[Coco] VCC VHD

Luis Fernández luis46coco at hotmail.com
Mon Feb 20 22:55:40 EST 2012

Interestingly OS9 detection method, which you proposeFor now I'm using$ 03 1 Track DD.TKS size (in sectors) = $ 11 DD.SPT 2 Number of Sectors per trackIt gives a good chance

> Luis,
> 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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