[Info-vax] OT: COCO stuff (WAS: OT: For MAC Lovers Only :-))

[glen herrmannsfeldt]

John E. Malmberg <wb8tyw at qsl.network> wrote:
> On 10/15/2012 7:13 AM, Bill Gunshannon wrote:

>> Thanks, even with all the problems it has been fun so far. Next on the
>> agenda is NitrOS9 on a COCO3 with 512K of memory and IDE disks.  :-)

> I think someone mentioned that a program was available the allowed 
> NitrOS9 to read and possibly write FAT formatted floppies.

> I did a test with a VMS system a while back and determined that the 
> TRS-80 Color Disk Basic ROM could read 512 byte sector disks.

> As an experiment, I was able to write data on to foreign mounted DDSS 
> 3.5 inch floppy on VMS and read the first sector using the COCO DISKI$ 
> routine.

Some years ago, I had a COCO running OS/9 and modified the disk I/O
routine to allow for double side 80 cylinder disks. 

Not so much harder to go to 512 byte sectors, but I didn't try that.

> Normally the COCO uses a track with 18 256 byte sectors.  The ROM 
> actually allocates 512 bytes, 256 for a read, and 256 for an optional 
> verification.  The ROM software just reads until it gets an end of 
> sector notification, so does not care about the sector size.

I haven't looked at the 1793 data sheet for a while now.
I do remember that one byte in the sector header indicates
the sector length. The ROM I/O routine is likely different
from the OS/9 routine. 

> The ROM only knows how to get at the first 256 bytes though.

> For writing, the two 256 byte buffers are also adjacent, and on 512 byte 
> sector media, 512 bytes are written.  Again the COCO ROM only knows to 
> put data in the first 256 bytes.

> So it should be possible to write a program on VMS or other platform 
> that will create a COCO disk directory structure and populate it with 
> files that an unmodified COCO can read and write for data transfer.

> You would have to play games by modifying the in-use sector map because 
> there are only 9 sectors available instead of 18.

I was once trying to figure out if it would be possible to write at
the 500kb/s rate for 8 inch (or 5.25in HD) drives. The loop reading data
from the controller and writing to memory isn't fast enough for that, but 
it could be done unrolling the loop. Not so bad for sector reading,
but for formatting it is big. (It has to be a whole track worth.)

-- glen