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A while ago I made up a Diagnostic Cartridge for a Commodore 64, by replacing the ROM in a spare Cartridge. I've have a few Commodore Plus/4 computers in for repair, so I looked around for a similar ROM image for the 264 series of computers. I found and excellent page from someone who wrote their own, Diag264. Next I found a donor cartridge, in this case a duplicate 'Atomic Mission' with a peeling label, and carefully removed the original ROM.