Posted By
JamesD
on 2014-03-31
21:31:00
|  Re: Sprite Card
A SID card just interfaces the chip to the Plus/4 buss and the CPU can directly write to the SID chip.
A sprite card is certainly possible but it would be much more complex.
The sprite hardware would need to replace the pixel output from the TED chip wherever part of a sprite has to be displayed. This is no easy task since the timing is handled internally with the TED but there are a few ways to do it.
There are registers the hardware could read to sync to the TED and the sprite hardware could keep a copy of all the TED registers to make sure it knows what timing to use. The complexity of this approach is pretty nasty and the upgrade would need it's own RAM.
The alternative would be to duplicate the entire TED chip in an FPGA and add the hardware there where the new hardware would have access to all the timers. From the add on perspective this is easier but it requires a lot of work up front to duplicate the TED.
There is an alternative that might be easier. A blitter could be added to the system with some custom modes for blitting C64 sized sprites as well as other memory operations. It would lock out the CPU while blitting but it would be much faster than using the CPU. Not as fast as sprites but with the faster clock speed it might be enough to port more C64 games. The important thing here is it's a relatively simple design compared to the sprite hardware. If an FPGA Plus/4 were implemented I think I'd want a blitter even if it did add sprites.
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