Limited support for XDF disk images

@jeffpar jeffpar released this Oct 29, 2014 · 3936 commits to master since this release

This release includes support for XDF disk images (specifically, the XDF disk images that were shipped with PC-DOS 7.00).

However, this support is referred to as "fake" XDF support, because it requires using JSON disk images created by DiskDump without the experimental "--xdf" option, which is an option that attempts to encode XDF sectors as they existed on the original diskettes (ie, with varying lengths and non-standard sector IDs).

"Fake" XDF support works by using conventional 80-track disk images with 23 sectors/track. No standard PC floppy disk format ever used 23 sectors/track, but in this case, by distributing the XDF track data across 23 conventional 512-byte sectors, the PC-DOS 7.00 Setup code that reads XDF disks succeeds. This was probably one of several fall-back options built into the PC-DOS XDF code.