Atari ST series Technical Reference Manual page 228

Giaccess()
T h is f u n c t i o n a l l o w s y o u to r e a d o r c h a n g e a n y r e g i s t e r in t h e P r o g r a m m a ­
b le S o u n d G e n e r a t o r (P S G ) c h i p .
C macro format
c h a r r e g v a l u e , v a l u e ;
i n t r e g n u m ;
r e g v a l u e = G i a c c e s s ( v a l u e , r e g n u m ) ;
Machine language format
m o v e . w # r e g n u m , - (s p )
m o v e . w # v a l u e , - ( s p )
m o v e . w # 2 8 , - (s p )
t r a p # 1 4
a d d q .l # 6 , s p
Inputs
r e g n u m w o r d
b y t e
v a l u e
Results
DO r e g v a l u e
See also
O n g i b i t ( ) , O f f g i b i t ( )
220
Giaccess
Read/Write Sound Chip
b y t e T h e n u m b e r s t o r e d in th e r e g i s t e r a t th e e n d o f th e
c a ll.
T h e n u m b e r o f t h e r e g i s t e r t o r e a d o r
c h a n g e . If y o u 'r e r e a d i n g th e r e g i s t e r , u s e
t h e r e g i s t e r n u m b e r ( 0 - 1 5 ) . T o c h a n g e th e
r e g i s t e r , u s e t h e r e g i s t e r n u m b e r +
( 1 2 8 - 1 4 3 ) .
T h e n e w 8 - b i t n u m b e r t o g o in t o t h e r e g ­
i s t e r .
Opcode = 28
1 2 8