Radio Shack TRS-80 User Manual page 142

Line
150
is
critical,
since
it
jumps
over a
RESET.
Without
this
jump,
the light blocks
would be erased
when
they
are
interrogated.
(Try deleting the
line
and
RUNning
to
see.)
Line 160
is
just a
foxy
display trick.
It
causes
a
blinking
light
to
"appear"
to
be scanning
all
points.
Actually,
line
160
is
just
turning blocks
ON
and
OFF
as the
Computer
interrogates
them.
Since
we
don't
want
to turn off
blocks that
are
already supposed
to
be
lit,
we
hop
over this line
when
a
real live
block
is
hit.
In
reality
then,
this
"roving
eye" never
actually "hits" an
"ON"
block.
Line 170
merely
closes the
FOR-NEXT
loops
started in line
110
Line
180
replaces the
heading that was erased
by
the
POINT
scanning
process.
Line
190 simply moves the cursor (which can be a
pesky
light in
graphics displays)
back
to the
far
upper-left corner,
to
get
it
out of the way.
It
could have
been
moved
to
any
point, or just
left
alone.
Line 200
is
the locking loop used
to
keep
READY
and
the
prompt from goofing
up the
display.
Pretty simple
when
taken
a line at
a time,
isn't it?
Oh
yes,
did
you
notice that
the
"moving
dot" turned off the original
heading? Did you
also
notice that
it
took two passes of the dot to equal the
width
of
one
printed character
(which
of course
fits
right in
with
what the layout sheet shows)?
The reason the heading
was
erased
is
that
we
deliberately
chose not
to protect
it
(like
we
protected the blocks)
from
the
RESET
in line
160
in
order to
make
the point.
You
could
write a
little
protective
line
if
you
wanted
to (or
reduce the
vertical
length of the scan to
avoid
it)
.
Alpha
or
Omega?
There
you
have
it
—
a
good
running
start
into graphics.
Go now
to part
C
where
you
will
find
more ready-to-run programs.
By
giving
them
careful
study,
you
will see
the four
graphics statements
in
use, plus
most
of
the rest
you
have learned
about
BASIC
program-
ming.
The
possibilities
from
here are
unlimited.
Learned
in
Chapter
22
Gr
aphfes Functions
':.,€*'
POINT
(X,Y)
PRINT AT
140