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

[Allen Huffman]

A few years ago, I had plans to do all my floppy archiving using DriveWire under OS-9. I was pretty sure OS-9 could read an RS-DOS disk (since I had the "rsdos" utility) thus as long as you knew how many sectors to read (35 or 40 tracks) you could do raw sector reads from OS-9, and write them to a "FILENAME.DSK" binary file and have a mirror image of the disk whether it be OS-9 or RS-DOS.

If I write in in BASIC for the CoCoSDC, I can read and write sectors using DSKI$ and DSKO$. I know I can BACKUP or clone a 35 track disk just fine. If the disk is 40 tracks, there are POKES to Disk BASIC that set it for 40 tracks then DSKI/DSKO can go to track 40 just fine.

But what about double sided OS-9 disks? There are POKES for Disk BASIC that let Drive 0 and 1 be the front sides, and Drive 2 and 3 be the back sides (or any other configuration). If I had those POKEs done, I might have DRIVE 0 be the front side of a disk and DRIVE 2 be the backside of the same disk. I could then DSKI$ from drive 0 and 2 to read it all.

But I forget the format of double sided sectors for an OS-9 disk. Does it put the first 40 tracks (front side) then the next 40 tracks (back side), or does it interleave them top/bottom/top/bottom or something like that? I guess I never thought about the physical sectors/sides since OS-9 treated everything as Logical Sector 0 to MAX.

Anyone remember?

Best,

		-- Allen