IBM PC





Due to the success of the IBM PC, the generic term Personal Computer became
common for all microcomputers compatible with IBM's specification (see IBM
PC compatible). The term is sometimes extended to mean all microcomputers.

   * For a discussion of generic "Personal Computers", see personal
     computer.
   * For details of the first generation of microcomputers that largely died
     out with the Personal Computer revolution, see home computers.

The IBM PC Concept

The original PC was an IBM attempt to get into the home computer market then
dominated by the Apple II.

Rather than going through the usual IBM design process, which had already
failed to design an affordable microcomputer (for example the failed IBM
5100), a special team were assembled to bypass normal company restrictions
and get soemthing to market rapidly. The project was given the code name
Project Chess.

The team consisted of just 12 people headed by William Lowe. They succeeded
- development of the PC took about a year. To achieve this they first
decided to build the machine with "off-the-shelf" parts from a variety of
different Original Equipment Manufacturers (OEM)s and countries. Previously
IBM had developed their own components. Second they decided on an open
architecture so that other manufacturers could produce and sell compatible
machines - the IBM PC compatibles, so the specification of the ROM BIOS was
published. IBM hoped to maintain their position in the market by royalties
from licencing the BIOS, and by keeping ahead of the competition.

Unfortunately for IBM, other manufacturers rapidly reverse engineered the
BIOS to produce their own royalty-free versions. (Compaq Computer
Corporation manufactured the first cloned IBM PC compatible in 1984). And
once the IBM PC became a commercial success the PC came back under 'normal'
IBM mangement control, with the result that competitors had little trouble
taking the lead from them.

Commercial Success

The first IBM PC was released on August 12 1981. Although not cheap, at a
base price of $1,565 it was affordable for businesses - and it was business
that purchased the PC. However it was not the corporate "computer
department" that was responsible for this, for the PC was not seen as a
'proper' computer. It was generally well educated middle mangers that saw
the potential - once the revolutionary VisiCalc spreadsheet, the "killer
app", had been ported to the PC. Reassured by the IBM name, they began
buying the machines on their own budgets to help do the calculations they
had learned at business school. The personal computer revolution was born.

IBM PC Models

The models of IBM's first-generation Personal Computer (PC) series have
names:

   * The original PC had a version of BASIC in ROM. The CGA (Colour Graphics
     Adapter) video card could use a standard TV for display. The standard
     storage device was cassette tape. Floppy disk was an optional extra; no
     hard disk was available. It had only five expansion slots; maximum
     memory using IBM parts was 256 K, 64 on the main board and three 64 K
     expansion cards. The processor was an Intel 8088 running at 4.77 MHz.

   * The original PC failed miserably in the home market, but was widely
     used in business. The PC XT was an enhanced machine designed for
     business use. It had 8 expansion slots and a 10 megabyte hard disk. It
     could take 256 K of memory on the main board. It was usually sold with
     an MDA (Monochrome Display Adapter). The processor was still a 4.77 MHz
     Intel 8088 and the expansion bus still 8-bit ISA with XT bus
     architecture.

   * The PC AT, announced August 1984, used an Intel 80286 processor,
     originally at 6 MHz. It had a 16-bit ISA bus and 20MB harddrive. A
     faster model, running at 8 MHz, was introduced in 1986. IBM made some
     attempt at marketing it as a multi-user machine, but it sold mainly as
     a faster PC for power users. Early PC/ATs were plagued with reliability
     problems, mostly related to the internal 20 MB hard drive. While some
     people blamed IBM's controller card and others blamed the hard drive
     manufacturer (Computer Memories International, or CMI), the IBM
     controller card worked fine with other drives, including CMI's
     33-megabyte model. The problems introduced doubt about the computer
     and, for a while, even about the 286 architecture in general, but after
     IBM replaced the 20-megabyte CMI drives, the PC/AT proved reliable and
     became a lasting industry standard. CMI quickly went out of business.

