[Coco] Re: 3.5" Floppy format

[Gene Heskett]

On Wednesday 04 February 2004 00:17, John E. Malmberg wrote:
>Theodore (Alex) Evans wrote:
>> Personally I feel that the "96 tpi" bit (bit 1 of byte $10 of
>> LSN0)
>
>I am missing quite a bit of the context for your post, and an
> assuming that this is something specific to the OS-9 operating
> system.
>
>As you point out, since all 3.5 inch drives are 135 tpi, no double
>stepping is ever desired, even if you are using only 35 or 40
> tracks.
>
>It is only 5.25 720K or 1.2M drives that should be concerned with
> double stepping when reading 360K floppies.
>
>Is this information stored on the actual floppy?  That would make
> many backup type operations a problem, as many of them do make
> complete images, and that includes copying images from 360K
> floppies on to the first 360k of a 3.5 inch drive with out changing
> any data.
>
>The device driver is the only place that I can see needing to know
> this information.

This information is stored in 2 places of LSN0 of the disk at format 
time, bytes $10 and IIRC $3F.  Look it up in the level 2 manuals, 
although its not *that* concise, mostly I suspect because when the 
manuals were written, 96tpi 5.25 disks were just then coming online.

As for the "backup" copy, that should not be a problem if the general 
guidelines are followed in that ONLY a 720k 96tpi disk is ever set to 
the quad density mode using dmode.  That "backup" would probably best 
be done with dsave where there is a DNS difference.

If the difference in size of the disk on the 3.5 format is a problem 
its very easy to dmode that drive to be a 40 tracker ($28 cylinders) 
and then a format, establishing to the backup program that the disks 
are indeed identical.

Conversely, if a 720k 5.25 disk is to be "backed up" to a 720k 3.5, I 
would first edit the LSN0 of the source disk to correct it, and set 
the 5.25" disks dmode DNS=1, not DNS=3 at which point everything 
should also be happy.  That situation would then be a straight image 
and backup will work just fine.  Or you could leave it alone and use 
dsave which won't care, and it will defrag the disk at the same time 
if SAS is set high enough, like to 40 or so.

In any event, you'll potentially get into a bit of molasses with the 
3.5" set for DNS=3.  If you do that then you'll not be able to backup 
any 40 trackers to the 3.5", at least not without fixing those two 
bytes in LSN0 again on the target 3.5" disk.  Best done with a run of 
dmode and a reformat. You aren't going to use the other half of the 
disk anyway once backup gets done with it.

Humm, OTOH, if both are set to DNS=3, that also would work, but what 
happens when a proper DSN=1 3.5" disk is inserted?  You'll get 
double-stepping, which is not the desired operation.

As I see it, the problem is going to need both disk editing with ded, 
and twiddling of the descriptor with dmode to establish each ones own 
personal standard.

>-John
>wb8tyw at qsl.net
>Personal Opinion Only

-- 
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Copyright 2004 by Maurice Eugene Heskett, all rights reserved.