For my next game I wanted to do a shooter, a very simplified Space Invaders, with ever-changing sprites. For these I used the same principle as with my very first video project in 2004, the 5x5pixler.
With a 1Mhz-baseline-PIC you get a very little screen resolution. (1 scanline = 64µS, only 48 are visible; 1 opcode cycle of a 12F508 uses 1µS, so with clever coding a maximum of 48 'pixels' per x-axis is possible.) Timing is everything.
The whole screen has to be drawn in one sweep, 50 times a second (PAL, non-interlaced), only at the end of the picture is some time left to calculate game relevant variables, f.e. the very simple 'AI' of the grey alien on top of the screen, the shooting etc. Similar to programming an Atari 2600 game.
The alien behaviour was very tricky but fun to program, it gets faster and harder to shoot. To make it something special, shots can be stopped and restarted by both human and computer. I'm quite proud of the final version, it's fun and challenging.
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