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2015-02-17 10:13

Heavy Stylus

Registered: Apr 2007
Posts: 62

Recent Diskmag with C65 FPGA Article?

Hey there! Not an ancient prod at all, but I read an article about someone creating an FPGA version of a C65 on a recent diskmag and cannot remember which one it was!

Any help would be mucho appreciated!

J/HS

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2015-04-05 18:44

hld_flt
Account closed

Registered: Nov 2014
Posts: 6

Sorry for extreme delay in response.

On a real C65 there is neither memory nor CPU time to effectively double-buffer in a game.

The DAT lets you specify an 8-bit X and Y coordinate in two registers, and then access the (upto 8) bitplane bytes for those coordinates directly.

Of course, you still have to do the bit fiddling, so changing a single pixel is still horribly slow.

Even assuming you could use DMA to write to the 8 bytes in 8 cycles, modifying a 256-colour pixel requires 8 cycles, and more like 100 or more cycles in reality. Since you have only 3.5MHz/50Hz = 70K cycles per frame, you are limited to updating never more than 10K pixels per frame using unrealistically optimistic appraisal. More realistically you could update not more than a few hundred pixels per cycle.

There was also as I recall some funny number of cycles you had to wait after setting the DAT address before you could use it for the new address.

I understand why they created it, but it was a partial mitigation of the bigger problems that bitplanes presented on the machine.

Of course, with the technology of the time, they might not have had too many other options, but I do :)

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