- Motorola 68008
The original 68000 had a 24-bit address bus and a 16-bit data bus. These relatively large buses made it hard to design a low-cost system around the 68000; they were difficult to lay out on a circuit board and needed a lot of supporting circuitry. A 16-bit data bus also required twice as many memory chips as an 8-bit one.
The 68008, introduced in 1982, was designed to work with low-cost 8-bit memory systems. Because of its smaller data bus, it was only about half as fast as a 68000 of the same clock speed. However, it was still faster than competing 8-bit microprocessors, because the 68008's internal architecture was based around a 32-bit architecture.
Except for its smaller data and address buses, the 68008 behaved identically to the 68000 and had the same internal organization and microarchitecture.
The 68008 was an HMOS chip with about 70000 transistors; it came in 8 and 10 MHz speed grades. There were two distinct versions of the chip. The original version came in a 48-pin dual in-line package and had a 20-bit address bus, allowing it to use up to 1 megabyte of memory. A later version came in a 52-pin plastic leaded chip carrier; this version provided a 22-bit address bus and could support 4 megabytes of memory.
Motorola ended production of the 68008 in 1996.
Notes and references
Motorola/Freescale processors Industrial Control Unit 6800 family 68000 family Embedded system 68k-variants68EC000 · 68EC020 · 68EC030 · 68EC040 · 68LC040 88000MC88100 (MC88110) Floating-point coprocessors Memory Management Unit PowerPC family
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