[Coco] Re: NitrOS9 on MESS?

[Gene Heskett]

On Friday 05 December 2003 11:24, Robert Gault wrote:
>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.

No argument with any of that, Robert.  I was under the impression that 
disk basic did use the index pulse while formatting a disk.  As it 
turns out, all this "housekeeping" is in the fdc, not the coco driver 
software.  Thanks for clarifying that.

-- 
Cheers, Gene
AMD K6-III at 500mhz 320M
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Copyright 2003 by Maurice Eugene Heskett, all rights reserved.