18 Mar
2012
18 Mar
'12
9:11 p.m.
On 18 Mar 2012 at 12:02, Chuck Guzis wrote:
Cool - I didn't know that - I thought it was "all or nothing" when it
came to 128/MFM.
What you can
do when formatting is:
- Lie about the sector size when supplying the data for the individual
ID headers.
- Fill in bogus head/sector data for sectors you don't want to use
(and will overwrite) later.
- Control the gap3 size for the entire track.
- Write fewer sectors than will fill the track
- Reset the FDC before it has completed writing the final sector
on the track.
It's the first that will work the best. In fact, all 765-family
controllers will FORMAT a 128-byte-per-sector track in MFM; it's just that
they don't do so well reading or writing them (depending on the controller
variation). But what the heck, if you're going to verify a
track by reading after
formatting, you might as well write it too, producing a track with 9 512
sectors and one 128-byte one.
But don't you run into problems with the header positions in some
layouts?
To do this on a general basis, you would have to be able to put
256, 512, 1024 etc. byte sectors into some multiple of 128 byte
sectors - for some of those it works out to significantly more space
than the same sized sector on a normally formatted track...
I agree that it might not be too hard to write a program to format a
disk with a known layout that works out reasonably with 128 byte
sectors "underneath", but I'd like to be able to do the best possible
general solution to mixed sector sizes.
The controller will normally stop formatting at the
index if it is
between sectors. If however, it's in the middle of a sector, it will
continue formatting until the end of the sector has been written, then
honor the end-of-track condition.
Have you ever tried resetting the FDC after it has written the last
header but before the index hole?
If this would work, it would let you have normal sector spacing for
the cases where the format is n large sectors followed by 1
small one ...
Dave
--
dave12 (at) Dave Dunfield
dunfield Firmware development services & tools: www.dunfield.com
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