Re: DOS 3.3 400k volumes

[pausch@saaf.se (Paul Schlyter)]

In article <vbednZ6OrbaIBHCgXTWcpg@comcast.com>,
Joel <joelbuckley54.nospam.@hotmail.com> wrote:
 
 
> While I haven't used the 3.5" Disk versions of DOS 3.3, I am familiar with
> the DOS VTOC (volume table of contents).
> 
> DOS 3.3 allocates 4 bytes to each track. Each bit stands for 1 256 byte
> sector.
> A 5.25 disk only uses 16 sectors per track out of 32.
> The older DOS 3.2.1 uses the same technique using only 12 sectors per track.
 
DOS 3.1 and 3.2.x both used 13 sectors/track
 
> But DOS 3.3 will support 32 sectors per track.
> A standard 5.25 disk uses only 35 tracks.
 
A _standard_ 5.25 disk used 40 tracks.  The Apple II used only 35 of
those 40 tracks.
 
> But the VTOC will support 50 tracks.
> This bitmap is stored in the VTOC (volume table of contents) at track 17
> ($11) sector 0
> starting at byte $38. From $38 to $FF is 200 bytes, 50 tracks max at 4 bytes
> per track.
> So maximum disk size is 50 tracks x 32 sectors x 256 bytes = 409,600 or 400
> x 1k (1024) bytes.
 
......................
> It would be possible to map each 400k volume to one side of an 800k disk.
 
Many years ago, I mapped two 320K DOS volumes to each side of 640K
floppy disks (5.25" 80-track double sided floppies).  I ended up
calling the second sides "drive 3" and "drive 4" -- which of coruse
caused some problems with software refusing to accept any drive
numbers other than 1 or 2 .... but DOS itself happily worked with this.
 
Then I stored most of my Apple II software on these 640K disks, instead
of on regular 140K Apple II disks.
 
I also mapped CP/M volumnes to these disks, which went much more
smoothly: here one could easily map the entire 640K floppy to one
single CP/M volume.
 
-- 
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