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IBM PC Family  -  Some Floppy Mathematics


"Floppy", not "Sloppy". smiley.gif

IMPORTANT:  The scope of this web page is limited to controllers and drives used in computers of the IBM PC family.


5.25" diameter floppy - Unformatted capacity

A diagram illustrating 'unformatted capacity' is here.


  [A]

Sides

 [B]

Tracks
per side		 [C]

Tracks
per floppy		 [D]

Data rate
(Kbits per second)		 [E]

Revolutions
per minute		 [F]

Revolutions
per second		 [G]

Kbits per track
(per revolution)		 [H]
Unformatted
floppy capacity
in Kbits		 [I]
Unformatted
floppy capacity
in KB		     Comments
160K floppy 1 40 40 250 300 5 50  2000  250  See here.
180K floppy 1 40 40 250 300 5 50  2000  250  
360K floppy in '360K' drive 2 40 80 250 300 5 50  4000  500  500 KB is sometimes on boxes.  See note 3.
360K floppy in '1.2M' drive 2 40 80 300 360 6 50  4000  500  See notes 1 and 2.
1.2M floppy 2 80 160 500 360 6 83.3r 13333.3r 1666.6r  1.6 MB is sometimes on boxes.
      [A] x [B]     [E] ÷ 60 [D] ÷ [F] [C] x [G] [H] ÷ 8  


[D]   Under the control of the BIOS, the floppy controller clocks the data in and out at this rate.

[E] and [F]   IBM PC family uses fixed-speed drives:  '120K/180K/320K/360K' drives always rotate at 300 RPM,  a '1.2M' drive always rotates at 360 RPM


3.5" diameter diskette - Unformatted capacity

A diagram illustrating 'unformatted capacity' is here.


  [A]

Sides

 [B]

Tracks
per side		 [C]

Tracks
per floppy		 [D]

Data rate
(Kbits per second)		 [E]

Revolutions
per minute		 [F]

Revolutions
per second		 [G]

Kbits per track
(per revolution)		 [H]
Unformatted
floppy capacity
in Kbits		 [I]
Unformatted
floppy capacity
in KB		     Comments
720K diskette in '720K' drive 2 80 160 250 300 5 50  8000 1000  "1.0MB unformatted" is often on boxes.
720K diskette in '1.44M' drive 2 80 160 250 300 5 50  8000 1000  "1.0MB unformatted" is often on boxes.
1.44M diskette 2 80 160 500 300 5 100 16000 2000  "2.0MB unformatted" is often on boxes.
      [A] x [B]     [E] ÷ 60 [D] ÷ [F] [C] x [G] [H] ÷ 8  


[D]   Under the control of the BIOS, the floppy controller clocks the data in and out at this rate.

[E] and [F]   IBM PC family uses fixed-speed drives.



Note 1 Double-stepping is required to be done by the BIOS, due to a 40-tracks-per-side floppy being in an 80-tracks-per-side drive.
That BIOS could be the motherboard BIOS, or for a 'smart' floppy controller, the controller's expansion BIOS.
Some motherboard BIOS' predate the '1.2M' drive, and therefore, do not know about double-stepping.
The 2M-XBIOS driver at here will contain double-stepping code.
   
Note 2 Reading is fine, but writing creates unreliability.  See here.
   
Note 3 • Also applies to '320K' floppies and '320K' drives.
• A photo of IBM describing a floppy drive as "320KB" is at here.
• A '320K' drive and a '360K' drive are the same thing.  If anything, IBM should have referred to them '500KB' drives instead (i.e. the unformatted capacity).
• A '320K' floppy: DOS has formatted a DSDD (2S2D) 5.25" floppy using 8 sectors per track  (introduced in DOS 1.1).
• A '360K' floppy: DOS has formatted a DSDD (2S2D) 5.25" floppy using 9 sectors per track  (introduced in DOS 2.0).