[Coco] OS-9 and RS-DOS disk structures

[Robert Gault]

Allen Huffman wrote:
>> On Dec 30, 2014, at 4:56 PM, Bob Devries <devries.bob at gmail.com> wrote:
>>
>> Allen,
>>
>> As far as I remember, OS-9 interleaves tracks.
>> Actually, a "track" on a double-sided disk is 2 x number of sectors per track as defined in LSN0.
>
> Meaning to OS-9, a double sided disk is seen as Track 1-40, each containing sectors 1-36???
>
> 		-- Allen
>
>

Allen,

The double sided OS-9 disks alternate tracks such that all the even tracks are 
on side one and all the odd tracks are on side two of a floppy. The default 
format for emulators is to alternate the "front" and "back" tracks which is easy 
to see if you examine the .dsk/.os9 images.

Given that a Coco requires track34 to be the boot track for OS-9 which normally 
would be sector $264 on a single sided disk, a double sided OS-9 disk will 
display the boot track starting at sector $4C8. It's twice the sector value as 
OS-9 alternates between the front and back sides of the disk. That results in 
sector $4C8 being on the front side at track34.

It is much easier to transfer files from Basic disks to OS-9 disks under OS-9 
control than it is under Disk Basic. You mentioned RSDOS and I can supply you 
with an OS-9 program I wrote, HRSDOS, which reads Disk Basic directories and can 
transfer files in either direction.

Robert