mtools: geometry description

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