Galaga uses 4-bit RAMs.  This means that in order to handle 8-bit data, it needs to group RAMs in sets of 2 – one for the lower 4 bits, and the other for the higher 4 bits.  When Galaga detects a RAM error during the self-test, it does not show the location of the failed RAM, they shows which RAM failed.

For example, if the chip handing the higher bits in RAM “bank” 3 fails the self-test, it will show RAM 3H and likewise if the chip handling the lower bits fails, it will show RAM 3L.  These may look like locations on the board somewhere, and this tends to trip some people up as they go looking for RAM chips at 3H/3L on the board.  Instead, here are the locations the different RAM errors refer to (on Galaga, all RAM errors point to the Video board):

RAM 0L – RAM at location 1K has failed
RAM 0H – RAM at location 1K has failed
RAM 1L – RAM at location 1K has failed
RAM 1H – RAM at location 1K has failed
RAM 2L – RAM at location 3E has failed
RAM 2H – RAM at location 3F has failed
RAM 3L – RAM at location 3K has failed
RAM 3H – RAM at location 3L has failed
RAM 4L – RAM at location 3H has failed
RAM 4H – RAM at location 3J has failed

Bosconian also shows H/L RAM errors that need to be mapped to RAM locations:

RAM 1L – RAM at location 2N on CPU board has failed
RAM 1H – RAM at location 2N on CPU board has failed
RAM 2L – RAM at location 2N on CPU board has failed
RAM 2H – RAM at location 2N on CPU board has failed
RAM 3L – RAM at location 1E on Video board has failed
RAM 3H – RAM at location 1E on Video board has failed
RAM 4L – RAM at location 1E on Video board has failed
RAM 4H – RAM at location 1E on Video board has failed
RAM 5L – RAM at location 1H on Video board has failed
RAM 5H – RAM at location 1H on Video board has failed
RAM 6L – RAM at location 1H on Video board has failed
RAM 6H – RAM at location 1H on Video board has failed