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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. See section Mformat for details.
- filtering
-
On some Unices 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. See section Supplying multiple descriptions for a drive 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).
- initial geometry
-
The geometry information (if available) is also used to set the
initial geometry on configurable device nodes. This initial geometry
is used to read the boot sector, which contains the real geometry. If
no geometry information is supplied in the configuration file, no
initial configuration is done. On Linux, this is not really needed
either, 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 don't use geometry configuration unless you
actually need it.
The following geometry related variables are available:
cylinders
-
cylinders
-
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
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
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