Spectravideo SVI 318 Service – Technical Manual page 103

REG I STER F ILE DESCRIPTION ( S ee Tab le 2 )
N ineteen reg i s t e r s in the CRTC can be acce s s e d by means of the data bus .
Reg i s t e r a ddress ing and l engths a re shown in Tab le 2 .
Add re ss Reg ister
The Add re s s Reg i s ter is a 5-b i t write-on ly reg i s t e r use d as an " ind irec t " or
" po i n t er" reg i s te r .
reg i s ters in the f i l e .
a dd res se d . When RS is h i gh , the Reg i s ter F i l e is acce sse d .
H or i z ontal T im ing Reg i s te rs RO , Rl , R2 and R3
F igu re 4-7 shows the vi s ib l e d i s p l a y area of a typ i c al CRT moni tor g iving the
point of re f e rence for horizont a l reg i s te rs as the l e f t most d i s p layed
cha rac t e r pos i t ion.
un i ts w i th r espect to t he re f e rence .
Hor i zon tal Tot a l Reg i s t e r ( RO)
H o r i z ont a l D i sp layed Reg i s t e r
( R l )
H o r i z ont al Sync Pos i t i on
Reg i s t e r ( R2 )
H o r i zont a l Sync W id th Reg i s t er
( R3 )
I t s contents a re the a dd ress o f one of the other
Whe n RS and CS are low, the Add res s Reg i s t e r i tse l f is
Hori zontal reg i s t e r s are p rog ramme d in "charac ter t ime"
Th i s
t he hor i z ontal f re quency of HS .
tot a l of di s p l a yed plus non-d isp layed
cha racter t ime un its m i nus one .
Th i s
t he number of d i s p layed cha rac t ers per
hor i zont a l l ine .
Th is
t he hori zontal sync pos i t ion on the
hor i zonta l l ine .
T h i s 4 b i t wr i te-only reg i s ter determines
the width of the HS pu lse .
apparent why th is wid th needs to
g ramme d .
widths must
the charac t e r c l ock per iod wh ich var ies .
I f HS wid th we re fixed as an interga l
numbe r of cha r�c ter times , i t would vary
w i th charac ter rate and be out of tolerance
for ce r t a in mon i tors .
grammab le feature a l lows compensat ing HS
width .
4. 70
b i t wri te-on l y reg i s t e r det erm ine s
8
b i t wri te-only reg ist er dete rm ine s
8
8
bit write-only reg i s t e r determine s
Howeve r , cons ider that a l l t iming
be
p rog rammed as· mul t iples of
The rate pro­
1 8
It i s the
be
It may not
be
pro­