|
I did some more research on this, and it seems using chip memory is the major issue with upgraded CPUs. If I understood correctly this is the situation:
The chipset controls the bus every alternate cycle, so the 7MHz memory speed on chip RAM is effectively reduced to 3.5MHz for the CPU -- no matter what kind of CPU is running. That's before any slow downs due to slow chipset operations.
That isn't an issue for the 68000 as it only accesses memory ever other cycle anyway.
An upgraded 68020 at 14MHz is then accessing chip memory at a maximum of 3.5MHz and that will be effectively lower when the chipset is busy as the CPU is forced to wait.
With any 16bit fast RAM installed, the 68020 (or higher) can access that at the full 7MHz without any delays from the chipset.
With 32bit memory installed on the CPU expansion card you do obviously gain the benefit of the full data width being used, but that would seem to have less of an impact than effectively running memory at 3.5MHz max.
If anything's incorrect there please do let me know.
|