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in article <SARREL.89Jul10202542@sioux.cis.ohio-state.edu>, sarrel@sioux.cis.ohio-state.edu (Marc Sarrel) says:

> A friend of mine just bought at 16MHz 68000 accelerator card for his
> SE.  How much performance difference is there between a 16MHz 68000
> and a 16MHz 68020?

It depends on the system.  If you're accelerating a 16 bit machine and not
adding any faster memory, a 68020 will in some cases go about maybe 15%-30%
faster than the 68000, depending on how well the instruction cache gets
utilized.  It's actually possible for a 68020 to go slower than a 68000 when
on a 16 bit bus, since the 68020 always prefetches 32 bits for instruction
processing, though sometimes only 16 bits worth actually gets used.  

Once you add some 32 bit memory to a 68020, you get real performance increases.
The 16MHz 68020 plus some reasonably fast 32 bit memory will get you about 
200%-400% speedup over your 7.8Mhz 68000, and a little better than twice the
performance of a 16MHz 68000 with fast 16 bit memory.  That's all integer
performance.

If you add a 68881 or 68882 to the 68020, and your software knows about math
chips, you may get a floating point performance increase from 25x-100x what
a plain 68000 is capable of.  I don't know how well the Mac software would
integrate any of this, but the hardware driven increases tend to be similar
among all 680x0 machines.

> Are 68000 accelerators a cost effective alternative?  

It depends.  Without faster 16 bit memory, it's going to boost your performance
by maybe 15% or so overall.  With the faster memory, it'll come close to 
doubling the performance, providing the Mac can be set up to use that fast
memory before its on-board slower memory.

> Can you get faster then 16MHz?  

Not likely.  The 16MHz (actually, 16.6667MHz; Motorola CPUs are rated by the 
time of the clock cycle, in this case 60ns) part is currently the fastest
68000 anyone makes.

> --marc

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