Source : Free On-Line Dictionary of Computing

IBM PC
     
         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)