(fdutils.info)Autodetection

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7 How autodetection works
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The principle of autodetection is rather simple.  When a floppy disk is
first accessed, and its geometry is not yet known, the floppy driver
tries out a list of up to 8 geometries (format descriptions) until one
is found that matches (i.e.  that makes it possible to read the first
sector or track).  This list of geometries is called the _autodetection
list_.  There is one autodetection list per drive type (as indicated in
the cmos).

   The autodetection list doesn't contain the geometry descriptions
themselves, but rather references to entries in the _geometry list_
(Note: geometry list).  Each list may contain up to 8 such references.
Each reference can be tagged with a 't' flag.  If this tag is set, the
floppy driver tries to read the whole track when trying out that
description; if it is not set, it only tries to read the boot sector.

   Reading the whole track is useful to distinguish among geometries
which differ only in the amount of sectors per track.  In order to do
this, put the geometry with the most sectors first in the list, and set
its 't' tag.  Use the 't' tag only in this case, as it makes
autodetection slower.

   Autodetection cannot distinguish between geometries that only differ
in the number of heads or in the number of tracks.

   Autodetection is meant to supply only a first approximation of the
actual format of the disk.  It supplies enough information to enable a
program such as 'mtools' to read the boot sector, which contains the
exact information.  'Mtools' then uses the information contained in the
boot sector to set the exact geometry.

   The autodetection list is set using the following command:
     floppycontrol --autodetect LIST

7.1 Example
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The following example restores the default autodetection sequence for a
3 1/2 ED drive:

     floppycontrol --autodetect 7,8,4,25,28,22,31,21

   The following example changes this sequence, so as to add the 1680KB
format (number 11).  As only 8 formats are allowed in the autodetection
list, we have to dump one entry (we chose the last, which is numbered
21).  The 1680KB format is identical with the default 1440KB format
except for the number of sectors.  Thus we must read the whole track in
order to distinguish it from the 18 sector format ('t' flag).
Furthermore, the 1680KB sector format should be detected first, as an 21
sector disk would also matches the standard format with its 18 sectors.

     floppycontrol --autodetect 11t,7,8,4,25,28,22,31

   The following example attempts to autodetect CBM 1581 disks along
with the more usual formats.  CBM 1581 disks are not among the
predefined formats.  Thus we first have to pick one of the predefined
formats and change it so it fits our needs.  We may for example pick one
of the rarely used 5 1/4 formats, such as h880, which bears number 20).
We first make a device node bearing the requested number (so that we
have a filename to pass to setfdprm), then we chmod it so it becomes
accessible to mortal users, finally we configure the geometry of the new
node, and enter it into the autodetection list.  We place it at the 4th
position, just behind the usual ED, HD and DD formats, and before the
more exotic extended formats.  Indeed, formats which are nearer to the
head of the list are autodetected faster, and hence more commonly used
formats should be put nearer to the beginning (1).

     mknod /dev/fd0cbm1581 b 2 80
     chmod 666 /dev/fd0cbm1581
     setfdprm /dev/fd0cbm1581 DD DS sect=10 cyl=80 ssize=512 fmt_gap=35 gap=12 swapsides
     floppycontrol --autodetect 7,8,4,20,25,28,22,31

   Some formats use more than 80 tracks.  It is not possible for the
kernel to autodetect the number of tracks in a reasonable time, so you
have to use a sufficiently recent version of mtools to set the number of
tracks according to the boot sector of the disk.  Mtools 3.0 and up are
OK. This doesn't obviously work with disks containing something else
than a MS-DOS filesystem.

   ---------- Footnotes ----------

   (1) except of course if several formats only differ in the number of
sectors per track, in which case the formats with the most sectors
should come first