Motorola 68000

(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 is actually a 32-bit architecture internally, but

16-bit externally. 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 an {orthogonal instruction set} and sixteen

{registers}, split into data and address registers. One

address register is reserved for the {Stack Pointer}. Both

types of registers 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. It was designed for expansion, including

specifications for {floating point} and string operations

(floating point later implemented in the {Motorola 68040}).

Like many other CPUs of its generation, it can fetch the next

instruction during execution (2 stage {pipeline}), the 68040

was fully pipelined (6 stages).

The 68000 was used in many {workstations}, notably early {Sun

2} and {Sun 3} machines, and {personal computers}, notably

{Apple Computer}'s first {Macintoshes} and the {Amiga}.

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].

(1999-03-24)