leo@ihuxl.UUCP (09/20/83)
My understanding of the multiple colors is that the video circuitry in the television is fast when responding to luminance but not fast when it comes to chroma (phase decoding is fairly slow). What happens when the computer trys to change color quickly is that the circuitry ends up compromizing on an intermediate value for the color (as it sees colors). This explains why you don't see the effect in black/green. In this case the intensity is changing but not the color (black being any color with intensity == 0). If I recall correctly, all colors on the apple are generate in this way. By the way, you can generate colors and strange shading patterns by poking into &HB2 (dec 178) and &HB3 (dec 179) and then defaulting colors on paint etc. (mostly paint). T. These are the default foreground and background colors, and writing invalid colors here results in unexpected bit patterns. I don't recall what pokes work best, and I suggest that you experiment. Leo R. ihuxl!leo P.S. to Mike Knudsen: the SYNC assembler instruction waits for an interupt. It causes the 6809 to stop until it receives an external signal and would be used in applications when the computer has interrupt driven I/O, and is in a state where it has to wait for more input. It is not very useful in the COCO, which has to scan for input.