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1. The Free On-line Dictionary of Computing (30 December 2018)

    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

   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.


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