IBM Personal Computer AT Manual To Operations page 286

Have your system unit serviced if you get any of the
following errors. Be sure your Key Lock key is with
your system unit.
• A parity check error.
• A ROM error.
• A screen with a blinking cursor and you are unable
to proceed.
• The same screen indefinitely and you are unable to
proceed.
Go to Step 1 to begin "System Testing".
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System
Testing