Add ability to convert FDS games to MiSTer

[euan-forrester]

I implicitly assumed they were the same as NES games, but it looks like maybe not: https://www.reddit.com/r/emulation/comments/sbpyhd/converting_srm_to_sav_a_free_and_easy_diy_method/

I wonder what the initial header of 0x00 is. It's unclear from that link how big the header is. We'll need to get a sample file from someone.

More information here: https://www.famicomworld.com/forum/index.php?topic=15522.0

It looks like the MiSTer save is just the whole disc image, and there appear to be a 16 byte header containing 0x00

It seems that emulators' save files may be diffs of the saved disc image vs the original disc image. This may make it difficult to add support for this since there doesn't seem to be a standardized format for emulators.

[euan-forrester]

So far we have this example from the MiSTer: Link.no.Bouken.-.The.Legend.of.Zelda.2.Japan.Rev.1.zip which does not load in an emulator.

I made this save with an emulator: Link no Bouken - The Legend of Zelda 2 (Japan) (Rev 1)-euan.sav.zip which is much shorter

And we've been given these files as well:

• Akumajou.Dracula.Japan.Rev.2.Disk.Writer.zip
• Esper.Dream.Japan.Rev.1.Disk.Writer.zip
• Ginga.Denshou.-.Galaxy.Odyssey.Japan.Rev.1.zip
• Zelda.no.Densetsu.-.The.Hyrule.Fantasy.Japan.Rev.1.zip

[euan-forrester]

Different patch formats:

.ips: Begins with the magic PATCH: https://zerosoft.zophar.net/ips.php
.bps: Begins with the magic BPS1
.ups: Begins with the magic UPS1: http://fileformats.archiveteam.org/wiki/UPS_(binary_patch_format) (this is the format that the emulator on my machine, Nestopia, happens to use)

[euan-forrester]

So the process for creating an emulator save from a MiSTer save would be:

1. Create a test save with the emulator
2. Determine the format of the save. Is it one of the patch file formats? Maybe the whole disc image?
3. Check if we need to remove the header from the MiSTer save by comparing it against the .fds file we're using. Also remove the footer
4. Once the MiSTer save has the header and/or footer removed, use a patching program to generate a patch between the MiSTer save and the original .fds image

To do it in reverse we'd need a sample MiSTer save to determine what header and/or footer to add.

Overall I think this is much too complex to add for a feature that no one has requested yet.