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.

Last updated: 1995-05-12

Nearby terms:

IBM Customer EngineerIBM discountIBM PCIBM PC ATIBM PCjr

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