Spectravideo MKI, Сервисное – Техническое руководство, Страница: 115 / 134

The CRC 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
and up to the
CRC cha r ac t e r s . The CRC re g i s t e r is p re s e t to one s p r i o r to d a t a
be ing sh i f ted
t h r o u gh
t he c i rcu i t .
ARITHMETI C/LOGIC UNIT ( ALU ) - The ALU is
a se r i a l comp a r a t o r , i
n
c r
eme nt
e
r
,
and
dec reme n t e r and is u s e d for reg i s t e r mo d i f i c a t i o n and compa r i s ons
wi t h the d i sk
recorded I D f i e l d .
TIMING AN D CONTROL - A l l compu t e r and
F l oppy D i s k In t e r f ace con t ro l s
are
gene r a t e d
th r
o u
gh th i s
l o g i c .
The inte rna l dev i ce t i m ing i s
gene r a t e d
f rom an ,
e x t e rna l
c ry s t a l c l oc k .
The FD 1 79 1 / 3 ha s
two
d i f f e rent modes
D DEN . Wh en
DDE
N
=
0 ,
doub l e
dens i t y
den s i t y
( FM )
i s a s sume d .
of ope r a t ion acc o r d i ng
to the s ta te o f
( NFD )
i s a s sume d . When
DDEN
=
1 , s ing l e
M.t\'1\
AM
DETECTOR - The add re s s ma rk detector de tec t s I D , data and index a dd re s s
marks
du r ing r ead and wr i t e opera t ions .
Whe neve r
a
Rea d o r W r i t e comma �d ( Type I I or II I ) i s re c e i ve d the FD 1 79 X
s amp l e s t he R ea dy inpu t . I f t h i s input i s log ic l ow t he command i s not
exec u t e d and an i n t e r rup t is gene ra t e d . A l l type
I
commands are per forme d
reg a r d l e s s o f t he s t a te o f t he keady inpu t . A l so , whe neve r a Type I I or III
command i � rece ive d , the TG43 s i gna l ou tput i s up d a t e d .
PRO CE S SOR INTERFACE
T he
i n t e r f ace to t he p roce s s o r i s accomp l i she d th rough the e i ght Data Acce s s
L i n e s
( DAL) and a s so c i a t e d cont r o l s igna l s . T h e DAL
a re
used to t rans f e r Data ,
S t a t u s , and Cont ro l wor d s ou t o f , or into the FD 1 793 . The DAL are three s t a t e
buf f e r s that are enab l e d as ou t p u t d r i v e r s whe n Ch ip Se l ect ( CS ) a n d Rea d
E na b l e ( RE )
are ac t i ve ( low log i c s t a t e )
o r
ac t a s inp u t rec e i ve r s when CS and
Wr i te
Enab le ( WE ) a re ac t i ve .
Wh en t ra ns f er o f da ta w i th 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 hos t
p roce s so r , t he d e v i ce a dd re s s i s decoded and CS i s ma de
low.
The a dd r e s s b it s
A l a n d AO , comb ined w i t h the s igna l s R E du r i ng a Rea d ope ra t i on or
WE
du ring a
W r i t e o pe r a t i on a re i n t e rp re te d as se l e c t ing t he f o l l ow ing reg i s t ers :
A1 -AO
0 0
0 1
1 0
1 1
READ ( RE )
S t a t us Reg i s ter
T rack Reg i s t e r
Sector Reg i s t er
Data Reg ister
4 . 82
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