Re: DOS 3.3 VTOC Puzzle: Precise Meaning of "Last Track Allocated"

[Andy McFadden <fadden@fadden.com>]

jutland1916@hotmail.com wrote:
> I have a follow up from some earlier questions about VTOC, etc. in DOS
> 3.3. What is the *precise* meaning of the phrase "last track allocated"
> for byte offset 30h in a standard VTOC?

Most recent allocation.  If you look at a DOS 3.3 disk over time, you'll
see allocations start at track 18 sector 15, march toward the end of
the disk, then start at track 16 sector 15 and march toward the start.

Files near the catalog track can be accessed faster.  They'll load
faster if they all sit on one track, so DOS like to put the T/S list in
sector 15 and add the file contents in descending sector order.

> Specifically, I'm interested in a file found at tracks 33 and 34
> decimal on a disk image. The VTOC for the image lists "last track
> allocated" as 16h (22 decimal). The file, however, *does* have a normal
> Catalog entry that points to track 21h (33 decimal) as its start point.
> This leads me to believe that it is not a file that operates
> independently of DOS, as someone had earler suggested. So ultimately
> I'm trying to reconcile the VTOC entry with the Catalog entry and the
> actual location of the file as visible with a hex editor.

The field is usually ignored by disk utilities, which just pack the files
in any way they see fit.  It's just an optimization that may or may not
be useful.  If it's wrong, it's ignored.

Read the description of the code at $B244 in Beneath Apple DOS,
specifically the code at $B272.  It references $B3EB, which holds the
"last track allocated" from the VTOC.

What is it you expect to gain by examining this field?

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