Tidy-up, bug fixes and the GCW-ZERO

I decided to give my 68K code a once-over before I move onto sprites; fill in any missing functionality in the execution path of the attract mode that I've already commented but not ported.

In the process I fixed a few minor bugs; the score and high-score display routines were always showing "000000", and the palette for the conveyor/cement level was wrong.

I also decided to finish off commenting and porting the level background rendering routines, which were decidedly simpler than the girder and ladder drawing routines; each level is a mixture of generic and special-case routines, the latter being simpler to reverse-engineer. As a result, here are the four levels as rendered before any sprites are added:

All 4 screens, fully rendered before adding sprites

I really am at the stage now where I need sprites to see what is happening in the code. Again, the conversion to Neo Geo sprite format should be straightforward, and shouldn't take me more than an hour or so. Then I need to write the sprite rendering routines for the Neo Geo hardware. One interesting aspect of the Donkey Kong hardware is that the sprite registers are shadowed in ordinary RAM and then DMA'd to the hardware registers each VBLANK interrupt. That actually makes my job really easy (for the initial implementation) as I can essentially do the same (although I'll be using the CPU to move the data). It's not necessarily how I'll implement the final version, but again, makes it very easy to debug and also rotate the screen!

I do have a particular issue with sprite placement due to the fact that I'm scaling the tiles (Neo Geo sprites) to fit the entire Donkey Kong screen on the Neo Geo display, but I'm pretty sure I have a solution for that issue.

As an aside, I received my GCW-ZERO yesterday and have spent a bit of time installing emulators - including the Neo Geo of course - and playing a few things. Whilst the emulations themselves are excellent, and the screen is nice and bright, the build quality is absolute, complete and utter GARBAGE! The controls are so horribly crappy and sticky, it's almost unusable. I'm not sure what my options are given that it's a Kickstarter project, but I'm sure as hell not happy about it! Consider this a Public Service Announcement.