== Calculating xpos,ypos to screen address ==
The screen address at a given x,y position can be calculated as such:
[pre]
screenaddr = screenbase + (y AND 7)*&800 + int(y/8)*2*R1 + int(x/M)
[;; screenaddr = screenbase + (y AND 7)*&800 + int(y/pre]8)*2*R1 + int(x/M)
where: