[Coco] Orchestra90-Dragon NEW version

Barry Nelson barry.nelson at amobiledevice.com
Sun Jul 17 02:30:45 EDT 2016

I would be using a CoCo SDC since Orchestra 90 talks directly to the disk controller hardware. It looks like the CoCo SDC has at least some support for the VDK format…


DSK Images
The DSK image format is named for the extension most commonly appearing on such files. Images in this format consist of a simple sector array with each sector being 256 bytes in length. This is the most common format used in the CoCo world. It is sometimes also referred to as the JVC format although that format allows for an optional header to precede the sector array. The CoCo SDC will recognize a JVC header only for the purpose of identifying the number of sides.

In order to be recognized as a simple DSK image, the file size must be an exact multiple of 256 bytes. The minimum file size is 82,944 bytes which is equal to 324 sectors or 18 tracks of a single-sided CoCo disk (enough to accommodate the Disk Basic directory track). If the file size is not a multiple of 256 then it is assumed the image contains either a JVC or VDK (Dragon) header. A sanity check of the header bytes is performed to determine if the file can be used.

The disk geometry associated with a DSK image is determined by the file size. For floppy images the number of sectors per track is always 18. There are either one or two tracks per cylinder (equal to the number of sides) and a maximum of 80 cylinders. The largest file size for a floppy image is 737,280 bytes or 2880 sectors (double-sided 80 cylinders).

An image with more than 2880 sectors is considered to be a hard disk. If a hard disk image is accessed using the floppy interface mode, only the first 1440 sectors can be used. In this situation those sectors are accessible as a single-sided 80 track floppy disk. The controller's LBA interface mode must be used to access sectors beyond the first 1440 in a hard disk image.

> Bill Pierce ooogalapasooo at aol.com 
> Sat Jul 16 18:50:09 EDT 2016
> Barry, I think you'll have problems with VDK in DW4 and/or VCC. I have no idea whether Mess even supports VDK at all. I have had many problems with VDK in VCC as well as DW4.
> VCC and DW4 both only have "limited" VDK support and will not read all forms of VDK. I may be wrong and those disk images may read fine, though I think the best bet would be to load the VDK images into XRoar and copy the files to a JVC format disk image.
> XRoar originally only supported VDK and it's still the native format, but JVC support has been greatly improved on the later versions of XRoar.
> In the end, VDK is actually a more "complete" disk image format, having the "between track" timing sections and such, but JVC with it's straight "raw file" format is just easier to work with and the reason they became the standard for VCC & DW4. Even the NitrOS9 repo produces JVC style disk images.

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