Atari ST series Technical Reference Manual page 408

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P o i n t e r to a t e m p o r a r y b u f f e r w h e r e t h e p r o c e s s o r c o n t e x t is s a v e d .
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T w o p o i n t e r s t o G E M D O S b u f f e r lis t h e a d e r s . E a c h p o i n t s t o a B u f f e r C o n ­
tr o l B lo c k (B C B ), w h i c h is la id o u t a s f o llo w s :
struct BCB
{
BCB *b_link;
int b_bufdrv;
int b_buftyp;
int b_bufrec;
int b__dirty
DMD *b__dm
*b__bufr
char
T h e f ir s t b u f f e r is u s e d to s t o r e d a t a s e c t o r s , t h e s e c o n d is u s e d t o s t o r e
F A T a n d d i r e c t o r y s e c t o r s .
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C o u n t e r f o r 2 0 0 H z t i m e r . D i v i d e d b y 4 f o r t h e 5 0 H z s y s t e m c lo c k . T h is
v a l u e is u s e d a s t h e s t a r t i n g s e e d t h e f ir s t ti m e R a n d o m ( ) is c a lle d .
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T h e d e f a u l t e n v i r o n m e n t s t r i n g , w h i c h c o n s i s t s o f f o u r 0 b y t e s .
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I n d i c a t e s w h i c h d r i v e s a r e c o n n e c t e d . E a c h o f t h e 3 2 b its o f t h e _ _ d r v b its
v a r ia b le c o r r e s p o n d s to a d i f f e r e n t d r i v e . B it 0 is a s s i g n e d t o d r i v e A : , b it 1
to d r i v e B :, a n d s o o n u p to b it 1 5 , w h i c h c o r r e s p o n d s t o d r i v e P : ( t h e
c u r r e n t v e r s i o n o f t h e S T o p e r a t i n g s y s t e m o n l y r e c o g n i z e s 1 6 d r i v e s ) . If
th e b it w h i c h c o r r e s p o n d s to a d r i v e is s e t to 1 , t h a t d r i v e is c o n n e c t e d ,
o t h e r w i s e , it is u n a v a i l a b l e . T h e v a l u e s t o r e d h e r e is t h e s a m e o n e r e ­
t u r n e d b y t h e B I O S f u n c t i o n D r v m a p ( ). N o t e t h a t if e v e n o n e f l o p p y is
c o n n e c t e d , b its 0 a n d 1 a r e b o t h a l w a y s s e t t o o n e . T h a t 's b e c a u s e if d r i v e
A : is c o n n e c t e d , t h e s y s t e m w ill u s e it a s a lo g ic a l d r i v e B : if n o p h y s i c a l
d r i v e B : is p r e s e n t .
400
APPENDIX K
sav__contxt
__bufl
f* ptr to next BCB *1
/* drive number, or -1 *1
/* buffer type */
/* record number in this buffer *1
/* dirty (buffer changed) flag *1
I* ptr to Drive Media Descriptor *1
/* ptr to buffer */
__hz__200
the_env
__drvbits
($4AE-$4B1)
($4B2-$4B9)
($4BA-$4BD)
($4BE-$4C1)
($4C2-$4C5)