Epson 386SX - Manual (Page 211)

640x200 Graphics Mode Check
To check your 640x200 graphics mode, select 6. The screen
displays three patterned squares against a contrasting
background, as shown below.
640X200 GRAPHICS MODE CHECK
Is the display correct ? (Y/N)
If the patterns on your screen are clear and distinct, select
Y.
If
any pattern is not clear, first check the adjustment of your
monitor, and then make sure that both ends of the cable are
plugged in firmly. If a problem still exists, select
N
to display the
error message.
Performing System Diagnostics C-19