[Coco] HDB-DOS Eprom

Gene Heskett gheskett at wdtv.com
Fri Mar 18 19:40:09 EDT 2016

On Friday 18 March 2016 17:06:01 Mathieu Chouinard wrote:

> > That is sector offset to the beginning of the HDBDOS/basic disks,
> > and allows 94,371,840 megabytes for os9, which is about 3/4 of what
> > can be handled with a cluster size of 1.  Above about 132 megs it
> > needs a cluster size of 2, then 4 at just above 250 megs, moving up
> > in powers of 2, so my 1Gb seagate I have as S1, is running with a
> > cluster size of 16, or $10 in hexidecimal.  We run out of cluster
> > size room at about 4Gb IIRC.
> is there some document that explain how to do it?
I don't know as there is an official document, but it works like this, at 
a cluster size of one, each bit in the allocation map represents a 
single sector. The map itself is limited to 65536 bytes, 64k IOW, so the 
map can represent 8 x 65536=522,944, 256 byte sectors, and that is 
133,873,664 bytes of disk.  Above that to double that to 267,747,328 
bytes, it needs a cluster size of 2, then double that again for 
535,494,656 bytes it needs a cluster size of 4. You just keep upping 
both figures by a power of two until you are out of a 1 byte cluster 
size, and you are something above 4Gb, at which point you are up against 
the 3 byte offset limit of a sector seek anyway, so you never hit a full 
256 sectors per bit in the map.  Your minimum allocation is one cluster. 
At a cluster size of $10, aka 16, one could use a sas of 2 or 4 and 
still have room for the average coco sized file in one FD.SEG entry.  We 
have had our files grow in average size over the years, so the default 
SAS=8 is occasionally a problem, so I have been advocating a SAS of $10, 
or for known huge files, set it for $FF temporarily, so you never run 
out of FD.SEG's, which can be an un-recoverable problem in some cases.

Once the file has been written and closed, os9 cleans up and returns any 
of that unused SAS to the system, so you don't lose anything by running 
a big SAS except the ability to stuff a disk to the last byte.

Does that clarify it for you?

> > So in this case the os9 "partition" ends at $59FFF sectors and the
> > disk0 for hdbdos starts at $5A000.  That leaves an awful lot of
> > unused disk above the 256th HDBDOS basic image, so theoretically we
> > could have 5 or so more /sh like descriptors named /si, /sj, sk,
> > /sl, /sm, each with the offsets pointing to a new set of 256 basic
> > disks, all  on a 1Gb drive. But I've not done it except to prove to
> > me the 2nd set works, I'm out of system ram as it is.
> Now this is interesting, too bad I have to work this weekend, I would
> I have loved try this in an emulator
> Mathieu
> --
> A te quaeso, sicut in Aeneidos libro quarto Aeneas, elocutus
> desiderium erigendi suum obeliscum in templo Venereo Didonis, ab ea
> quaesivit, tuae domi an meae?

Cheers, Gene Heskett
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

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