Computer_word
  Standard work unit of the CPU architecture.
  Size of a word varies between architectures. 
    Size of CPU's basic work unit - numeric value.
    Size of CPU's work registers.
    Size of CPU's internal bus structure.
    Size of single memory fetch.
    Common size of an instruction.
    Chicken and egg.

  Early systems designed for specific problems used custom word sizes,
    Often used different word sizes for instruction and memory fetches.

  Currently, word is most commonly either 16 or 32 bits in size.
    Systems with 16 bit words usually recognize 32-bit "double word".

  Systems that evolved from early 8/16-bit CPUs (1970s) use a 16-bit word.

    MOS 6502 
      8 bit internal bus structure, but used pair of registers to use 16-bit 
       words such as memory addressing.  ~ 1975
      Apple 2, Atari, Commodore, variety of early game systems.
      Modern variations used in embedded systems such as medical equipment.

    Intel 8086
      16 bit architecture. ~ 1976-1978
      8088 was a "downgrade design" which allowed use of existing 8-bit bus
        based motherboards. ~ 1979
      Modern Pentiums and AMDs are the great-grandchildren of the i8086.

    Motorola MC6809, 6800
      8 bit architecture.
      Commodore SuperPET, Acorn System 2, latter TRS-80s, traffic-signal 
      controller, pinball machines.
  
    Zilog Z80
      8 bit. ~ 1976
      Earliest(?) "personal" CP/M business system. Osborne 1, TRS-80
      Embedded systems.
      The Z8 series is an off-shoot that functions more as an embedded 
      RISC micro-controller.

  Systems designed as 32-bit or larger from start recognize a 32 bit word.
 
    IBM 360 - Mainframe. ~ 1965
      Pre-monolithic (one stone) so no numbered CPU silicon chip.  

    Motorola 68000 - Lisa and original Macintosh, Commodore Amiga ~ 1979

    RISC architectures  (32/64 bit)
      ARM (ARM designs and licenses variations but does not manufacture.)
       - ~ 1985 first introduced.
       - currently most popular RISC CPU. (cell phones)
       - large variety of designs.
        
      MIPS - PlayStation, Nintendo, home routers. ~ 1985
        - Embedded systems most common modern use. 
        - Residential gateways and routers.

      PowerPC (Apple-IBM-Motorola) IBM primary. ~ 1992
       - RISC instruction set architecture defined/designed first.
       - Hardware manufactured to implement architecture.
        Super-computers, Macs (2003-4), Sun, Nintendo, PlayStation, XBox
        Embedded and High-performance systems.

      SPARC (Scalable Processor Architecture) Originally, SUN's ~ 1987
       - Sun and other Unix workstations, servers, super-computers.
       - latest models 2017  

* Note that the software (language) definition of the size of a numeric 
   storage unit, such as short, signed integer, long integer, and word, are 
   independent of the CPU's register organization.