Mostly working.

Usual story, busy with work blah blah.

I have managed to get it mostly working though - except for the shields. The rotation code is brain-dead brute-force, and given that the VBLANK ISR now takes most of the frame to execute, there's going to have to be a lot of optimisation if it's going to be playable, let-alone add colour and sound. But I still think it can be done.

I should note that I do appreciate the coding/optimisation suggestions tendered in the comments, and will certainly be visiting those again when the time comes.

But the important part is that I've now wrapped my head around what needs to be done. Tonight, for example, I racked my brains trying to find which routine wiped the invaders as they moved - in particular when they hit the edge of the screen and move down. Finally it dawned on me that they don't get erased, but in fact get redrawn by the shifted sprite routine - with a shift value of 0. This has the effect of wiping the 8 pixels above the invader (each and every time).

There's still a chance that I'll get all this done and then need to return to the original plan though...