CRT
CONTROL
The
16-bit words
programmed
into
the
Screen RAM are
accessed
by
the
refresh addresses from
the
CR
TC.
These
addresses are
incremented
every character clock period
during
the
active
scan
line
period
but
repeat
the
same
sequence of addresses over
sixteen
adjacent scan lines
(as
illustrated
in
Figures 2
and
3).
The
CRTC
automatically
cycles
through
the
same
sequence of addresses each screen
refresh period,
unless
the
CR
TC
start
address is updated.
In
the
text
mode,
the
CR
TC
is programmed to cycle
through
2000 different refresh addresses to
map
the
whole
text
screen of 25 lines of 80 characters.
These
repetitively
access
the
16-bit words programmed
into
the
first 2000
word address locations
in
the
Screen RAM.
In
the
graphics mode,
the
CRTC
is programmed to cycle
through
1250 different refresh addresses to
map
the
whole
graphics screen of
50
columns
by
25 rows.
These
repetitively access
the
16-bit words
programmed
into
the
first 1250
word
address locations
in
the
Screen RAM.
The
16-bit
data
programmed
into
the
Screen RAM is
dependent
on
the
selected mode.
Text
Mode
In
the
text
mode,
the
Screen RAM is programmed
with
16-bit words, each
word
consisting of a
II-bit
font
pointer
and a 5-bit
attribute
code.
The
font
pointer
is used to select
characters stored
in
the
System
RAM.
The
attribute
code is
supplied
to
the
video interface
to
determine
the
display
attributes
of
the
character
accessed frpm
the
font. Changing
a character
on
the
screen is achieved by
writing
a different
16-bit word
into
the
appropriate address location of
the
Screen RAM.
The
11
least significant
bits
of
the
16-bit
word
(DO
to
DI0)
in
the
Screen RAM are
the
font pointer.
The
11
bits
of
the
font
pointer
are
combined
with
the
raster
address lines from
the
CRTC
to
form a
word
address, to access a character row
in
the
System
RAM (The
II-bits
are shifted 5 binary places
in
significance to
bits
5
to
16
and
combined
with
the
raster
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