Atari ST series Technical Reference Manual page 201

Get Screen RAM Physical Base Address
Physbase()
T h is f u n c t i o n r e t u r n s th e s t a r t i n g a d d r e s s o f s c r e e n d i s p l a y m e m o r y .
C macro format
l o n g s c r a d d r ;
s c r a d d r = P h y s b a s e ( ) ;
Machine language format
m o v e . w # 2 , - (s p )
t r a p # 1 4
a d d q .l # 2 , s p
Inputs
N o n e
Results
DO s c r a d d r l o n g
See also
L o g b a s e ( ) , S e t s c r e e n ( )
Physbase
A p o i n t e r t o t h e b e g i n n i n g o f t h e 3 2 K m e m o r y
b l o c k t h a t is c u r r e n t l y d i s p l a y e d .
Opcode = 2
193