When I ordered the the PCB boards for the C64 test harness 586220, I got a lot of extra boards for free. So I decided to complete all the additional boards. Upon completion I checked them all on a good machine with the 586220 diagnostic cart. In fact multiple times. Once after I’d finished building them. And one more time before I shipped each one to it’s new owner.
It was a bit annoying that the diagnostic cartridge spends most of it’s time running various RAM checks which are not of relevant in my case. It would be great if the RAM checks could be skipped. Fortunately, World of Jani has a source code which was disassembled from a ROM dump of the cartridge. So I loaded it into C64Studio. The locations where the SCREEN RAM, RAM TEST 1 & 2 and COLOR RAM are performed were not hard to find, since the source code was already well documented. Basically I changed the code at four locations in order to skip the checks and print the string “SKIPPED” to the screen:
And this is how it looks like in action. The time from start to where the interrupt is checked was cut down from 38 seconds to 3 seconds:
Source Code and Binaries
The source code of the modified cartridge is available on github: nharrer/c64_diagnostic_586220plus_noramcheck.
The .crt or .bin files can also be downloaded there.