About to bite the bullet...

It's time to book the fat lady who sings. After consulting my Donkey Kong code, I copied the "magic" algorithm for transforming the sprite Y coordinate and viola - sprites appeared! Still perhaps not pixel-perfect due to the placement of the underlying tilemap, but close enough for now.

Score table with sprites

It runs slow, there are glitches, but most of the implementation is done. Bullets are missing but everything else is in place. It's actually playable now.

Scramble, on the Neo Geo in tate mode

The game really slowed down when I got the sprite update code working. Not sure why because there wasn't a lot I added but maybe just enough to tip the NMI execution time over the 16ms mark. It could also be because of the brute-force hack I did to minimise glitches (still) caused by a race condition with the Neo Geo VRAM registers. So there's still a bit of optmisation that can be done. I have no doubt I can ultimately get it running at 60fps.

The scrolling was trivial to implement on the Neo Geo. Initially I defined the tilemap as a single 32x32 tile (scaled to 50%) chained sprite, with a single origin. But today I divided it into 3 chained sprites, the middle sprite being the scrolling portion of the display. So scrolling the playfield now requires an update to just a single sprite Y coordinate register - too easy.

I'll need to go back and create a tile for the bullet sprite; on the arcade game the bullets are not rendered from ROM data like other sprites but rather a dedicated circuit on the PCB. Then I can add the code to assign another 4 Neo Geo sprites to the bullets and the functionality should then be complete.

From there it's a matter of handling the race conditions (I think I just need to disable interrupts briefly) and optimise the VRAM access code and that should be it. I might look into adding sound if I can find some samples as there seems to be resources around to get a sound driver up and running.

I'm also thinking of options and strategies for rotating the display. I already have rotated tiles and sprites in the CROM.

UPDATE: added bullets.