A year ago, I created a kernel to display 13 characters per line, each of which could independently be one of two colors. Rather a neat trick, but it required a bunch of straight-line code. The only way to change the contents of a line of text was to modify the code that displayed it. On a SuperCharger, that could work, though displaying eight lines of text would use up most of the RAM; I dismissed the kernel as an interesting curiosity.
Discussions of Sudoku led me to wonder whether the kernel technique I used there could be used without RAM expansion if it didn't have to show so many characters. It turns out that it is.
Thus, my flicker-free Sudoku kernel demo. The cursor flickers in a crude effort to make it transparent, but otherwise no fliker is necessary. Each cell may be independently colored white or purple using the RESET button, and each cell's content may be changed using the fire button. There are no palette restrictions.
Can anyone explain how this works?