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.
IBM PC and PS/2 models
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
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.