Re: please help--tracks, sectors, VTOC, and disk geometry

[David Wilson <david@uow.edu.au>]

jutland1916@hotmail.com writes:
>Moving from the center to the edge of the disk, are tracks numbered in
>ascending or descending order? Is track 34 closest to the center of the
>disk or closest to the outer edge?

Track 34 is the inner (smallest diameter and circumference) track while
track 0 is the outer (longest and nearest the edge) track.

>Moving clockwise around the disk, are sectors numbered 0-15 or 15-0?

Is that logical or physical sector numbering? :-) :-)
I must admit I do not know which way the floppy disk spins (and of course,
because the data is written on the underside of the diskette, it rather
depends which side of the floppy envelope you are looking at).

>I have a disk image whose VTOC indicates that the last allocated track
>was 22, moving inward ($16, $01), yet there is good data beyond that
>(at tracks 33-34). How can that happen?

I would have to look that one up in "Beneath Apple DOS".
Is it because DOS uses free space on the diskette from track 18 to 34 then
back to 16 and down to 1?

>Does the VTOC retain a record of "erased" data on the disk that has not
>yet been overwritten?

>Why are the VTOC and Catalog sectors always at track 17? Is there a
>mechanical reason for this, something related to the position of the
>read/write head, given that it's the middle track of the drive?

As has already been pointed out, it is an optimisation to have the VTOC
centrally located as this reduces the average seek time to find it.
Unfortunately this does not scale to different disk sizes which is why
ProDOS goes back to having the directory at the start of the disk.
-- 
David Wilson  School of IT & CS, Uni of Wollongong, Australia