On Wed, Jan 18, 2017 at 11:29 AM, Hagstrom, Paul via vcf-midatlantic < vcf-midatlantic@lists.vintagecomputerfederation.org> wrote:
Just a couple of quick clarifications on Apple II disk images:
On the Apple II .dsk/.do/.po files can't be protected, they're standard 16-sector disks (because those container types only store the data, not the structural information on the disk, and "the data" is only defined properly if its comes from a standard 16-sector structure).
.do and .po files differ in the order in which the sectors are located in the data stream for each track. Again, since no structural information is present in the image file (specifically, the headers, which contain the sector number and volume number), you have to know where to look in a track to find sector 9, since the only thing that defines "sector 9" is where it is in the track.
.dsk files are usually the same as .do files, that is, the sectors are in the DOS order. There is no difference between .do and DOS-ordered .dsk files, it's just that .dsk is underspecified. Some .dsk files are in ProDOS order instead. So, it's better to use the explicit .do and .po extensions on disk images so the emulator doesn't have the chance to guess wrong about what order the sectors are in. As a corollary, if booting a .dsk-type image crashes to the monitor, it's sometimes worth changing the extension to .po and trying again (and, if that doesn't work, to .do and trying yet again), since if the sectors are out of order, a crash on boot is expected behavior. In practice, nearly everyone uses .dsk to mean .do, so 90% of the time if you get a crash on boot due to sector ordering, changing the extension to .po will be the solution.
.nib ("nibble", as opposed to reassembled data bytes) files contain the structural information that .dsk/.do/.po files omit, so if the volume number is important or if the sectors are in yet another order or marked in a different way (like, a 13-sector DOS 3.2 disk), the .nib file retains that information. But that means that most of the time utilities designed for standard format won't be able to manipulate sectors/files on such images.
-Paul
ah ok, this makes sense now thanks for the tips Paul I'll have to add this info to my Apple notes Dan