Atari ST series Technical Reference Manual page 240

Vsync()
T h is f u n c t i o n s i m p l y w a i t s u n til t h e v e r t i c a l b l a n k i n t e r r u p t is f in i s h e d , a n d
r e t u r n s . I t c a n b e u s e d to s y n c h r o n i z e s c r e e n d r a w i n g w i t h t h e v b la n k , o r
a s a s i m p l e d e l a y m e c h a n i s m .
C macro format
V s y n c ( );
Machine language format
m o v e . w # 3 7 , - (s p )
t r a p # 1 4
a d d q .l # 2 , s p
Inputs
N o n e
Results
N o n e
232
Wait for Vertical Blank
Vsync
Opcode = 37