[Coco] The 6502 sprints ahead of the 6809: 100Mhz 6502

[RETRO Innovations]

On 10/14/2021 10:20 AM, Dave Philipsen wrote:
> This is an interesting design and nice packaging in the 40-pin DIP. However, in order to run as fast as it does, apparently it copies external ROMs to its internal RAM and then uses that internal RAM exclusively when it runs and then only accesses external stuff for I/O and it slows itself down to do that. So I doubt that it really ‘sprints ahead’ of the 6809 in that regard because we have the 6809 available in FPGA that could likely do the same thing. Likely this device is limited to 64K and would not have the ability to access a larger amount of RAM like the CoCo 3 does (512k, 2M, 8M) without some trade offs.
>
> So it isn’t exactly a drop-in replacement for a 6502 since it can’t deal with external RAM and it would have to be customized for different environments.

The presentation goes over many of these items:

  * It is indeed limited to 64K internal RAM, but has a capability to
    handle bank switching.  Author notes it is not tested and isn't sure
    it's needed for the use case, as it does slow down (I assume it
    copies all the 64 K of the current bank into internal, flushing it
    each time the banking changes.
  * The customization (some of it anyway) can be done via DIP switches
    on the top of the unit.
  * Point is made, though, that the exact same HW design could be loaded
    with 6809 core, if a 100MHz 6X09 core exists.  I don't think one
    does at this point.

Thus, I think, at least until someone cleans up the timing on the 6X09 
cores so they can do 100MHz, the OP has a point.


Jim