Lines
63,64:
On
these
lines,
the
RAM
is
permanently
selected
and
writing
to
the
RAM
is
disabled
.
This
makes
the
RAM
look
like
a
ROM
which
stores
video
data
.
Line
65:
The
outputs
from
the
RAM
are
disabled
when
the
video
is
blanked
since
there
is
no
need
for
pixels
during
the
blanking
intervals
.
This
isn’t
really
necessary
since
no
other
circuit
is
trying
to
access
the
RAM
.
Line
71:
The
address
in
RAM
where
the
next
four
pixels
are
stored
is
calculated
by
concatenating
the
lower
nine
bits
of
the
line
counter
with
bits
7,6,5,4,3
and
2
of
the
pixel
counter
.
With
this
arrangement,
the
line
counter
stores
the
address
of
one
of
2
9
=
512
pages
.
Each
page
contains
2
6
=
64
bytes
.
Each
byte
contains
four
pixels,
so
each
page
stores
one
line
of
256
pixels
.
The
pixel
counter
increments
through
the
bytes
of
a
page
to
get
the
pixels
for
the
current
line
.
(Note
that
we
don’t
need
to
use
bits
1
and
0
of
the
pixel
counter
when
computing
the
RAM
address
since
each
byte
contains
four
pixels
.
)
After
the
line
is
displayed,
the
line
counter
is
incremented
to
point
to
the
next
page
.
Lines
73,74:
The
register
that
holds
the
byte
of
pixel
data
from
RAM
is
cleared
when
the
VGA
circuit
is
reset
.
The
register
is
updated
on
the
rising
edge
of
each
pixel
clock
.
Lines
80
-
82:
The
pixel
register
is
loaded
with
data
from
the
RAM
whenever
the
lowest
two
bits
of
the
pixel
counter
are
both
zero
.
The
active
pixel
is
always
in
the
lower
two
bits
of
the
register
.
Each
pixel
in
the
RAM
data
byte
is
shifted
into
the
active
position
by
shifting
the
register
two
bits
on
each
rising
clock
edge
.
Line
86
:
The
register
that
holds
the
red,
green,
and
blue
color
gun
control
bits
is
set
to
zero
when
the
reset
input
is
high
.
Line
87:
The
color
register
is
clocked
on
the
rising
edge
of
the
pixel
clock
since
this
is
the
rate
at
which
new
pixel
values
arrive
.
Lines
88
-
96:
This
truth
-
table
defines
the
color
gun
control
bits
which
are
stored
in
the
color
register
as
a
function
of
the
pixel
value
and
the
blanking
input
.
When
the
pipelined
blanking
input
is
low
(lines
87
-
90),
the
color
displayed
on
the
monitor
is
red,
green,
blue,
or
white
depending
upon
whether
the
pixel
value
is
00,
01,
10,
or
11,
respectively
.
When
the
pipelined
blanking
input
is
high
(lines
91
-
94),
the
color
register
is
loaded
with
zero
which
will
display
the
color
black
on
the
monitor
.