The Free On-line Dictionary of Computing (18 March 2015):
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
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
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
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
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.