@node Acronyms, Interesting formats, Compile-time configuration, Top
@appendix Acronyms

@table @emph

@item 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.

@item 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
protection).

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).

@item 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.


@item 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.

@item 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.

@item DS (Double sided)
Self explanatory.

@item SS (Single sided)
Self explanatory

@item MSS (Mixed size sectors)
Mixed sector size formats are formats which use sectors of several
different sizes on a single track. @xref{Mixed size sectors}, for
details.

@item 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. @xref{2M}, for details.

@item 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.

@item rps (Rotations per second)
See above.


@item 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.

@item 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. @xref{XDF}, for details.

@item 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. @xref{XXDF}, for details.

@item 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:
@example
http://www.moria.de/~michael/floppy/floppy.ps
@end example
contains more detailed information about FM and MFM encoding.

@item 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:
@example
http://www.moria.de/~michael/floppy/floppy.ps
@end example
contains more detailed information about FM and MFM encoding.

@item kb (kilobit)
1000 bits

@item 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 controller.

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

@item KB (K-Byte)
1024 bytes. Sometimes also noted K.

@item 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.

@item 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.

@item 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 controller.

@end table
