[Coco] VCC VHD

Robert Gault robert.gault at att.net
Sun Feb 19 17:49:47 EST 2012

Luis Fernández wrote:
> Hi all
> I'm trying to get my utility cocodskutil (and I have to change the name),
> see the VHD that can be used in DC with the basic RGB-DOS and see that contains 2 zones.Do not want to call partitions as there is no header that defines them (or have not found)
> may be in the same header of OS9/NITRO.
> I do not find much more information.
> The information meeting is for virtual hard disk formats of Windows and TRS-80Mod I-III and apparently are not
> equal. Test the creators of disks and partitions, but not for this type of partition.In the end it parses and saw that has a simple OS9 disk to the top and excessive insize that ultimately lays 255-256 DECB Disks.Now my program can see them  (both partitions and disks), but do not know if it is aformat invented, or is actually a OS9 partition. Waste is spectacular, full well,  the OS9 partition has a file 15000-1700, and only occupy 13% down and 30% for the 256 final DECB disks,
> which is why itcompatibly. but
> Well the question is if anyone has some more information from this file.or assume it is like narrating?
> I and I have all of OS9. (It is possible that this in)


The .vhd format used in VCC, MESS, and the JVC emulators models the original 
RGBDOS software used with the KenTon SCSI hard drive system. Real SCSI hard 
drives depending on their sizes could have up to 256 Disk Basic drives and an 
OS-9 partition all on the same hard drive.
The .vhd format, however, has a default composition based on software written by 
Alan DeKok. Unless changed by a user, the OS-9 partition will be in the lower 
LSNs of the drive and of $5A000 sectors. After that will be 256 35-track Disk 
Basic drives/sections. All sectors are of 256-bytes.

In conjunction with the .vhd format, the RGBDOS emulator version of DOS has an 
offset of $5A000 in the ROM so that Disk Basic is forced to start with this 
offset when reading .vhd drives.
While possible, it is very difficult to be certain whether any specific .vhd 
drive has an OS-9 section or not. Even if you looked for an OS-9 disk structure 
that would not be proof but might be sufficient for most .vhd drives. For 
practical purposes, just assume $5A000 OS-9 sectors as I doubt many users have 
changed the format of the .vhd drive.

A reasonable method for checking the presence of an OS-9 drive would be to look 
for .. and . as the first entries in the root directory of the drive. If you 
find this data, the odds are very high that it actually is an OS-9 disk.

DD.DIR - points to the root directory file descriptor
LSN0, bytes $08-$0A

Directory file descriptor FD.SEG -points to the first sector of the directory
bytes $10-$12 of the file descriptor point to first sector.

Directory entries - each entry is $10 bytes and the first two entry names are
.. and .  . So, any directory starts with
location size content
$00       2    $2EAE
$10       2    $AE

There is no method for proving a disk is a Disk Basic disk. Still, all .vhd 
types should have 256 "disks" of 35-tracks of 256-byte sectors starting at the 
sector seen in DD.TOT. This term is used by OS-9 and indicates the number of 
OS-9 sectors on the disk.

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