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_SD (Single density)_
     Single density disks use a data transfer rate of 125 kb/s and are
     no longer in use today, because of their low capacity.

_DD (Double density)_
     Double density disks normally hold 360KB (5 1/4) or 720KB (3 1/2)
     of data per disk.

     Double density disks have only a hole on one side (for write

     Double density uses a data transfer rate of 250 kb/s or 300 kb/s
     depending on the drive type: 5 1/4 high density drives use 300
     kb/s when writing to a double density diskm 250 kb/s is used in
     all other circumstances. The reason why double density disks in
     high density drives need a higher data transfer rate is because
     these drives rotate faster (360 rpm instead of 300 rpm).

_HD (High density)_
     High density disks normally hold 1200KB (5 1/4) or 1440KB (3 1/2)
     of data per disk.  High density 3 1/2 disks are marked as such by
     the presence of a second square hole, just opposed to the write
     protect hole.  3 1/2 high density disks are the most commonly used
     type of disks today.

_QD (Quad density)_
     Quad density is a hybrid between double and high density.  It only
     exists for 5 1/4 disks, and holds 720KB of data.  It can be
     obtained by formatting DD disks in a HD drive.  QD uses double
     density for the density along the tracks (data transfer rate), and
     high density for the density perpendicular to the tracks (spacing
     between tracks, and thus number of tracks).  This came to
     existence because these two aspects are limited by two different
     factors: the density along the track is limited by the quality of
     the media, whereas the density perpendicular to the tracks is
     mainly limited by the drive mechanism (this density, expressed in
     bits per inch comes nowhere near the limits of the media, even with
     HD).  Thus quad density is an easy way to double the capacity of an
     ordinary double density disk, just by formatting it in a HD drive.

_ED (Extra density)_
     Extra density refers to a disk density that can normally hold
     2880KB of data per disk.  Extra density disks only exist as 3 1/2
     disks.  ED disks are marked with a second squared hole opposed to
     the square hole, which is a little bit closer to the middle of the
     edge than that of HD disks.  This format never really took off,
     because it only was released when storage media with a much higher
     capacity, such as CD-Roms, tapes and Zip disks became popular.

     ED uses a data transfer rate of 250 kb/s.

_DS (Double sided)_
     Self explanatory.

_SS (Single sided)_
     Self explanatory

_MSS (Mixed size sectors)_
     Mixed sector size formats are formats which use sectors of several
     different sizes on a single track. Note: Mixed size sectors for

_2M (2 Megabytes)_
     2M is a high capacity format developped by Ciriaco de Celis.  The
     basic principle is the same as MSS: mix sectors of several sizes
     on a same track, in order to minimize both slack space and header
     overhead.  2M is different from MSS in that it uses a normal 18
     sector format on its first track. Note: 2M for details.

_rpm (Rotations per minute)_
     All 3 1/2 drives and 5 1/4 DD drives run at 300 rotations per
     minute, whereas 5 1/4 HD drives run at 360 rotations per minute.

_rps (Rotations per second)_
     See above.

_tpi (tracks per inch)_
     Expresses how close cylinders are to each other.  Usually, 5 1/4
     double density disks have 48 tpi, whereas 5 1/4 high density and
     quad density disks have 96 tpi.  3 1/2 disks use 135.5 tpi.

_XDF (eXtended Density Format)_
     XDF is a disk format used for the OS/2 distribution disks.  Its
     operating systems are similar to 2M and MSS disk, but it is faster
     due to a more creative arrangement of sectors. Note: XDF for

_XXDF (eXtended XDF)_
     XXDF is an Linux enhancement for XDF. It can store 1992 KB of data
     on an ED disk instead of just 1840 available with the regular XDF
     format. Note: XXDF for details.

_MFM (Multi Frequency Modulation)_
     MFM is a low level encoding of disk data. It is used for DD, HD
     and ED disks, i.e. virtually all disks that are available today.
     The PC hardware can only read MFM and FM disks.  The doc at:

     contains more detailed information about FM and MFM encoding.

_FM (Frequency modulation)_
     FM is a low level encoding of disk data. It was used for SD disks,
     and is now considered to be obsolete.  The doc at:

     contains more detailed information about FM and MFM encoding.

_kb (kilobit)_
     1000 bits

_kb/s (kilobit per second)_
     We express the raw data throughput to and from the disk in this
     unit, which is also used in the documentation of the floppy disk

     Byte. A byte is 8 bits, and is the smallest individually
     addressable unit of data.

_KB (K-Byte)_
     1024 bytes. Sometimes also noted K.

_KB/s (K-Byte)_
     We express the usable data throughput to and from the disk in KB/s.
     Roughly, 1 KB/s = 8 kb/s. However, the usable data throughput is
     always lower than the raw throughput due to header overhead,
     interleaving and seek overhead.

_MB (Megabyte)_
     Initially, 1 megabyte was 1024*1024 bytes (i.e. 1048576 bytes).
     However, when talking of floppy disk capacity, we understand it as
     1000KB, that is 1000*1024 bytes, i.e. 1024000 bytes.

_MB/s (million bytes per second)_
     We express (high) raw data throughput to and from the disk in kb/s,
     which is also used in the documentation of the floppy disk

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