Monday, 3 February 2014

Neo Geo sprite limitations, RTFM and finally working!

It's been a mixed bag of frustrations, red herrings and re-reading documentation over the last few days but I'm very excited to announce that I have finally seen Donkey Kong running on my NGCD with all graphics intact!

Along the way I suspected issues with MAME/MESS, NeoCD/SDL and Raine emulators. In the end I can report that MAME/MESS appears 100% accurate with NeoCD/SDL actually rendering sprites that the hardware is simply not capable of (and sending me off on a wild goose-chase).

After a few experiments with furtek's sprite experimenter demo in MAME, and subsequently studying the SNK Neo Geo Hardware Manual in more detail, it became apparent that I was trying - in an attempt to fix the flickering issue - to produce a scaled display that the hardware wouldn't support. As time consuming as it is, the upshot of all this is that I learned a bit about the sprite hardware.

I won't bore anyone with intimate details, but in the end it was kyuuu on #neogeodev that suggested perhaps I was updating the VRAM too fast, and that was indeed the case. My background rendering had back-to-back writes to SCB1, whilst the documentation specifies a 12 CPU clock cycle delay is required.

So now I really have no excuse not to get back to finishing the port... except for work pressures and design of the flash cart...

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