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Originally Posted by ross
Just to get along 
You're both right, simply the 6502 if it had a 16-bit bus would access the memory at twice the speed of 68k.
But since we are in the real world, the 6502 actually reads the same data at (if it ever existed) same speed.
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There is such a thing as a 16 bit 6502, it's the 65816 (as found in the SNES). It does just that
The 65816 does have a problem though, it requires rather fast memory to be able to run at full speeds. Which is partly why the better known 65816 based designs tended to run at low clock speeds (Apple II-e @2.8Mhz, SNES @3.5Mhz) until much later on when faster memory came available (but the 65816 was long since out of fashion).
The C64 SuperCPU running at 20Mhz has really fast memory access speeds for instance, but is a 1996 product.
Honestly, I really rather like the 6502. Truly. Even have a special section on them on my website.
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Originally Posted by meynaf
Makes me wonder if the two can really run at the same speed.
Isn't a 8-bit bus in some way simpler than a 16-bit one ?
Besides, the 68000 can wait for data to come if something behind is busy. The 6502 can't - it is the master of the bus at any time and leaves the unused cycles for the dma, and if it were pushed to use every possible cycle you wouldn't get any display at all anymore.
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The 6502/65C02 was commonly available up to about 4Mhz.
Later on there where some faster versions (including a custom built 1987 Hudson Soft version used in the PC-Engine which ran at 7.16Mhz - though some don't consider that to be a 6502 as it included a bunch of extra instructions and other special hardware features).
So it is possible, but wasn't actually done until memory speeds had increased.
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Quote:
Originally Posted by ross
Really? I did not know this. Of course nobody would use those instructions, but I did not think there could be such a side effect.
I thought it was simply a read, of the float value on the bus, and then a writing of 0 in $dff096/9a (so nothing).
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The HRM states that reading from write-only chipset values will cause unintended effects (or horrors if you wish
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