View source for Programming:Calculating xpos,ypos to screen address in assembly

== Calculating xpos,ypos to screen address in assembly ==

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)

[/pre]

where:
x and y your 2D coordinates

R1 = value of CRTC register 1 (eg. 40 with the default firmware settings)

M = 2 (for screen mode 0, 2 pixels/byte)
M = 4 (for screen mode 1, 4 pixels/byte)
M = 8 (for screen mode 2, 8 pixels/byte)

screenbase = &C000



[[Category:Programming]]