150 User's
Handbook
to
the
Atari 400/ 800
Computers
EXAMPLE
10 OPEN #1,
13,0,
"E:"
20
T = 3.14
30
PRINTT
40
POSITION 0,0
50
INPUT #1, X
60
PRINT X
70
END
The previous example contains a program that
uses
the display
screen
as
an
input
d
ev
i
ce
. Line 10 opens I/ O channel
number
one
for
the
editor
(device "E:"). The second
argument
of
the
OPEN statement
(13)
indicates that the display
is
used
for
input
and
output.
The second
line
of
the
program
assigns
the
value
3.14 to the variable T. Line
30
causes the value
of
T
to
be displayed
on
the screen. Since the OPEN statement clears the screen and
resets the cursor, the
value 3.14
is
displayed at the
upper
left
hand
corner
of
the
screen.
The
POSITION statement at
line
40 returns the cursor to the
upper
left
hand
corner
of
the
screen. The INPUT statement at
line
50
chooses the device
on
I/ O channel 1.
As
a result, the
screen
is
used
to
input
a value
for
the variable
X.
When
an
I N PUT statement
is
u
sed
with
the screen,
the
value that
follows
the cursor
is
u
se
d
for
input.
Since
the
value 3.14 appears
at the
top
of
the
screen, and the cursor is also at the
top
of
the
screen, the
value 3.14
is
assigned
to
X. Line
60
displays
the
value
of
the variable
X.
The
output
of
this
program
is
the
va
lue 3.14 displayed
twice
. The
number
is
repeated because
it
is
printed
at lines 30 and 60.
Screen
The OPEN statement
for
the screen (device "S:")
is
used
to
choose a graphics
mode
. The
third
argument
of
the
OPEN
statement indicates the graphics
mode
(Othrough
8)
. Thesecond
argument
indicates
if
the screen is u
sed
for
input
or
output,
or
both.
Also, the second
argument
determines
if
the display
has
a
text
window
and
if
the display
is
cl
ea
red
when
the
OPEN
statement
is
executed.
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