Spectravideo SVI 318 Service – Technical Manual page 115

T he C R C i n c l u d e s a l l i n f o rma t i o n s t a r t i n g w i t h t h e a d d r e s s ma rk
CRC cha r a c t e r s . The CRC re g i s t e r is p re s e t to on e s p r i o r to d a t a
t he c i r cu i t .
t h r o u gh
ARITHMETI C / LOG I C UN IT ( ALU ) - The ALU i s
dec r e me n t e r and is u s e d for r e g i s t e r mo d i f i c a t i o n a n d compa r i s o n s
r e c o r d e d I D f i e l d .
TIMING AN D CONTROL
th r gh th i s
g e n e r a t e d o u
e x t e rn a l
c r y s t a l c l oc k .
The FD 1 7 9 1 / 3 ha s
two
DDE
D DEN . Wh en N
=
( FM )
den s i t y i s a s sume d .
AM
DETECTOR - The a dd re s s ma rk d e t e c t o r de t e c t s I D , d a t a and i n d e x a dd re s s
du r ing r ead and wr i t e o p e ra t i o ns .
m a r k s
a
Whe neve r Rea d o r W r i t e comma �d ( Type I I or I I I ) i s re c e i ve d the FD 1 7 9 X
s amp l e s t he R ea dy i n pu t .
exec u t e d and an i n t e r r up t is gene ra t e d .
reg a r d l e s s o f t he s t a t e o f t he k e a dy i n pu t .
command i � rece i v e d , t h e TG43 s i gna l ou t p u t i s up d a t e d .
PRO CE S SOR INTERFACE
i n t e r f a c e to t he p r oce s s o r i s accomp l i s he d t h rough t he e i gh t Data Acce s s
T he
( DA L ) and a s s o c i a t e d cont r o l s igna l s .
L i n e s
S t a t u s , and Cont ro l wor d s ou t o f , or i n t o th e FD 1 793 .
b u f f e r s t h a t a r e enab l e d a s ou t p u t d r i v e r s whe n Ch i p S e l ec t ( C S ) a n d Rea d
E na b l e ( RE ) a r e ac t i ve ( l ow l o g i c s t a t e )
E n a b l e ( WE ) a re a c t i ve .
W r i t e
Wh en t ra ns f e r o f da ta w i t h t he F l op py D i sk Con t ro l l e r i s re q u i re d by t he
p roce s s o r , t he d e v i ce a dd re s s i s d e co d e d and CS i s ma de
A l a n d AO , comb i n e d w i t h t h e s i g na l s R E du r i ng a Rea d op e ra t i on o r
W r i t e o pe r a t i on a re i n t e rp re te d a s se l e c t ing t he f o l l ow ing reg i s t e r s :
A 1 -AO
0 0
0
1
1 0
1 1
- A l l compu t e r and
The i n t e rna l de v i ce t i m i ng i s
l o g i c .
d i f f e r e n t modes
of ope r a t i o n ac c o r d i ng
0 , d e n s i t y
doub l e ( N FD )
I f t h i s input i s log i c l ow t he command i s not
READ ( RE )
S t a t us Reg i s t e r
T ra c k Reg i s t e r
Sector Reg i s t er
Data Reg ister
4 . 82
a s e r i a l comp a r a t o r ,
F l o p p y D i s k I n t e r f ac e co n t r o l s
i s a s sume d . When
M.t\'1\
A l l type comma n d s a r e p e r forme d
I
A l so , whe n e ve r a Type I I o r I I I
T h e DAL
a re
The DAL are three s t a t e
ac t a s inp u t rec e i ve r s when CS and
o r
low.
WRITE ( WE )
Command Reg i s te r
T rack Reg i s t e r
Sec tor Reg i s te r
Data Reg i s t e r
and up to the
be i n g sh i f t e d
,
n eme n t r and
i c r e
w i t h the d i s k
a re
f rom an ,
gene r a t e d
to the s ta te o f
1 , s ing l e
DDEN
=
u s e d to t ra n s f e r D a t a ,
hos t
b it s
The a dd r e s s
WE
du r i ng a