[Coco] Re: NitrOS9 on MESS?

[Robert Gault]

Gene Heskett wrote:

> On Friday 05 December 2003 00:03, Roger Taylor wrote:
> 
>>At 08:44 PM 12/4/2003 -0500, you wrote:
>>
>>>>>cc3disk.  The fact that the bug occurs when used with a VHD -
>>>>>thus bypassing cc3disk - confirms this observation.
>>
>>Unless there is a duplicate set of problem code in either module for
>>disks and vhds.
>>
>>
>>>>>bug, but what kind of timing bug would cause this bug.  As for
>>>>>"index hole signal code", can you elaborate on what that is?
>>
>>I've always read that M.E.S.S. emulates the hardware as much as
>>possible.  Perhaps it emulates the pulses generated by the passing
>>of the index hole of a real floppy disk when a .dsk is mounted and
>>accessed.  Anyway, I think someone reminded me that it is only used
>>when formatting a floppy disk on a real CoCo controller. 
>>Personally, I never had a need to know much about the index hole of
>>a disk.
>>
>>
>>>I only know of one drive and driver that ignores the index pulse,
>>>and thats all 68k based amigas ever built, because they do read,
>>>and write, a whole track at a time with no regard to the index
>>>pulse.
>>
>>Then how did it prevent from writing a track over the hole itself? 
>>:)
> 
> 
> The index hole is close to the center hole of the disk, and the heads 
> never reach that far inward so its a non-issue.  This hole has no 
> bearing on the snips and such in the edges of the sleeves, or the 
> slider covers in a 3.5" diskette.
> 
> 
>>>  It
>>>reads the whole track and a bit more into a buffer, and searches
>>>the buffer for the right "sector" address.
> 
> 
OK, I can see that I need to present some details on the Western Digital 
controller or we'll all go away with very wrong ideas on how floppy 
drives work.

1) Disk Basic does not make use of the index marker status bit for any 
operation just the busy status. Check "Disk Basic Unravelled" to verify.
2) The disk controller uses the index pulse for all operations. Type I 
commands have a wait 6 index pulses at the beginning of the sequence 
before any head movement if "enable spin-up" is set. Type II commands do 
the same thing. Type III commands (includes write track {i.e. format}) 
has the above 6 pulse delay and also uses the index pulse in two other 
places in the command block diagram so that tracks are NOT placed at 
random on the disk. See Western Digital Manual to verify.
3) The index hole is not detected by the drive heads. It is detected by 
an LED/detector combo that passes current whenever the index hole 
unblocks the beam path.

If the index hole LED becomes loose and falls out of position, you won't 
be able to format a disk. I've had this happen on a 5.25" drive.

Does this have any impact on Disk Basic? None at all. Could this have an 
impact on MESS? Maybe, depending on how closely the hardware is mimicked.