Another quickie; cracked a few more variables in the last session which revealed an interesting if subtle detail to the game.
I had previously deduced the meaning of one variable to be the amount by which the background had scrolled left/right (dy), so it could be used to update, for example, the map and objects superimposed on it. Turns out that whilst I was close, I wasn't quite right.
There are actually two (2) variables updated when the ship is moved left/right; the abovementioned and another variable, updated by a slightly different amount. The other variable is actually dy for the background and ground-based objects and the first variable is dy for airborne objects, such as bullets and aliens.
What this means is there is a subtle parallax scrolling effect when moving the ship left/right! It is hard to notice when playing, but the effect is that your bullets (and certain aliens) will move left/right slightly less than the background moves.
[I should note that I use dy for left/right movement rather than dx because the screen is rotated, and the offset is made to the sprite register that is documented as the 'Y' register. Ultimately it's less confusing to reference the hardware than the physical orientation of the monitor, especially if the hardware is used for multiple games, some horizontal!]
It is getting harder and harder to progress, but I'm yet to finish a session without making at least some progress. A few variables seem to be initialised but never used (like the letter 'S'), but it's not straightforward to prove they're never used. In some cases the best you can do is set a watchpoint in MAME and play the game...
Here are the areas I have visited but have yet to complete:
- Background map generation/rendering; the data appears to be packed into blocks of meta-data
- Alien movement; alien movement is all table-driven
- Planet/Area data that affect difficulty; some of them are simple spawn timers or alien types but I haven't decoded every single parameter as of yet
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