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a
cOMbination of hardware and
<joftware.
The
eRTC
activates its cursor
output when the address of
the
character
being fetched
during a
DMA
c
y
c:
1
t·~
Mat
c:
h
t:~
s
t
h
0~
c: () n
t
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n
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~!;
0
f
:i. t
~:;
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n t
f.~ ('
n
a 1
c
\J l' SOl'
add ('
E~ 5 S
(' f.-~ (.:)
i
s t
(~~
(' .
1hi5 output is
active for ali scan lines.
The software Maintains and
updates this register in the
I:RTC
corresponding
to the position of the
c
u
l'
!:i () r () nth
(o?,
d:i.
~:;
pIa y .
I n
0
f'
d
(.? l'
t ()
1"' ,
a 1<
(-? t
h
(~~
C U l' 5 0
r' b:l. i
n 1<
t
h
(:? !:; ()
·f
twa r
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i:\
1.
t
(~~ l'
nat
f::O
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w ('
i
t
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a
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a
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5 0 l'
add
l' €.~ S !:)
c\ n d
t
h
(7~
nan i n
IJ
1.
a :i. d () n
~:.~
.
Th(::!
CU1'!:;or' !:;:i.qnal>
CUR>
OU'!:put
froM
th(-:~
CRTC
is
gatf.~d
w:i.th anothf::or'
s:i.qnal,
UL.Tli"IE)
to
pl'odlJC:(7~
d
CU1'S;Ol'
~:;iqnal)
NCUR>
whic:h is
activE~
on
the 1. :3
t
h !:;
c:
a n
1.:i. n
E~
.
l.l
L TIM E
:L~:>
cI
(~,
cod
f:~
d f
r'
0
M the !!i
C
i:\
n
l:i. n
(-?
c
0
u
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t b
Y U '3
<7
i:'\
n
d U 4
1.
i. .
T h i
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f;;
i q
n a 1.
i:\
1
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0
<:~
n
i:\
b
1.
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S
t
h
f.-~
unci
~~
('
1. :i n
(7~
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f\
h
c\ n c
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, ....
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n
t
clu
l'
:i.
n
g
t
h
t~
1.
~~
'l
h
!:i
c:
a n lin
~;~
.
In the norMal situation>
where the cursor does not lie in an underline
f
i
<-:-~
:I. d
>
t
h
E~
N C Lm
s
i q n
e1
1.
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0
b
~\
COM
e
N C LJ R SO R w h i c h
:i.~:;
f(~~d
to th(-'::o
s(~l~?ct
C
input
()f
th(~
char'act(~r
Mult:i.p1(-?xor.
This input
q()<-:-~s,
low to
c':\c:tivatE~
the
C:U1'S'H'
which for nor'Mal
chc':H'acter'<..:;
(not copy
b
:i.
t )
~:; (-;~
1
(-?
C
t
s
t
h
(-?
D ()
(H'
D
:1.
:i.
n put s
w
h
i c haT' e t:i e d
1 ()
W .
T
h
:i.!!i c a u
~:i e~:i
t
h co?
dot
s t "
€-~
c\
M
t
0
b
€-~
act:i
V ~\
f
0 l'
the
13
~:)
can
l:i. n
t:1
0
·F
the c:
h
i:\ l'
iH:
t
€-H'
P
0
sit
:i. () n
:l
n
w hie h
t
h
(.;~
C
U l'
!;
0 T'
1
i
(-:~
s .
I
n
f~
f of'
(-?
C
t t his
0
I~
,
s
t
h
E~
cur !:,
0
r wit h
the
character in the cell
(a non'Mdestructive cursor).
If
the
copy
bit
c :i. r"
c u
:i.
t
i !;
act
i v
(~~
how
(:?
v
(-:H' ~
the
D
;:~
0
f
D
~5
i
n
put
~;;
0
f
t
h (o?
C
h
a r
a c:
t
EH'
M U
1
tip
1
~~
x
0
r
i:H' t:~
~j.
t:11
E~
c t
£01
d .
T 1\
E~ ~:; E~
i n put s,
p
l' 0
v ide t
h
~~
i
n v
(-?
r t
(~~
d
S EH'
i
(-:~
s
0
f
dots frOM the
copy bit shift register.
In essence
this inverts
the
1.3th scan linB of
the character when
the cursor
is active.
This is
n(-:~C(~S!:ic'H'y
r'<~th(~l'
than
th(-:~
DR'c!
cur!:ior used
ab()vf.~
dUf-::O
to
th(-?
fi~ct
thi~t
SOMe of the characters May
ha~e
all dots of
the 13th scan line lit and
th(::~
c:ur~)or'
would
n(o?v(~r'
bf.~
!:;(7~(-?n.
l)(H
STI~ETCH
Thf.~
dots
al'(~
inver't(-?d by
U~~L3~
the CI'\iH'iH:1;(-:H' Multiplexo1') to pr'ovide
<~n
active high dot streaM output.
This dot streaM is then passed through
(~j.
and
its
a!a<..:;oc:i.ated
cir'l::uitry
which
perfor'M~:;
i~
"dot
str'etch"
function.
This;
dot stl'etch
i~;
u<.:,<-:~cI
to
pr'ovidt:~
an elongated
ac:t:i.vf.~
d()t
which
has a
More pleasant appearance when displayed.
It essentially
" f
d
t
t
f.-H\
!!,
up" t
h
E~
d ()
t~,
c: ()
,v,
p () s;
:i.
1\
(;J
i:\
c
h
a
r'
act
t-:H' .
The
<.:;
wit
chi
n q
tiM
e
0
f
the
transistor frOM
saturation to cutoff is
dependent
upon the paraSitic
collector
to base
capacitance
and the external
capacitor C2.
This
capacitance
liMits the
switching speed>
essentially
stretching
the
aMount of tiMe the transistor is active
(in saturation).
Capacitor C3
is included to
cOM~ensate
for paraSitic
base to eMitter
capacitance.
Note that an inversion is
int~oduced
by this dot stretch circuit.
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