On Sat, 8 Feb 2003, Eric Smith wrote:
> Pete wrote:
> > You must be thinking of some different 6502 to the rest of us :-) As
> > Sellam said, no 6502 opcode takes less than two clock cycles to
> > execute, and most take more (up to 7): the only 2-cycle instructions are
> > the ones with implied addressing, like RTS, CLI, TAX, ...
>
> Not RTS, that takes a bunch.
6 according to my chart.
> I've spent some time working on a reimplementation of the 6502 in a
> Xilinx FPGA. It's actually fairly difficult to design to match the
> exact number of clock cycles for each instruction. It's much easier
> if you allow instructions to take more cycles, which is the approach
> taken by the OpenCores version. That still lets you run it much faster
> than the real thing, but is no good for things that depend on the
> exact cycles counts, such as the Apple II RWTS routines (low-level
> disk access).
Yes, that could really mess up some timing-critical code.
Sellam Ismail Vintage Computer Festival
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Received on Sat Feb 08 2003 - 14:30:01 GMT