Question about IPF images of self-modifying games

Hi all,

I have the CAPS .ipf image of a game called "Moebius - The Orb of Celestial Harmony". This is a game that saves the state of your character to the original game disk. How is this accomplished? I noticed that in my \WinUAE\SaveImages directory, there is a file called Moebius_save.adf, that is created whenever the "Write Protected" box is checked with the .ipf image mounted, and deleted whenever the .ipf image is ejected. Also, the original .ipf file never seems to be modified, regardless of the state of the "Write Protected" check box. What exactly is happening with these images? The Moebius_save.adf file doesn't seem to be a normal .adf file either. It is much larger than normal disk images (about 2 megabytes), and also is not bootable, whereas the original .ipf file is. Can anyone shed some light on this mystery for me?

Thanks a lot!

[derSammler]

IPF files are always write-protected, there's not way for the emulator to modify them. If a game tries to modify a disk in such a case, WinUAE will create an Extended ADF (that's a MFM disk image) and all modifications are stored there instead.

Hi, thanks for the reply.

So, when I play Moebius from a previously saved game, how does the emulator know when to read from the .ipf file and when to read from the MFM .adf file? It must use a combination of the two, because the game seems to always successfully get past the disk-based copy protection, but it also reads the save game properly.

Thanks again!

[derSammler]

how does the emulator know when to read from the .ipf file and when to read from the MFM .adf file?

I think the emulator knows which parts of the save image are used. If the game tries to access these parts of the IPF file, the emulator loads that data from the save image instead.

[IFW]

Wk is correct.
If you want more details just read the api documentation and the part about how to deal with saved/modified disk data.

Hi everyone,

Thanks a lot for explaining this to me. It makes perfect sense and is extremely elegant. However, the use of the difference file under WinUAE seems to be keyed by filename only, so that if I am running FunGame.ipf and write-enable it, a difference file called FunGame_save.adf will be written. If I then go to a different directory containing a different game that happens to be named FunGame.ipf, it seems as though it will attempt to open and use the FunGame_save.adf difference file written for the first game. Is this correct? If so, while the chances of this happening are quite small, there is potential for such a "collision", no? Unless there is some sort of universally unique identifier associated with every .ipf image. In that case, I assume that after the first FunGame_save.adf was written, once the second FunGame.ipf was inserted, the application would create something like FunGame_save01.adf. Anyway, thanks for taking the time to help me understand this better.

[IFW]

Yes, ipf files have an ID, disk id and revision number embedded.
Whether this is checked by uae or not for save images I can't tell.

Very interesting. Thanks for all the information!

[Toni Wilen]

Save images are not caps-specific. All read-only disk formats can use save-images (even zipped adf's)

Caps id can be added easily but unfortunately it would make old save images "incompatible"...