Motorola 68000

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 linearaddress 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 ZilogZ8000).

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 ZilogZ8000 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 AppleComputer'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)