Atari ST series Technical Reference Manual page 229

Clear a Bit on Sound Chip I/O Port
Offgibit()
A t o m i c a l l y c l e a r s a s i n g l e b it o f t h e P o r t A I /O r e g i s t e r o n t h e P S G s o u n d
c h i p .
C macro format
in t b i t n u m ;
O f f g ib i t(b itn u m );
Machine language format
m o v e . w # b i t n u m , — (s p )
m o v e . w # 2 9 , - ( s p )
tr a p # 1 4
a d d q .l # 4 , s p
Inputs
b i t n u m w o r d
Results
N o n e
See also
O n g i b i t ( )
Offgibit
T h e n u m b e r o f th e b it ( 0 - 7 ) to c h a n g e .
Opcode = 29
221