17 Jul
2005
17 Jul
'05
12:28 p.m.
Hi Allison,
Is there a calculation, NO. Unless your formatting your
own, then
it's use biggest gaps that allow sector data and a resonable end gap
on off speed drives. Most of the time you have to know how
that disk was formatted originally to arrive at the correct values.
Fortunatly there is a fair amount of wiggle room if you do not run
too close (too close is below) to minimums.
I know how the disk was originally formatted to the extent that I know
the sector size and the number of sectors on the track.
I had assumed that it worked the way you described ... my problem is that
I cannot make sense of the values in the NEC supplied table.
Using the IBM System 3740 (FM) and System 34 (MFM) disk format specifications,
I tried working the NEC tables backward to get the "number of available bytes
on a track" which they started with. I worked it through for both 8" and
5.25"
drives in each format type, and the problem is that the results are not
consistant. Using a given drive type and encoding method, for each sector size,
number of sectors/track, and format GPL values in the NEC table, I get a "total
bytes on track" values which are quite different - within a few 100 bytes, but
you can make them all come out much closer by adjusting the gap length.
In other words, I want to do a calculation something like:
Total number of bytes assumed to be available on the track
- track overhead
- (#sectors * per-sector overhead) [not including gap]
- (#sectors * data bytes/sector)
/ #sectors
= actual sector gap length (for format)
However the values from the NEC table do not fit such a calculation, because
for each sector-size/number-of-sectors in the NEC table, you work out a "total
number of bytes assumbed to be available on the track" which is only within a
few 100 of the value worked out for other sector-size/number-of-sectors
combinations at the same drive type and encoding method.
For example, if you begin with a "total number of bytes assumed to be available
on the track" which you worked out from 512 byte sectors, and reverse the
calculation for 1024 byte sectors, the gap length you arrive at is different
from the one in the NEC table. Another way to look at
it is that by tweaking the
sector GPL, you can always get a "total number of
bytes" which is within
number-of-sectors/track" of the "optimim value" you were aiming for,
however the
values in the NEC table often result in "total number of bytes" values which
are
a couple of 100 away from each other.
Unfortunately the 765 based FDC in the PC is very limiting, and does not allow me
to read the raw format information, nor can I determine the GPL that was used on
the original disk - the only other option is to calculate a suitable GPL based on
the format information.
If no calculation is possible, can anyone point me at a more complete table of
suggested GPLs? The NEC table has some large holes, for example 9x512 and 10x512
are missing.
Regards,
Dave
--
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
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