One timely interruption

Not a lot of time to work on Space Invaders over the last few days, but coming along nicely. I definitely think I'd like to finish it now before returning to Knight Lore.

As-is it loops through the attract screens, complete with punctuated text rendering, although the animations (involving the little invader) are yet to be coded. The demo displays the shields and the line across the bottom of the screen before continuing onto the attract mode.

Source, including data, is around 1,500 lines already. In comparison, the IDA disassembly of the arcade original Z80 code is around 4,200 lines. The translation from 8080 to 6809 has been quite straightforward thus far; I'm using a fairly strict register mapping convention and I've only been forced to use shadow memory registers (for BC) once or twice.

The game uses interrupts heavily, having two (60Hz) interrupts on specific scan lines which drive most of the game logic. I've only enabled one of them - line 224 which is effectively the VBLANK interrupt - and it simply decrements a timer counter (used in the attract mode) at this point.

Rather than attempt to use a HBLANK interrupt and count scan lines for the other interrupt, I'll start the timer in the VBLANK interrupt. The exact timing of the second interrupt isn't critical so it should be accurate enough for the code to run without any noticeable difference.

Once I finish the portions of the attract mode that aren't interrupt-driven, I'll have to start fleshing out the ISR's to see anything else happen.