[Coco] Biggest OS-9/Nitros-9 HD?

Gene Heskett gene.heskett at verizon.net
Sat Sep 24 03:38:23 EDT 2005

On Friday 23 September 2005 21:44, Diego Barizo wrote:
>   I've been reading the OS-9 manual, and I understand that the FAT
>3 bytes - 24 bits - to keep track of the files sectors.
>That would give 16777216 sectors, and at 256 bytes a sector, 4194304
>or 4 Gb
>Is that the maximum size of HD partition that OS-9 can work with?
>What is the biggest HD any of you has ever used with OS-9?
>Also, the manual says something about the files being able to be as
>as the media. Did I get it right? Could OS-9 in 1985/6 work
>(theoretically) with files over 2 GB, something that Windoze machines
>could not do until the release of WinXP some 23 years latter?

No, there is an even smaller limitation to os9/nitros9, and thats the
fact that the fat table is limited to 65535 bytes as it exists on the
disk.  At one sector per fat table bit, thats a hair over 131 megs as
a maximum disk size limit IF the cluster vale is 1, where each bit
represents a sector of the disk.

That said, the code is in rbf.mn to allow a larger disk to be used via
raising the sectors per cluster by some power of 2, eg 2, 4, 8, 16,
32 etc.  This sectors per cluster value is contained in each disks
Logical Sector Number zero (LSN0) DD.BIT, the 2 byte integer at $06,
so it can be a very large value, so large that I believe the  3 byte
seek to limit is probably the real limit as I do not believe that is
also scaled by the DD.BIT value. 

In my rbf.mn work with that, I was limited by my own hard drive being
the only one I had at the time, a maxtor 7120s with about 130 megs
available, so it wasn't available for 'playing' with but I did format
and use 784k (85 track DS) floppies with rather extensive amounts of
testing at sector per cluster values up to 8, all of which worked as
expected.  At a value of 8, 1gb drives could be used, at 16, 2 gb, and
at 32, then 4gb drives could be used.  Note too that this is almost
exact how messydos expended that filesystem to handle larger drives. 
This value is not (that I know of) something that can be adjusted by

Note that at the time, the format command was not equipt to handle
this from the command line, and I had to create the multiple sector
per cluster disks once they had been formatted via the use of dEd,
moving things around as needed.  As that took about an hour per disk,
thats why the testing stopped at 16 sectors per cluster.  At that
size, the minimum size of the allocation for a 1 byte file was 4096
bytes of actual disk space used.  So there is a maximum number of
files per disk limit too, but at the end of the day, most file systems
in common use today will have limits based on similar circumstances. 
Its large enough that I have not run into that limitation here in
linux, using the ext3 filesystem with any of the drives I have, one of
which is currently a 200gb drive I'm using for virtual tapes with
amanda, the Advanced Maryland Automatic Network DIsk Archiver, aka my
backup program.

I believe that Boisy has now made the format command capable of doing
this for nitros9, so if the command syntax were known, it should be
pretty easily done nowadays.  Correct me please if I wrong on that
point, Boisy or ??

Cheers, Gene
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