[Coco] Another Coco Virtual Disk Util

[Walt Zydhek]

Ok, I have added support for $82 in DD.FMT. New VHD images will be created
with $82 in DD.FMT. When opening a VHD, if $82 is in DD.FMT, 1 head will be
used. Otherwise it will use DD.FMT to determine 1 or 2 heads. I Have a few
CHDs that were created with 2 heads so opening them as 1 head causes a
failure to load. So OS9 does not support multiple head hard drives? They
stick to a single head of the drive?

-Walt Zydhek

-----Original Message-----
From: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] On
Behalf Of Robert Gault
Sent: Friday, October 18, 2013 8:39 AM
To: CoCoList for Color Computer Enthusiasts
Subject: Re: [Coco] Another Coco Virtual Disk Util

Walt Zydhek wrote:
> Are you saying that ALL VHD's should be one head regardless of what 
> DD.FMT reports? That would be an easy fix, and I can force it to 
> always use $82 for DD.FMT if it's a VHD if that would be the proper use.
>
> -Walt Zydhek

Keeping in mind that this refers ONLY to .vhd emulated hard drives, yes the
assumption is that there is just one head. More to the point, DD.FMT can't
say much of anything about hard drives as it is limited to one or two heads
while real hard drives can have more than that.
DD.FMT most likely is a holdover from floppy drives where the choice is only
one or two heads.

The problem is determining if the emulated disk is a floppy or a hard drive.
The only indicator in LSN0 is the highest bit of DD.FMT and that does not
help if there is no OS-9 section on the .vhd. For safety, we really should
always use .vhd if the drive is hard but that has not been done
consistently.

There is also a problem determining where the "Disk Basic" (ie. 35 track
drives) start on a .vhd image. The original specs had the .vhd image
partitioned at
$5A000 bytes regardless of whether the first partition was formatted for
OS-9 or not.
This spec also required the RGBDOS ROM to contain this offset value of
$5A000 and that is what determines where RGBDOS starts reading the .vhd
image. If you mess around with the .vhd OS-9 size or change the offset value
in RGBDOS, you can get cross contamination of the partitions.
In short, the .vhd image is not a good determinator of where the 256 35-T
drives start.

Robert


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