[Coco] HDB-DOS Eprom

[Gene Heskett]

On Friday 18 March 2016 15:59:17 Tormod Volden wrote:

> On Fri, Mar 18, 2016 at 7:55 PM, Robert Gault wrote:
> > HDBDOS will work with real hard drives and hard drive images (.vhd).
> > The most prevalent .vhd image used with the emulators JVC, MESS, and
> > VCC assumes the first part of the .vhd is for OS-9 and the latter
> > for Disk Basic. This hard drive image consists of $5A000 bytes
> > allocated to OS-9/NitrOS-9. The rest of the image consists of 256
> > Disk Basic "disks" which are 35-track, single sided.
>
> Out of curiosity, what is the origin of the $5A000 value?
>
> Tormod

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.

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.

Cheers, Gene Heskett
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