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Related to IBM PC: IBM PC XT, Lenovo


International Business Machines Personal Computer.

IBM PCs 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.

Note: "IBM PC" is used in this dictionary to denote IBM and compatible personal computers, and to distinguish these from other personal computers, though the phrase "PC" is often used elsewhere, by those who know no better, to mean "IBM PC or compatible".

There are hundreds of models of IBM compatible computers. They are based on Intel's microprocessors: Intel 8086, Intel 8088, Intel 80286, Intel 80386, Intel 486 or Pentium. The models of IBM's first-generation Personal Computer (PC) series have names: IBM PC, IBM PC XT, IBM PC AT, Convertible and Portable. 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 speed of the CPU (microprocessor) is the most significant factor in machine performance. It is determined by its clock rate and the number of bits it can process internally. It is also determined by the number of bits it transfers across its data bus. The second major performance factor is the speed of the hard disk.

CAD and other graphics-intensive application programs can be sped up with the addition of a mathematics coprocessor, a chip which plugs into a special socket available in almost all machines.

Intel 8086 and Intel 8088-based PCs require EMS (expanded memory) boards to work with more than one megabyte of memory. All these machines run under MS-DOS. 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.

Although IBM sells printers for PCs, most printers will work with them. As with display hardware, the software vendor must support a wide variety of printers. Each program must be installed with the appropriate printer driver.

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.

The 1981 PC had 360K floppy disks. In 1984, IBM introduced the 1.2 megabyte floppy disk along with its AT model. Although often used as backup storage, the high density floppy is not often used for interchangeability. In 1986, IBM introduced the 720K 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 can be added to existing PCs.

Fixed, non-removable, hard disks for IBM compatibles are available with storage capacities from 20 to over 600 megabytes. 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. Removable hard disks that hold at least 20 megabytes are also available.

When a new peripheral device, such as a monitor or scanner, is added to an IBM compatible, a corresponding, new controller board must be plugged into an expansion slot (in the bus) in order to electronically control its operation. The PC and XT had eight-bit busses; the AT had a 16-bit bus. 16-bit boards will not fit into 8-bit slots, but 8-bit boards will fit into 16-bit slots. Intel 80286 and Intel 80386 computers provide both 8-bit and 16-bit slots, while the 386s also have proprietary 32-bit memory slots. The bus in high-end models of the PS/2 line is called "Micro Channel". EISA is a non-IBM rival to Micro Channel.

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

Intro CPU Features PC Aug 1981 8088 Floppy disk system XT Mar 1983 8088 Slow hard disk XT/370 Oct 1983 8088 IBM 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

PS/2 range

Intro CPU Features Model 1987-08-25 8086 PC bus (limited expansion) Model 1987-04-30 8086 PC bus Model 30 1988-09-286 286 PC bus Model 1987-04-50 286 Micro Channel bus Model 50Z Jun 1988 286 Faster Model 50 Model 55 SX May 1989 386SX Micro Channel bus Model 1987-04-60 286 Micro Channel bus Model 1988-06-70 386 Desktop, Micro Channel bus Model P1989-05-70 386 Portable, Micro Channel bus Model 1987-04-80 386 Tower, Micro Channel bus

IBM PC compatible specifications

CPU CPU Clock Bus Floppy Hard bus speed width RAM disk disk OS bit Mhz bit byte inch byte Mbyte

8088 16 4.8-9.5 8 1M* 5.25 360K 10-40 DOS 3.5 720K 3.5 1.44M

8086 16 6-12 16 1M* 20-60

286 16 6-25 16 1-8M* 5.25 360K 20-300 DOS 5.25 1.2M OS/2

386 32 16-33 32 1-16M** 3.5 720K Unix 3.5 1.44M 40-600

386SX 32 16-33 16 1-16M** 40-600

*Under DOS, RAM is expanded beyond 1M with EMS memory boards

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

See also BIOS, display standard.
This article is provided by FOLDOC - Free Online Dictionary of Computing (


A PC made by IBM. In 1981, the first IBM PC was introduced with a colossal 16KB of RAM. However, the company was not that optimistic, projecting that 250,000 IBM PCs would sell in the first five years. In fact, three million were sold in that period, and the PC industry, first DOS and later Windows, would become the world's largest computer base.

Good Bye IBM PC
In 2004, IBM exited the desktop and laptop arena when it sold its personal computer division to Lenovo, a major Chinese PC manufacturer. IBM retained its x86-based server market with its System x series until 2014, when Lenovo also acquired the line. See System x.

Names and Numbers
IBM's first PCs had names such as PC, XT and AT, while its next-generation PS/2 line was given model numbers. For historical purposes, early IBM model designations are listed below. See PC, IBM-compatible PC, ThinkPad, IBM and Lenovo.

The Original IBM PC
This first model had two floppy disks and 16KB of RAM. Today, a smartphone has 150,000 times as much memory and more. (Image courtesy of IBM.)

IBM PS/2 Model 50
The PS/2 introduced the Micro Channel bus, VGA graphics and the 3.5" floppy disk. See IBM PS/2. (Image courtesy of IBM.)

IBM ThinkPad
Very popular, IBM's ThinkPads introduced novel features such as the pointing stick and Butterfly keyboard. This model 755CD came out in 1994. See ThinkPad. (Image courtesy of IBM.)

FIRST GENERATION - IBM PCYearModel       Intro. CPU   Features

 PC (#1)     1981   8088  floppy disk
 XT          1983   8088  slow HD
 XT 286      1986    286  slow HD
 XT/370      1983   8088  370 emulation
 AT          1984    286  hard disk
 3270 PC     1983   8088  3270 emulation
 PCjr        1983   8088  floppy home use
 PC Portable 1984   8088  floppy portable
 Convertible 1986   8088  floppy laptop

 Unless otherwise noted, models
 used the Micro Channel bus.

          YearModel    Intro. CPU  Features

 25       1987   8086, PC bus
 30       1987   8086, PC bus
 30-286   1988   286, ISA bus
 35 LS    1991   386SX, ISA, diskless
 35 SX    1991   386SX, ISA
 L40 SX   1991   386SX, ISA, laptop
 40 SX    1991   386SX, ISA
 N45 SL          386SL notebook
 50       1987   286
 50 Z     1988   286, faster 50
 N51 SX   1992   386SX, notebook
 N51 SLC  1992   386SLC, notebook
 55 SX    1989   386SX
 55 LS    1990   386SX, diskless
 56 SLC   1992   386SLC
 56 SX    1992   386SX, upgradable
 57 SLC   1992   386SLC
 57 SX    1991   386SX, 2.88MB floppy
 CL57 SX  1992   386SX, notebook
 60       1987   286
 65       1990   386SX
 70       1988   386
 70 486   1989   486
 P70      1989   386 portable
 P75 486  1990   486 portable, 22 lbs.
 76       1994   486SX & DX, OS/2, SCSI
 76i      1994   486SX, OS/2, IDE drive
 77       1994   DX4, OS/2, SCSI, ECC RAM
 77i      1994   DX4, OS/2, IDE, ECC RAM
 80       1987   386, tower
 90       1990   486, XGA, SCSI
 90       1992   486DX2/66
 95       1990   486, tower
 90 SX    1991   486SX
 95 SX    1991   486SX, tower
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.
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