Radio Shack TRS-80 User Manual page 110

In the vertical
direction, there are
48
light-block addresses
numbered from
to 47.
is
at
the top
and 47
is
at
the bottom.
The
statement
"SET
(X,Y)" whitens
the
block which
is
the
Xth
block
from
the
left in
the
horizontal direction
and the
Yth one
down
from
the top in the
vertical
direction.
And,
you've figured
out
that
RESET
works
the
same
way
except that
it
"turns the
light off".
Let's try
it
out. This
program
will
lighten
any one
block of your
choosing.
Type:
10
INPUT
"HORIZONTAL ADDRESS
(0
TO 127)
IS";X
20
INPUT
"VERTICAL ADDRESS
(0
TO 47) IS"
(Y
30
CLS
40 SET
(X, Y)
and
RUN
many
times using various values of
X
and Y.
What
happens
if
X = 150? Try
it.
How
about
if
Y
= 60?
Try
it,
too.
The
block just
moves
off
the end
(or
bottom)
of the screen
and
starts
over with the address count.
We
call
this
"wrap
around".
You may
have noticed that
if
a
block
is lit
in the
upper left-hand corner, the
READY
and
the
prompt
(>)
destroy
it.
Try
X
= 6 and Y
=
6.
Then
X
=
5
and
Y =
5.
We
can avoid this
problem by not returning control to the
prompt
—
by
adding
99
GOTO
99
at
the
end of the
program. After running the program,
this line
locks the
Computer
in
an
endless loop.
To
break the loop,
press
|
BREAK]
key.
You
should put an endless
loop
at
the
end
of every graphics program.
Do
it
here,
then try
X
=
5
and
Y
=
5.
Now
try
X
=
and
Y =
0.
Remember
the
|BREAK|
key
to stop a properly
"locked out"
graphics
program,
before starting
another.
While
we
have a key
that
RESETs
every block on
the screen to
"OFF"
in
one operation
(the JCLEAR
J
key),
we
don't have a
similar
key
to turn
them
all
"ON".
However,
we
can
easily
write a
program
that "lights",
"whitens" or "paints" the entire
screen.
It
uses
one
Clear (not really a
must, but always
a
good
habit to use
one
for graphics
programs),
two
FOR-NEXT
loops
and one
endless "locking loop".
Type
this:
10
CLS
108