from
The Free On-line Dictionary of Computing (8 July 2008)
Motorola 68000
68000
MC68000
<processor> (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)