Sunday, 17 April 2016

Space Invaders

And so it begins... with the Knight Lore ROM image emailed off to John for CocoFEST, I've decided to take a short break from filmation games until it's time to produce cartridges, in which case I'll need to look at code optimisation. However I've been in contact with another programmer who is in the final stages of porting Pentagram to the Atari 8-bit machines, and he's given me some invaluable insight into where the code can be improved. In fact, he has Pentagram running on the Atari slightly faster than the ZX Spectrum, and a lot of the code is still auto-generated from Z80 to 6502!

I was just playing around with the arcade version of Space Invaders, seeing how that would look on the Coco3. I fired up MAME and took a memory dump mid-game, played with the layout a bit, and then loaded it into the Coco3 under MESS.

Space Invaders (mock up) on the Coco3 under MESS

It looks pretty good to me, and tonight I couldn't resist starting on an actual port. Whether or not I decide to see it through just now will depend on how quickly it proceeds in the initial stages, but if my experience with Knight Lore is any indication, it might not take long to get things well along.

I'll retain the original video memory mapping (including rotated screen) until the game is all-but-ported, to simplify the process. I've done rotation on-the-fly in my TRS-80 Model 4 'bootleg', so it's not a big deal. Running MESS with the 'rol' option will give me the correct display during development. One thing I do need to do though is bit-reverse all the graphics data, as the pixels are laid out in the reverse order within each video RAM byte.

Space Invaders in 6809 (WIP)

I've also decided to work with the disassembly on Computer Archeology, rather than my own reverse-engineering,  for reasons I won't bother to explain.

I'm hoping to knock this one over fairly quickly, given all the reverse-engineering has already been done and it's a relatively small - and simple - program. I'm not going to bother producing a cartridge for Space Invaders; rather I'll make disk and ROM images freely available for download.

No comments:

Post a Comment