compatible in 1984). And once the IBM PC became a commercial success the PC came back under the usual IBM management control, with the result that competitors had little trouble taking the lead from them. (In this regard, 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).

Commercial Success

The first IBM PC was released on August 12 1981. Although not cheap, at a base price of $1,565 it was affordable for businesses — and it was business that purchased the PC. However it was not the corporate "computer department" that was responsible for this, for the PC was not seen as a 'proper' computer. It was generally well educated middle managers that saw the potential — once the revolutionary VisiCalc spreadsheet, the "killer app", had been ported to the PC. Reassured by the IBM name, they began buying the machines on their own budgets to help do the calculations they had learned at business school.

IBM PC models

The models of IBM's first-generation Personal Computer (PC) series have names:

  • The original PC had a version of Microsoft BASIC —IBM Cassette BASIC — in ROM. The CGA (Colour Graphics Adapter) video card could use a standard TV for display. The standard storage device was cassette tape. A floppy disk drive was an optional extra; no hard disk was available. It had only five expansion slots; maximum memory using IBM parts was 256 KB, 64K on the main board and three 64K expansion cards. The processor was an Intel 8088(second-sourced AMD's were used after 1983) running at 4.77 MHz. IBM sold it in configurations with 16K and 64K of RAM preinstalled.
  • 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 256K of memory on the main board. It was usually sold with a Monochrome Display Adapter (MDA) video card. 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, announced August 1984, used an Intel 80286 processor, originally at 6 MHz. It had a 16-bit ISA bus and 20MB harddrive. 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, mostly related to the internal 20 MB hard drive. While some people blamed IBM's controller card and others blamed the hard drive manufacturer (Computer Memories International, or 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-megabyte CMI drives, the PC/AT proved reliable and became a lasting industry standard. CMI quickly went out of business.

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 didn't 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" (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 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 an 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.

This was the original IBM PC character set:

    0 1 2 3 4 5 6 7 8 9 A B C D E F
10   §
20   ! " # $ % & ' ( ) * + , - . /
30   0 1 2 3 4 5 6 7 8 9 : ; < = > ?
40   @ A B C D E F G H I J K L M N O
50   P Q R S T U V W X Y Z [ \ ] ^ _
60   ` a b c d e f g h i j k l m n o
70   p q r s t u v w x y z { | } ~
80   Ç ü é â ä à å ç ê ë è ï î ì Ä Å
90   É æ Æ ô ö ò û ù ÿ Ö Ü ¢ £ ¥ ƒ
A0   á í ó ú ñ Ñ ª º ¿ ¬ ½ ¼ ¡ « »
E0   α ß Γ π Σ σ µ τ Φ Θ Ω δ φ ε
F0   ± ÷ ° · ²  

Storage media

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. In 1988 IBM introduced a drive for 2.88 megabyte "DSED" diskettes in its top-of-the-line models; it was an instant failure and is all but forgotten today (but survives as a possible "size" choice in disk-formatting utilities).

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 bootstrapping, called a BIOS. In addition, the original IBM PC came with BASIC in ROM (Cassette BASIC ). Later, BASIC and BASICA were distributed on floppy disks but ran and referenced routines in the PC's ROM.

IBM PC and PS/2 models

PC range
Model name Introduced CPU Features
PC Aug 1981 8088 Floppy disk system
XT Mar 1983 8088 Slow hard disk
XT/370 Oct 1983 8088 System/370 mainframe emulation
3270 PC Oct 1983 8088 With 3270 terminal emulation
PCjr Nov 1983 8088 Floppy-based home computer
PC Portable Feb 1984 8088 Floppy-based portable
AT Aug 1984 286 Medium-speed hard disk
Convertible Apr 1986 8088 Microfloppy laptop portable
XT 286 Sep 1986 286 Slow hard disk, but zero wait state memory on the motherboard. This 6mhz machine was actually faster than the 8mhz AT's (when using planar memory) because of the zero wait states
PS/2 range
Model Introduced CPU Features
25 August 1987 8086 PC bus (limited expansion)
30 April 1987 8086 PC bus
30 August 1987 286 PC bus
50 April 1987 286 Micro Channel Architecture bus
50Z June 1988 286 Faster Model 50
55 SX May 1989 386SX MCA bus
60 April 1987 286 MCA bus
70 June 1988 386 Desktop, MCA bus
P70 May 1989 386 Portable, MCA bus
80 April 1987 386 Tower, MCA bus
IBM PC compatible specifications
CPU Clock
width (bits)
width (bits)
disk drive
Hard drive
8088 4.77-9.5 16 8 1 (1) 5.25, 360K
3.5, 720K
3.5, 1.44M
10-40 DOS
8086 6-12 16 20-60
286 6-25 1-8 (1) 5.25, 360K
5.25, 1.2MB
20-300 DOS, OS/2
386 16-33 32 32 1-16 (2) 3.5, 720K
3.5, 1.44MB
40-600 UNIX
386SX 16
  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.

See also


External link

  • Google Groups thread from 1982 indicating that IBM PCs with 16 K RAM were actually manufactured and sold. (The statement that 16 K machines were sold is hard to believe and hence frequently challenged).

This article was originally based on material from the Free On-line Dictionary of Computing and is used under the GFDL.

Last updated: 10-24-2004 05:10:45