   * IBM Convertible
   * IBM Portable
   * IBM PCjr.

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.

Technology

Electronics

The main circuit board in an IBM PC is called the motherboard. This carries
the CPU and memory, and has a bus with slots for expansion cards.

The bus used in the original PC became very popular, and was subsequently
named ISA. It is in use to this day in computers for industrial use. Later,
requirements for higher speed and more capacity forced the development of
new versions. The EISA was developed as a backward compatible standard, but
due to high complexity and medium performance it dod not really catch on.
Instead, the more specialized PCI or AGP busses are now used for expansion
cards.

The motherboard is connected by cables to internal storage devices such as
hard disks, floppy disks and CD-ROM drives. These tend to be made in
standard sizes, such as 3.5" (88.9 mm) and 5.25" (133.4 mm) widths, with
standard fixing holes. The case also contains a standard power supply unit
(PSU) which is either an AT or ATX standard size.

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.

Keyboard

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.

Character set

The original IBM PC used the 7 bit ASCII alphabet as the basis, but in
addition this was extended to am 8 bit somewhat haphazardly collected set of
characters unique for the IBM PC. This set was not really suitable for
international use, and soon a veritable cottage industry emerged providing
variants of the original character set in various national variants. In IBM
tradition, these variants were called code pages. These codings are now
obsolete, being replaced by more well thought out schemes for character
coding, like the ISO 8859-1 and Unicode.

Storage media

Technically, the standard storage medium for the original IBM PC model 5150
was a cassette port. Being pretty much obsolete even by 1981 standards, very
few, if any, IBM PC probably left the factory without a floppy disk drive
installed. The 1981 PC had one or two 360 kilobyte 5 1/4 inch single sided
double density floppy disk drives.

In 1984, IBM introduced the 1.2 megabyte dual sided 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 720
kilobyte 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.

The first IBM PC that included a fixed, non-removable, hard disks was the
XT. Hard disks for IBM compatibles are now 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.

Software

All IBM PCs includes a relatively small piece of software stored in ROM and
used mainly for bootstraping, called a BIOS. In addition, 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

 Model name Introduced  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 PortableFeb 1984   8088 Floppy-based portable
 AT         Aug 1984   286  Medium-speed hard disk
 ConvertibleApr 1986   8088 Microfloppy laptop portable
 XT 286     Sep 1986   286  Slow hard disk

                      PS/2 range

 Model Introduced  CPU             Features
 25   August 1987 8086  PC bus (limited expansion)
 30   April 1987  8086  PC bus
 30   August 1987 286   PC bus
 50   April 1987  286   Micro Channel Architecture bus
 50Z  June 1988   286   Faster Model 50
 55 SXMay 1989    386SX MCA bus
 60   April 1987  286   MCA bus
 70   June 1988   386   Desktop, MCA bus
 P70  May 1989    386   Portable, MCA bus
 80   April 1987  386   Tower, MCA bus

                      IBM PC compatible specifications

        Clock     CPU    System                Floppy
  CPU   speed     bus      Bus       RAM        disk   Hard drive Operating
        (MHz)    width    width   (megabytes)  drive   (megabytes)  System
                 (bits)   (bits)
                                              5.25,
 8088 4.77-9.5           8                    360K     10-40

                                  1 (1)       3.5,                DOS
                                              720K
 8086 6-12                                    3.5,     20-60
               16                             1.44M
                         16                   5.25,

 286  6-25                        1-8 (1)     360K     20-300     DOS, OS/2
                                              5.25,
                                              1.2MB

 386                     32                   3.5,
      16-33    32                 1-16 (2)    720K     40-600     UNIX
                                              3.5,
 386SX                   16                   1.44MB

  1. Under DOS, RAM is expanded beyond 1M with EMS memory boards
  2. Under DOS, RAM is expanded beyond 1M with normal "extended" memory and
     a memory management program.