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