After poking around HPFS to determine the amount of space reserved by HPFS,
compared to the size of the drive (the only part that varies by size is the
directory band), I ran into something interesting.

On a completely empty drive, the difference between the total drive size,
and the free space (as reported by DosQueryFSInfo()) has a gap of 2MB not
accounted for by HPFS structures.  What I first did was make sure that the
partition was indeed the size stated by DosQueryFSInfo().  I verified that
by writing every sector of the drive to a file on another drive.

What I did next was read all the freespace bitmaps (programmatically, of
course), and add up the free sector count that way.  The result was that
there were 4096 free sectors not counted by whatever DosQueryFSInfo() does
(via HPFS.IFS, no doubt).  This number is the same on every drive I tested.

What I'd like to do now is distribute the program that does this
comparison, to get a wider range of data.  Maybe HPFS386 doesn't have this
problem, or a different version of HPFS, etc.

The problem is that I'm not certain how to handle drives larger than 4GB.
The freespace bitmap list is four sectors long, which allows for 512 data
bands (about 4GB).  Also in the HPFS SuperBlock is a pointer to a spare
freespace bitmap list.  What's not there is the length of this spare list.
If it's only four sectors, like the primary list, then HPFS dies at 8GB,
without another list.

Unfortunately, I don't have any HPFS partitions larger than 4GB that I can
poke around on.

Does anyone know what the length of this secondary list is, or where to
find the next list, if it's only four sectors (and so on)?

Mike Ruskai
thannymeister@yahoo.com


... Best way to dispose of the Borg: Give them Windows 3.1x

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