Motorola 68000
68000
MC68000

    (MC68000) The first member of Motorola, Inc.'s
   family of 16- and 32-bit microprocessors.  The successor to
   the Motorola 6809 and followed by the Motorola 68010.

   The 68000 has 32-bit registers but only a 16-bit ALU and
   external data bus.  It has 24-bit addressing and a linear
   address space, with none of the evil segment registers of
   Intel's contemporary processors that make programming them
   unpleasant.  That means that a single directly accessed
   array or structure can be larger than 64KB in size.
   Addresses are computed as 32 bit, but the top 8 bits are cut
   to fit the address bus into a 64-pin package (address and data
   share a bus in the 40 pin packages of the 8086 and Zilog
   Z8000).

   The 68000 has sixteen 32-bit registers, split into data and
   address registers.  One address register is reserved for the
   Stack Pointer.  Any register, of either type, can be used
   for any function except direct addressing.  Only address
   registers can be used as the source of an address, but data
   registers can provide the offset from an address.

   Like the Zilog Z8000, the 68000 features a supervisor and
   user mode, each with its own Stack Pointer.  The Zilog
   Z8000 and 68000 are similar in capabilities, but the 68000 is
   32 bits internally, making it faster and eliminating forced
   segmentations.

   Like many other CPUs of its generation, it can fetch the next
   instruction during execution (2 stage pipeline).

   The 68000 was used in many workstations, notably early
   Sun-2 machines, and personal computers, notably Apple
   Computer's first Macintoshes and the Amiga.  It was also
   used in most of Sega's early arcade machines, and in the
   Genesis/Megadrive consoles.

   Variants of the 68000 include the 68HC000 (a low-power HCMOS
   implementation) and the 68008 (an eight-bit data bus version
   used in the Sinclair QL).

   ["The 68000: Principles and Programming", Leo Scanlon, 1981].

   (2003-07-11)