23 Aralık 2007 Pazar

Motorola 68000

Pre-release XC68000 chip manufactured in 1979.

The Motorola 68000 is a 16/32-bit CISC microprocessor core designed and marketed by Freescale Semiconductor (formerly Motorola Semiconductor Products Sector). Introduced in 1979 as the first member of the successful 32-bit m68k family of microprocessors, it is generally software forward compatible with the rest of the line despite belonging to the 16-bit hardware technology generation. After twenty-seven years in production, the 68000 architecture remains a popular choice for new designs.

The 68000 grew out of the MACSS (Motorola Advanced Computer System on Silicon) project, begun in 1976 to develop an entirely new architecture without backward compatibility. It would be a higher-power sibling complementing the existing 8-bit 6800 line rather than a compatible successor. In the end, the 68000 did retain a bus protocol compatibility mode for existing 6800 peripheral devices, and a version with an 8-bit data bus was produced. However, the designers mainly focused on the future, or forward compatibility, which gave the M68K platform a head start against later 32-bit instruction set architectures. For instance, the CPU registers are 32 bits wide, though few self-contained structures in the processor itself operate on 32 bits at a time. The 68000 may be considered a 16-bit microprocessor which is microcoded to accelerate 32-bit tasks. The MACSS team drew heavily on the influence of minicomputer processor design, such as the PDP-11 and VAX systems, which were similarly microcoded.

In the mid 1970s, the 8-bit processor manufacturers raced to introduce the 16-bit generation. National Semiconductor had been first with its IMP-16 and PACE processors in 1973-1975, but these had issues with speed. The Intel 8086 in 1977 quickly gained popularity. The decision to leapfrog the competition and introduce a hybrid 16/32-bit design was necessary, and Motorola turned it into a coherent mission. Arriving late to the 16-bit arena afforded the new processor more integration (roughly 70000 transistors against the 29000 in the 8086), higher performance per clock, and acclaimed general ease of use.

The original MC68000 was fabricated using an HMOS process with a 3.5-micron feature size. Initial engineering samples were released in late 1979. Production chips were available in 1980, with initial speed grades of 4, 6, and 8 MHz. 10 MHz chips became available during 1981, and 12.5 MHz chips during 1982. The 16.67 MHz "12F" version of the MC68000, the fastest version of the original HMOS chip, was not produced until the late 1980s.

The 68000 had many high-end design wins early on. It became the dominant CPU for Unix based workstations, found its way into heralded computers such as the Amiga, Atari ST, Apple Lisa and Macintosh, and was used in the first generation of desktop laser printers. In 1982, the 68000 received an update to its ISA allowing it to support virtual memory by conforming to the Popek and Goldberg virtualization requirements. The updated chip was called the 68010. A further extended version which exposed 31 bits of the address bus was also produced, in small quantities, as the 68012.

To support lower-cost systems and control applications with smaller memory sizes, Motorola introduced the 8-bit compatible MC68008, also in 1982. This was a 68000 with an 8-bit data bus and a smaller (20 bit) address bus. After 1982, Motorola devoted more attention to the 68020 and 88000 projects.

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