Commodore 128D - Page 181

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Lines 20 and 30 do most of the work in this program. Line 20 makes
the computer wait until a key is pressed before executing any more
of the program. Note that when the command immediately after
THEN is a GOTO, only the line number is necessary. Also note that a
GOTO command can GOTO the same line it is on. Line 30 tells the
computer to go back and wait for another key to be pressed unless
the F1 key has been pressed.
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USING C64 MODE—Using the Keyboard in C64 Mode