CRT
CONTROL
The
refresh addresses generated by
the
CR
TC
are of
two
types, refresh address lines
(MAO
to MAIO), and raster
address lines
(RAO
to
RA4).
The
refresh address lines access
the
data
stored
in
the
Screen RAM (the font cell
pointer
plus
the
character
attributes
in
the
text
mode/the
graphics cell
pointer
in
graphics mode).
The
raster address lines are
combined
with
the
pointer
to
access
the
System
RAM
to
select
the
row of
the
font/
graphics cell, for display.
The
CRTC
is
programmed
to operate
in
a raster scan
mode defined as
interlace
sync
and
video. In
this
mode,
the
display raster
on
the
screen
is generated
using
two
fields, an
even field
and
an
odd field, consisting of even
and
odd
scan
lines respectively. (Similar
to
the
method
employed
within
a standard
domestic
television). Half
the
rows of each
font/graphics cell are displayed
in
each
field.
In
the
even
field, as
the
electron
beam
sweeps
out
the
even
scan
lines,
the
even
rows
of a font/graphics cell are
accessed for displaying
on
the
screen. In
the
odd field, as
the
electron
beam
sweeps
out
the
odd
scan
lines,
the
odd rows
of a font/graphics cell are accesssed for displaying
on
the
screen. Each frame of video
information
is
thus
defined by
two consecutive fields.
The
movement
of
the
electron
beam
across
the
screen is
controlled by
the
two
outputs
from
the
CRTC,
Hsync
and
Vsync.
Hsync
controls
the
horizontal
sweep of
the
beam
across
the
screen
during
each
field (line scan rate). Vsync
controls
the
vertical
retrace of
the
beam
back
to
the
top of
the
screen
at
the
end
of
each
field (field rate).
The
active display period
on
the
screen is indicated by
the
Display Enable
(DE)
output,
which
is supplied
to
the
video
interface,
memory
control
circuits,
and
the
Parallel
Interface.
On
the
video interface,
the
DE signal enables
the
video signal
output
during
the
active periods,
and
inhibits
the
output
during
line
and
field flyback periods.
The
DE
output
is
also
used
as
an
enablelinhibit
signal for
the
memory
control
circuits.
During
the
active display
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