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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.

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

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.

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 was 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 could be added to existing older model PCs.

Fixed, non-removable, hard disks for IBM compatibles are 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.

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.

Article based on IBM PC at FOLDOC.