For the first time I now have something "gamelike" running on Dodo! I now have software to render a game background and a sprite. The walking cycle on this character is a bit rough, and could use several more frames, but in concept it is working. I have a sleep of 255ms per frame so I can slow it down to see what is happening. Funny that I finally have something going too quick on this slow little 1mhz machine. Furthermore, there is still a lot of room for optimization with what I have implemented thus far. I am now confident I will be able to implement a playable game on this thing.

The other good news is that I finally got around to verifying that interrupts are working. I successfully configured the 6522 to use one of timers to fire a regular interrupt. This was the one item I designed on the PCB without having ever tested it. I also have the 6551 wired for interrupt as well but I am not sure I will ever use it. The timer interrupt is important because I will use it to pump my game loop to ensure that it runs at a consistent FPS.

Interrupt lines on for these series of chips need to be carefully designed. The old style IRQ outputs are all open drain on the original chips so that they can be wired together. The newer chips with the 'C' in the middle are no longer open drain. I have a mix, my 65C22 is not open drain but my old 6551 ACIA is. I fed both interrupts into an AND gate and there is a pullup for the ACIA.