Epson 10020 - Manual (Page 165)

The checks made on the 8088 CPU chip are extremely
comprehensive. They ensure that the CPU instruction set is
functioning correctly.
If an error is reported, write down the error code and message,
or print them out, and contact your Customer Care Center.
Attempting to correct system board errors yourself may violate
your warranty agreement.
Memory Check
Use this option to check the computer’s built-in memory. The
program checks the DIP switches to determine the amount of
main memory. If any settings are incorrect, check the DIP
switches to make sure they match the installed memory.
Note
This option does not check expanded memory above the
640KB memory limit imposed by MS-DOS.
For this check, the program writes specific data into memory
and then reads it back. The data is written and read in blocks of
64KB. A parity check is also made on each block. A count of
memory is displayed after each block that is tested without an
error. The final message is usually:
640 KB OK
If an error is reported, write down the error code and message,
or print them out, and contact your Customer Care Center.
Attempting to correct memory errors yourself may violate your
warranty agreement.
D-8 Performing System Diagnostics