Re: An 18-sector DOS 3.3?

[Michael J. Mahon <mjmahon@aol.com>]

<bpiltz@gmail.com> wrote:
> After seeing that Prince of Persia, and a handfull of other Apple
> programs have a customized 18-sector disk format, I ask the inevitable:
> Did anyone back in the day attempt to rewrite the DOS 3.3 RTWS to utilize
> an 18-sector disk format? It does not at all seem like this would be a
> trivial task, although getting 180kb per 40-track 5.25 disk side would be
> extremely tempting. How about an 18-sector ProDOS? I know a few have
> tried half-tracking and 3/4 tracks to increase capacity of a 5.25" DOS
> 3.3 disks, but did THAT ever get anywhere?
> 
> Seems that there is a lot of inefficiency and redundancy on Apple's 6:2
> GCR encoding scheme.
> 
> Back in the day, I got over 1100 kb on an "dual-sided, dual density" '360
> kb' floppy by using full-tracks and 1 sector per track, 88 tracks per
> disk, and 2350 kb on a 1.44 HD floppy with the same method, and 90
> tracks! (God, I loved Alps floppy drives, 10 extra tracks!) That was, of
> course, MFM encoding on a PC.

An 18-sector track is essentially a read-only track, since it doesn't
contain long enough self-sync gaps to allow reliable read-address to
write-data transitions.

Nor does it allow for typical speed variations in drives, which call for
suitable self-sync padding to prevent a re-written sector from overwriting
the next sector's address marks. 

In short, an 18-sector format is a "write whole track"-only format. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon