Now that it is possible to read from D81 files on the SD card, I wanted a way to pick from the images on the SD card, and make the floppy controller think I have inserted it into the internal drive.
So I wrote a little program that reuses much of the FAT32 code from the kickstart ROM, and allows you to say Y or N to each image in turn. Once you say Y to an image, it makes it available for use.
I haven't debugged whether it sets the disk change line properly, but it does work, directing access to that disk.
I wrote it initially as a program to be loaded, but that is a bit annoying, so instead I include it in the Kickstart ROM, which then copies the program to $C000, so that it can be entered with SYS49152 from C64 mode.
Below you can see a screen shot of it in action:
Clearly this could do with some serious visual improvement, but the core functionality is there. What is nice, is that only the core SD card access routines need to be in this wedge, as it could load a larger program direct from the SD card (optionally after saving the memory it will occupy), to give a nice looking interface with support for DOS subdirectories and FAT32 long file names.