Monday, 16 May 2016

Seeing double

Next-to-no time to work on Space Invaders lately, but managed to find 10 minutes here and there a few times since the last post, and tonight I finished another intermediate task.

The idea was to duplicate the screen, to achieve a few goals. One was to identify each of the routines in the code that will require extending to render the rotated screen. FTR there are 11 such routines. In each case, I simply added code to the end of the routine to (also) re-render the video to the Coco3 display. This code will ultimately rotate the graphics on-the-fly. It should be obvious that during the process, I had to take note to preserve and restore any registers that were assumed to be a certain value on return from the rendering routine.

Another goal was to gauge how much processor time was still available during the interrupts. The good news is that, whilst the VBLANK interrupt now encroaches on the visible display (as it no doubt does on arcade hardware) there still seems to be plenty of headroom for further complexities.

Next step is to add the rotation. Looking at my old Space Invaders TRS-80 Bootleg Project code, it was all done brute-force using a few assembler macros. No harm in at least giving that a go; it should mean there's not a lot of code required to achieve all of the rotations!

UPDATE: A little more progress. Three (3) of the eleven routines are complete; the two clear routines and the routine to draw the line along the bottom. None of these required any graphics rotations and were more-or-less simple rewrites.

As for the remaining eight (8) routines, they've all been modified to draw the graphics at the correct screen location. One of them, the routine that draws the shields, will have to be modified in the caller as well as it sets the video address of each individual shield.

The Coco3 screen in its correct orientation

Right now it's crashing, but that's not totally unexpected.

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