This is the second version of my diagnostic for the SuperPET expansion RAM.
It writes the patterns $00, $55, $AA, $FF to every location in every bank. It then attempts to identify bad bits by comparing the bytes written to the bytes read back.
The "X"s indicating bad bits are sticky; once an "X" appears on the screen it is never overwritten. This allows you to run the test for an extended period to see any bits that ever errored in each bank.