Radio Shack TRS-80 Model 100 Basic Manual page 127

As before, (]J should move the asterisk left and (]J should move the asterisk right.
Hold down
m
until the asterisk reaches the top line of the display. If you attempt to
move the asterisk higher, a "warning beeper" sounds and the asterisk stays on the top
line.
Similarly if you
try
to move beyond any display boundary, the beep will sound and
the asterisk will stop moving. Try this by moving the asterisk to the four comers with
the appropriate keys.
Line 238 If
m
is pressed, the PRINT@ position is decreased by 40 to move up one
line.
Line
240
If
00
is pressed, the PRINT@ position is increased by 40 to move down
one line.
Line 258 If the new PRINT@ position (AT) will be valid, that is, between 0 and 318,
the condition
AT>=0
AND
AT<=318
will be true, and execution jumps to Line 300. The condition uses the logical operator
"AND" to combine the two logical expressions
AT>=0
AND
AT<=318
into a third logical expression. A logical expression is either true or false. For
example, the expression:
AT>=0
is True if AT is greater than or equal to zero. It will be False if AT is less than zero.
Similarly, the combined expression
AT>=0
AND
AT<=318
will be True if AT is greater than or equal to zero and also less than or equal to 318.
In general, if Ll and L2 are two logical expressions, then the logical expression:
L1 AND L2
is True if both Ll and L2 are True, and False otherwise. Logical expressions may also
be combined with the OR logical operator. The expression
L1 OR L2
is True if either Ll or L2 is True, and False only if they are both False.
You may have wondered why the upper limit on the PRINT@ position was 318
instead of 319, which is the extreme lower right comer of the display. This was done
to prevent scrolling which would occur if the asterisk was printed in the comer.
Even the use uf a semiculun after the PRINT@ will nut prevent the scrolling, which
occurs automatically when a character is printed in position 319. You can verify this
for yourself by changing Line 250 to:
250 IF AT>=0 AND AT<=318 GOTO 300
and moving the asterisk into the lower right comer.
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