Atari 400 User Manual page 51

The letters will wrap around to a third line, if you
hold down the X key long enough.
SCREEN 3*
But you are in for a surprise. As you approach
the right edge (at the end of the third line) you
will hear a beep.
Hold
down the X key until the
letters wrap around to the fourth line and notice
when you hear the beep.
SCREEN 4*
The beep really means nothing at this point; it
only
matters when you are programming.
In
that case, the beep signals that you've reached
what the computer considers the end of a line.
For now, if you go past the right edge, the
cursor will wrap around, but the new line will
be the first of another set of three lines. The
machine will beep again near the end of
every third line .
*If you are in Memo Pad Mode on a 400 or 800,
your screen matches the photos in this exercise,
with ATARI COMPUTER - MEMO PAD at the
top. If you have a BASIC cartridge in your 400,
800, or 1200XL, your screen looks the same as
the photos, except for the READY message at
the top of your screen.