Specifications
The
VerticaL
Sync
frequency is determined
by
the dispLay
mode,
the size of the
Video
Display
Memory
area,
and
the master clock frequency.
The
exact Vertical
Sync
period
is
given
by
tv
=
(96(n+2)
+
88)
*
te,
where
n is the
number
of
scan
lines being displayed,
and
te
is
the period of the e-clock.
On
the
QUARK,
this
is
670ns.
In
ALphanumerics
mode,
the
number
of dispLayed lines
is
the
Video
Display
Memory
size (in bytes) divided
by
16,
whiLe
in Graphics
mode,
it
is
the
Video
DispLay
Memory
si
ze
di
vi
ded
by
96.
For
a
60Hz
Vertical
Sync
frequency, the
Video
RAM
area should
be
4k
in
ALphanumeric
Mode,
and
24k
in Graphics
Mode
(assuming
that the master clock frequency
is
23.86176Mhz,
as
is
the
case for the
"60Hz"
versions
of
the
QUARK).
For
a
50Hz
Vertical
Sync
frequency, the
memory
sizes should
be
5k
and
30k,
respectively (assuming the
24.8MHz
master clock frequency
used
on
"50Hz"
versions of the
QUARK).
If
it
is
not
necessary for the Vertical
Sync
frequency to
be
exactly
60Hz
then
one
is
free
to
pick
the
starting
address of one's choice.
The
starting
address of the
Video
Display
Memory
may
be
pLaced
on
any
1k
address boundary, although
in
Graphics
mode
only
3k
boundaries
will
result
in
a prpoer HorizontaL
Sync
signal.
For
example, a
60Hz
board
could
be
operated in the
Graphics
mode
with a
Video
Display
Memory
size of
27k
instead
of
24k.
This larger
size
wouLd
result
in
a
vertical
sync
frequency of
53.4Hz,
with a
total
of
288
Lines
per frame, instead of
256.
The
horizontal
sync
frequency
wouLd
remain
unchanged
at
15540Hz.
See
Table
VI
in the
Appendix
for
SAM
Control Register values
needed
to
select
various
Video
Display
Memory
sizes
and
locations.
Alphanumeric
Mode
-----------------.-----
------._-_.-------------------
In
Alphanumeric
Mode,
data stored
in
the
Video
Display
Memory
are interpreted as
eight-bit
character codes.
These
codes
are fetched
from
the
96
scanned
locations in
each
half-page of
the
Video
Display
Memory
and
presented
to
the input of the Character Generator.
The
Character
Generator contains the patterns
which
represent the characters to
be
displayed
on
the
CRT.
The
standard character set for the
QUARK
uses characters
formed
from
a 5-by-7 matrix of
dots. This matrix
is
imbedded
in
a larger 8-by-8
background
field.
The
background
field
is
effectively part of the character;
in
the standard
QUARK
character
set
the top
row
of dots
and
the
first
two and
the
last
columns of dots
for
each
character
are
bLank
to
provide
the
necessary
space
between
adjacent characters.
The
standard character set
incLudes
128
"normal
video"
characters
and
128
"reverse video" characters,
and
is
desi
gned
to
use
seven-bit
ASCII
codes.
The
reverse video characters are essent ia L
Ly
a
second
set of
128
characters
where
the
dots forming
both
the character
and
the
background
are inverted
from
the corresponding
normaL
video
characters.
In
the
standard
character
set,
a
reverse
video
character
is
dispLayed
whenever
bit
7 (the
most
significant bit) of
any
8-bit
vaLue
stored in the
Video
Display
Memory
is
set.
The
remaining seven
bits
form
the
ASCII
code
for
the
character
that
is
to
be
displayed.
There
are
no
extra
dot
columns
or
dot
rows
between
the 8-by-8
background
fields
of
each
character, that
is,
that the 8-by-8 fieLds
completeLy
fi
Ll
the displayable area of the screen.
Thus
it
is
also
possible
to
create
graphic
characters
(as
distinguished
from Bit-mapped
Graphics,
below)
which
aLlow
continuous Lines, bars,
and
other figures
to
be
displayed
on
the
screen
in
Alphanumeric
Mode.
Megatel
Computer
Technologies Toronto,
Canada
Page
H/W-11