[Coco] Another Coco Virtual Disk Util

[Robert Gault]

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