Atari ST series Technical Reference Manual page 362

T h is c o m m a n d s e t s t h e o r ig i n o f t h e y c o o r d i n a t e s y s t e m a t t h e b o t t o m ,
w h i c h m e a n s t h a t d o w n w a r d m o v e m e n t ( t o w a r d s t h e u s e r ) is t r e a t e d a s
n e g a t i v e , a n d u p w a r d m o v e m e n t ( a w a y f r o m t h e u s e r ) is t r e a t e d a s p o s i ­
ti v e .
Input parameters
N o n e
354
APPENDIX I
$0F Set y Origin at Bottom