CRT
CONTROL
attributes.
These
allow screen characters to be
in
either
normal
or
reverse video
with
the
addition of
any
combination
of
the
following display features, as required:
(a)
Highlight (High
intensity
characters-).
(b)
Strikethrough
(Horizontal
line
through
the
centre
of
the
character).
(c)
Underline. (Horizontal
line
under
the
character).
In
the
graphics mode,
the
screen is
memory-mapped
directly
into
the
System
RAM (i.e. each individual pixel
on
the
screen corresponds
to
a
bit
written
into
the
System
RAM).
The
resolution
of
the
screen available
in
this
mode
is
800 pixels
wide
by
400
pixels high, requiring
just
under
40
Kbytes (800 x 400 bits) of
System
memory
to
map
every
pixel.
All
information
displayed
on
the
screen is generated using"
a raster scan
imaging
system,
with
the
CRT
control
circuitry providing
the
video drive signal,
and
the
synchronising
pulses
to
control
the
movement
of
the
CRT
electron beam.
Connections
to
the
Display
Unit
are supplied via a 9-pin
male
D-type
connector
located
at
the
rear of
the
System
Unit.
DESCRIPTION
General
A simplified
view
of
the
CRT
control
circuitry
is
illustrated
in
the
block
diagram Figure
1,
which
is used
in
the
following paragraphs
to
provide
an
introduction
to
the
principles involved
in
generating
the
video signal for
the
two
different display modes. Subsequent
sections
of-the
chapter
explain
the
operation
of
the
circuitry
in
more
detail.
The
block diagram
in
effect, presents
the
view
of
the
circuitry as
seen
from
the
CRT
Controller
(CRTC),
and
is
simplified
by
the
omission
of:
(a)
The
system
data/address/control
bus
connections
to
the
Screen
and
System
RAM.
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