[Coco] Re: NitrOS9 on MESS?

[Robert Gault]

Nathan Woods wrote:

> Roger Taylor writes:
> 
>> At least you know that it's occuring while RBF is writing out to a
>> storage device.  So if this does not happen on a real CoCo, and the code
>> is  exactly the same, then it's M.E.S.S... probably a timing issue or 
>> the index hole signal code.  ??
> 
> 
> Truthfully, I do not know.  This particular bug defied almost all of my 
> assumptions.  At first, I assumed that it was either a FDC timing bug or 
> a 6309 core bug.  However in my investigations, it seems that the orders 
> to write to the bad sector come from outside cc3disk.  The fact that the 
> bug occurs when used with a VHD - thus bypassing cc3disk - confirms this 
> observation.  Since this bug also occurs with NitrOS-9 6809, the 6309 
> core is not to blame.
> Generally any bug that appears in MESS but not the real hardware is by 
> definition, a MESS bug.  Probably the only bugs for which this is not 
> the case is if some CoCo code was relying on something totally whacked 
> out, such as reading from undefined IO memory or the contents of memory 
> when power is turned on.  I'm inclined to believe that the latter is not 
> the case.  It could be a timing bug, but what kind of timing bug would 
> cause this bug.  As for "index hole signal code", can you elaborate on 
> what that is?
> 
The controller reports the passing of the floppy disk index hole past 
the drive detector as a bit in Status for type I commands. It also would 
be used by the Western Digital controller to synchronize read/writes in 
some fashion.

The only time this info was used in Coco code was in programs that 
tested drive rotational speed. So, it would be invisible to Disk Basic 
or OS-9.

It might be an issue with the MESS emulation of the controller. It would 
be most likely to show up in NitrOS-9 operations because of tighter 
timing during disk I/O. However, the bug in question does not depend on 
the floppy controller and is also present in .vhd use.