Motorola 68000


Motorola 68000

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

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

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

Like the Zilog Z8000, the 68000 features a supervisor anduser mode, each with its own Stack Pointer. The Zilog Z8000 and 68000 are similar in capabilities, but the 68000 is32 bits internally, making it faster and eliminating forcedsegmentations.

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

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

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

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