Atari ST series Technical Reference Manual page 233

Start Sound Interrupt Processing
Dosound()
T h e D o s o u n d ( ) f u n c t i o n e n a b l e s a n i n t e r r u p t - d r i v e n r o u t i n e t h a t c a n p l a y
m u s i c o r s o u n d e f f e c t s in t h e b a c k g r o u n d .
C macro format
lo n g c o m m a n d l i s t ;
D o s o u n d ( c o m m a n d l i s t ) ;
Machine language format
m o v e .l c o m m a n d l i s t , - ( s p )
m o v e , w # 3 2 , - (s p )
t r a p # 1 4
a d d q .l # 6 , s p
Inputs
c o m m a n d l i s t l o n g
T h e c o m m a n d s t r u c t u r e is:
First Byte
0 - 1 5 L o a d a
b y t e v a l u e in to
t h e r e g i s t e r
s p e c if ie d b y th is
c o m m a n d b y t e .
( 0 - 1 5 )
1 2 8 S t o r e a
v a l u e in a
t e m p o r a r y
r e g i s t e r f o r u s e
b y c o m m a n d
n u m b e r 1 2 9 ( s e e
b e l o w ) .
1 2 9 L o a d a
r e g i s t e r w i t h th e
v a l u e s t o r e d in
th e t e m p o r a r y
r e g i s t e r (b y
c o m m a n d 1 2 8 ).
I n c r e m e n t th e
v a l u e in th e
r e g i s t e r e a c h
t i m e r ti c k , u n til
a n e n d v a l u e is
r e a c h e d .
Dosound
A p o i n t e r t o a d a t a s t o r a g e a r e a t h a t c o n t a i n s a
n u m b e r o f q u e u e d s o u n d c o m m a n d s , o n e o f w h i c h
is e x e c u t e d e a c h 1 /5 0 o f a s e c o n d .
(Command Number)
Second Byte
T h e b y t e v a l u e
t o lo a d i n t o th e
r e g i s t e r ( 0 - 2 5 5 ) .
T h e b y t e v a l u e
t o s t o r e in th e
t e m p o r a r y
r e g i s t e r ( 0 - 2 5 5 ) .
T h is is t h e
s t a r t i n g r e g i s t e r
v a l u e f o r
c o m m a n d 1 2 9 .
T h e n u m b e r o f
th e r e g i s t e r to
u s e . ( 0 - 1 5 )
Third Byte
I n c r e m e n t v a l u e
( 0 - 2 5 5 ) . A d d e d
to r e g i s t e r v a l u e
e a c h t i m e r tic k .
C a n b e p o s i tiv e
( 0 - 1 2 7 ) o r
n e g a t i v e .
Opcode = 32
Fourth Byte
F i n a l r e g i s t e r
v a l u e .
C o m m a n d e n d s
w h e n th is v a l u e
is r e a c h e d .
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