68030
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Yes, the DRAM nibble mode supported is as you described. I don't have a
data sheet on the 68851. Perhaps I'll call Motorola office here to get one
as I'm curious too what it looks like.
My reasoning on the relative speeds of the 68020 and 286/386 is… From
various comparisons I've done the 286 and 68000 seem to be about the same
speed for equal clocks. ie my 12Mhz 68K is 2x a 6Mhz 286. The 386 takes the
same nb of clocks as a 286 for instructions that are the same between the
two. Any speed up comes from the faster 386 clock (16Mhz) and better code
possible due to 32 bit ops and additional address modes. An overall gain of
2-3 would be as much as I'd expect between 286 & 386 and I don't believe
Intel claims much more than this anyway.
The 68020 takes a lot fewer clock cycles than a 68000 in most cases. For
example, the 4 instructions for the loop in the code contest loop takes 42
cycles on 68000 and 27 (cache case) on 020. This is for garden variety
move, add, dbra ops. Others have much greater differences such as muls
(70/27) and divs (158/44). Several 20Mhz 0 wait state VME bus 020 boards
are available and I just saw a Plessey ad for a 25Mhz 0 wait board. Factor
in the higher clock speed of the 020 as well as the 020's new instructions
and addressing modes and it runs MUCH faster (~4x) than a 68000 and a 286
and still somewhat faster than a 386. For floating point the 68881 is a lot
faster than the 287. I don't know if there is a 387 yet or how fast it is.
I've seen no claims on the ratio between the 020 & 030 but it must be non
trivial given the new cache and bus interface.
I don't remember exact dates but the 68020 has been available in qty for
8-12 months longer than the 387. A long time in this business. It's been
less than 2 years since I sent you the DIFF program.
A coworker suggests that as soon as someone makes an 030 board to plug into
a 020 socket that I get the CSA 020 upgrade for my 68000 and then plug the
030 upgrade into that! That would make the old Amiga move right along. 🙂