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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" floppy - Unformatted capacity

A diagram illustrating 'unformatted capacity' is here.

    = Controller dictates this
    = Drive dictates this

  [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
360K floppy in 360K drive 2 40 80 250 300 5 50  4000  500  500 KB is sometimes on boxes
360K floppy in 1.2M drive 2 40 80 300 360 6 50  4000  500  See note 1
1.2M floppy in 1.2M drive 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:  '180K' and '360K' drives always rotate at 300 RPM,  '1.2M' drive always rotates at 360 RPM


3.5" diskette - Unformatted capacity

A diagram illustrating 'unformatted capacity' is here.

    = Controller dictates this
    = Drive dictates this

  [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 floppy in 720K drive 2 80 160 250 300 5 50  8000 1000  "1.0MB unformatted" is common on boxes
720K floppy in 1.44M drive 2 80 160 250 300 5 50  8000 1000  "1.0MB unformatted" is common on boxes
1.44M floppy in 1.44M drive 2 80 160 500 300 5 100 16000 2000  "2.0MB unformatted" is common 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 by BIOS due to a 40-tracks-per-side floppy being in an 80-tracks-per-side drive.