Atari ST series Technical Reference Manual page 189

Tickcal()
T h is c a ll r e t u r n s t h e n u m b e r o f m i l l i s e c o n d s b e t w e e n t i m e r tic k i n t e r r u p t s .
F o r t h e c u r r e n t S T m o d e l s , th i s v a l u e is 2 0 m i l l i s e c o n d s .
C macro format
l o n g tic k le n ;
ti c k le n = T i c k c a l ( );
Machine language format
m o v e . w # 6 , - ( s p )
t r a p # 1 3
a d d q .l # 2 , s p
Inputs
N o n e
Results
DO ti c k le n l o n g
Tickcal
Get Timer Calibration
T h e n u m b e r o f m i l l i s e c o n d s b e t w e e n s y s t e m t i m e r
u p d a t e s .
Opcode = 6
181