Weird disk writing problem

[ericj]

Copy Master only works with a game doctor. If you don't have one, you can't use it.

[Tomy]

hello,

Yes, your disks are original disk. Then I have no idea why you face that problem 🤔

Copymaster can copy to disk or emulate by FDSSTICK. But it need special Hardware to run it.

[P]

Does the Game Doctor ever connect to the Expansion port of the RAM Adapter in some way? The reason I ask is because I have been analyzing the Copy Master disk and it does write to $4026 several times. $4026 is the output register of the Expansion port, and there's no reason to write here if nothing is connected to it.

[Tomy]

Hello,

Good find. But I'm sure it is no use for end user. I beleive it is test code for BUNG internal use. Or maybe they have device for it ??

Can you provide more info for it ?

[P]

I don't really have much information about it. The Nesdev wiki have everything I know about the FDS. I did a quick disassembly of the Copy Master FDS disk image and saw it writing the content of RAM register $00F9 to $4026 in the initialization code and in a few other places, and thought it was weird. I checked the wiki and $00F9 is used by the BIOS to hold the last value written to $4026, so maybe it's just a compatibility thing done in case an Expansion port device is used or something like that. In one place it writes a value pulled from stack instead of writing the content of  $00F9 though.

The reason I'm looking at Copy Master is to figure out how it measures the drive speed. If I could just learn how it's done, I believe I should quite easily be able to make a homebrewn drive speed measuring program so that people don't need to own a Game Doctor and Copy Master for disk drive maintenance in the future. It would be an FDS file that can be loaded via an FDSStick, or be written to an empty disk.

Although I can make my own Famicom programs I'm not really an engineer and unfortunately I suck at disassembly and reverse-engineering. I found the init code I think, but it jumps to a place, cat-knows-where, and I don't know how to proceed with it. It doesn't help that the disk doesn't run in most emulators. I guess I should ask the people at Nesdev for some tips.

Copy Master is quite different from a normal FDS disk though. There are only two files, and no KYODAKU-file that the BIOS normally requires (though there are ways to trick it) for example. I guess this is normal for Game Doctor disks.

[P]

Loopy reminded me that the FDSStick can be used for head alignment. He seems to think motor speed is less important.