[Coco] VHD 256 or 255 disks

[Robert Gault]

nickma at optusnet.com.au wrote:
>
> There seems to be a lot of confusion about how Virtual Hard Drives
> work, me included, that I wonder how many people have been turned off
> from using them and learning about OS-9.
>
> Maybe this is an opportunity for someone to develop a small book about
> Hard Drives on the CoCo with emphasis on the VHD and Drivewire.
>
> I'd be prepared to buy such a book if it was clearly written and takes
> the mystery and frustration out of the whole process and gets people
> back on track to producing software rather than fighting to make
> everything work in the first place.
>
> Nick
>

Oh boy!!

Well there actually is quite a bit of information already out there but it is 
scattered. What you need to keep in mind is that the DOS you need to understand 
is RGBDOS and it was custom burnt for a customer's hard drive system. You told 
Ken-Ton what size drive you wanted to buy and how it should be divided between 
OS-9 and Disk Basic. You then received a ROM with a custom offset value at 
$D938-$D93A.

When Alan DeKok created the emudsk hard disk driver for NitrOS-9, he knew that a 
fixed standard would be required to prevent chaos. So, he fixed the offset at 
$5A000. That meant he could distribute emudsk and h0 without any problems and 
all .vhd images should work.
So now comes Drivewire and HDBDOS with an offset of $000000 for the default and 
there is total confusion.

By the way, Cloud-9 still custom burns HDBDOS ROMs for hard drive systems with 
the offset based on the size of the drive and the customer's wishes.

The Ken-Ton hard drive systems worked with any Coco. What was interesting was 
that with the Coco3 and it being in full RAM mode, it was now possible to POKE 
the offset value.
So, let's say you wanted a certain amount of OS-9 but 400 Disk Basic drives on 
your hard drive. Starting with the offset in ROM, you just POKEd in as many 
multiples of $27600 as needed with each one adding a extra 256 Disk Basic drives.
Well there was no way to do that with old Seagate drives but with modern hard 
drives there is no limit to the number of Disk Basic driveletts you can have.

The standard .vhd image is about 129MB which gives you a large OS-9 section and 
256 35T drives. How many people need more than 256 Disk Basic disks? Still if 
you have gegabyte hard drives, you could certainly increase the .vhd at least 10 
fold.
There is a limit to the size of both sections (OS-9 and Disk Basic) as the total 
number of sectors must be describable with 3 bytes; $FFFFFF.

Drivewire now has adjustable offset values that take the place of that in the 
ROM so the ROM offset can be $000000. Just keep in mind that you may want to 
share your .vhd image among several emulators, JVC, MESS, VCC, as well as your 
Coco. That means maintaining some standard for .vhd so that emudsk or whatever 
OS-9 driver is used to access the image will be sure to work without having to 
creating a slew of custom descriptors.

Robert