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Programming the Commodore 64: the Definitive Guide
Mirk writes "Back in 1985 it was possible to understand the whole computer, from the hardware up through device drivers and the kernel through to the high-level language that came burned into the ROMs (even if it was only Microsoft BASIC). The Reinvigorated Programmer revisits R. C. West's classic and exhaustive book Programming the Commodore 64 and laments the decline of that sort of comprehensive Deep Knowing."
One of the most comprehensive protections at that time was called "V-Max!" which stood for Verify Maximum. What were called "nibblers" for disc copy software couldn't touch it even though those nibblers represented the ultimate in disk copy technology at the time. There were two ways to copy V-Max, the first was to get a dedicated hardware copying unit. The second was to apply a bit of knowledge with a debugger cartridge: the V-Max protection was a turn-key system you gave them files and they wrapped the protection around it and provided a fast-loader at the same time. So what you would do is fill all of memory (the whole 64K) with a value you knew say: $AF. Then you would load a V-Max file from the disc, it's loader would automatically take over and while it was loading you would enter your debugger cartridge and change it's exit point to point to itself. So instead of $0800: RTS you would make it $0800: JMP $0800. Then you would wait for the V-Max loader to fully load the file. Then a quick button press on your debugger cartridge and use the memory monitor to find where the file loaded by seeing what memory was NOT $AF. Then from the debugger cartridge save that memory block out again. Completely de-protected file. Since V-Max used standard kernel-load vectors the program itself needed no further modification, the protection was completely gone you just lost the fast-loader function. Which you then re-added yourself into a chunk of memory wherever the game didn't use it. Relocatable code was best for that. Later versions of V-Max also did on-the-fly decompression of files so occasionally while stripping the protection you would run into a situation where your destination disk ran out of space versus the original protected disk. Again, that was worked around by inserting your own custom loader into the kernel load-vectors which also did decompression. V-Max was impossible for copy software of the day to copy but with a little bit of knowledge and a debugger cartridge it was absolutely trivial to defeat.
Shh.
There doesn't appear to be any section on custom high-speed communication with the external floppy drive unit. IIRC, you could upload a small program to the drive, and then you could in particular read data from the drive a lot faster than the 'OS' normally supported. This technique was also used to do copy protection for a bunch of titles, primarily by stepping the drive head 1/2 between tracks then doing reads. Production disk duplication could write to both the track & between tracks [or could write a wide enough track to cover the whole area], but regular floppy drives couldn't write both [you could either write on the track, or between tracks].
Not that I was interested in this stuff or anything.
Sleep your way to a whiter smile...date a dentist!
I was playing a game with some DRM (either StarForce or SecuROM) and it wouldn't run if I had a debugger present. I asked them why and they were all like "Anyone who has a debugger and is playing the game is a hacker." That's RIGHTLY earned state of paranoia.
And why did I spend time removing protection systems? Funny that part is: I owned an MSD floppy drive which was completely incompatible at a machine-language level with the 1541 drives everyone else owned and that all the game-makers wrote their protection systems for. So my floppy drive would load any of the software of the day. I literally bought a game, had to hack away the protection, and then I could play it on my computer. Of course no one will believe me when I say this but damnit, its the truth! Now get off my lawn.
Shh.