The original 68000 (introduced in 1979) has a 24-bit address bus and a 16-bit data bus. These relatively large buses made it hard to design a low-cost system based on 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 internally was based on 32-bit data registers and a 16-bit data bus.
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 70 000 transistors; it came with a speed grade of 8 and 10 MHz. 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 RAM.
Notes and references
- comp.sys.m68k Usenet posting, May 16, 1995; also see other posts in thread. The end-of-life announcement was in late 1994; according to standard Motorola end-of-life practice, final orders would have been in 1995, with final shipments in 1996.