mtools: geometry description
3.5.3 Disk Geometry Configuration
---------------------------------
Geometry information describes the physical characteristics about the
disk. Its has three purposes:
formatting
The geometry information is written into the boot sector of the
newly made disk. However, you may also describe the geometry
information on the command line. 
mformat, for details.
filtering
On some Unixes there are device nodes which only support one
physical geometry. For instance, you might need a different node
to access a disk as high density or as low density. The geometry
is compared to the actual geometry stored on the boot sector to
make sure that this device node is able to correctly read the
disk. If the geometry doesn't match, this drive entry fails, and
the next drive entry bearing the same drive letter is tried.
multiple descriptions, for more details on supplying several
descriptions for one drive letter.
If no geometry information is supplied in the configuration file,
all disks are accepted. On Linux (and on SPARC) there exist device
nodes with configurable geometry (`/dev/fd0', `/dev/fd1' etc), and
thus filtering is not needed (and ignored) for disk drives.
(Mtools still does do filtering on plain files (disk images) in
Linux: this is mainly intended for test purposes, as I don't have
access to a Unix which would actually need filtering).
If you do not need filtering, but want still a default geometry for
mformatting, you may switch off filtering using the `mformat_only'
flag.
If you want filtering, you should supply the `filter' flag. If you
supply a geometry, you must supply one of both flags.
initial geometry
On devices that support it (usually floppy devices), the geometry
information is also used to set the initial geometry. This initial
geometry is applied while reading the boot sector, which contains
the real geometry. If no geometry information is supplied in the
configuration file, or if the `mformat_only' flag is supplied, no
initial configuration is done.
On Linux, initial geometry is not really needed, as the
configurable devices are able to auto-detect the disk type
accurately enough (for most common formats) to read the boot
sector.
Wrong geometry information may lead to very bizarre errors. That's
why I strongly recommend that you add the `mformat_only' flag to your
drive description, unless you really need filtering or initial geometry.
The following geometry related variables are available:
`cylinders'
`tracks'
The number of cylinders. (`cylinders' is the preferred form,
`tracks' is considered obsolete)
`heads'
The number of heads (sides).
`sectors'
The number of sectors per track.
Example: the following drive section describes a 1.44M drive:
drive a:
file="/dev/fd0H1440"
fat_bits=12
cylinders=80 heads=2 sectors=18
mformat_only
The following shorthand geometry descriptions are available:
`1.44m'
high density 3 1/2 disk. Equivalent to: `fat_bits=12 cylinders=80
heads=2 sectors=18'
`1.2m'
high density 5 1/4 disk. Equivalent to: `fat_bits=12 cylinders=80
heads=2 sectors=15'
`720k'
double density 3 1/2 disk. Equivalent to: `fat_bits=12
cylinders=80 heads=2 sectors=9'
`360k'
double density 5 1/4 disk. Equivalent to: `fat_bits=12
cylinders=40 heads=2 sectors=9'
The shorthand format descriptions may be amended. For example, `360k
sectors=8' describes a 320k disk and is equivalent to: `fat_bits=12
cylinders=40 heads=2 sectors=8'
Info Catalog
mtools: location information
mtools: per drive variables
mtools: open flags