henry (03/15/83)
An attempt to clear up the debate about how wide the 68000 is... The 68000's guts are *basically* 16 bits wide. There is a single main 16-bit internal bus, but under a number of circumstances it can be segmented into a pair of 16-bit buses (by breaking it at a switch in the middle). This gives a limited ability to do two 16-bit transfers simultaneously, and the 68000 microcode does make use of this to speed things up. But in general it is still a 16-bit machine; check the timing differences between 16-bit and 32-bit instructions if you doubt me. What Motorola's propaganda about the design being 32 bits wide inside means, is that the *architecture* is logically 32 bits wide, so you could implement a 32-bit-wide 68000. This is presumably what the 68020 will be. (Will it actually have thrilling things like a 32-bit multiply instruction? Who knows...) Anyone who wishes to understand the architecture and implementations of the 68000 should be aware of a key fact: originally the 68000 was a 16-bit architecture with >16-bit addresses. This is why the distinction between data and address registers: at the start, the data registers were 16 bits but the address registers had to be wider. The 32-bit data registers, and assorted 32-bit instructions, were largely afterthoughts added after a fair bit of the design had already solidified. Henry Spencer U of Toronto