IBM PC
<computer> 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.
(1995-05-12)
Nearby terms:
IBM compatible « IBM Customer Engineer « IBM
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