Floppy Disk size (in BIOS)

[adholioshake]

Assuming that a sector on a floppy disk is 512 bytes long and the floppy is a standard 3.5" disk:
- How many sectors are there per head ?
- How many heads per cylinder(or track??) ?
- How many cylinders on the disk ?

 

[adholioshake]

I found an interrupt that told me:

  mov ah,08h ; get params...
  mov dl,00h ; ...for floppy drive
  int 13h

This gives the following results (in decimal):
max. sector = 18
max. cylinder = 79 (80 altogether inc. 0)
max. head = 1 (2 inc. 0)
This appears to be coherent for a 1.44Mb floppy:

512 x 18 x 80 x 2 = 1474560 bytes
(/1024) = 1440 kb
(/1000) = 1.44 Mb

Just to check - is the following diagram (of a floppy, as best as I can do...) correct or am I mistaken as to what cylinder, sector & head are:

 #====================#
 !        oooo        !
 !      oo  : oo      !
 !    oo    :   oo    ! <-sector=between..
 !  oo      :     oo  !
 !  o       :......o  !
 !  o      ~       o  !
 !  oo    ~       oo  ! <-cylinder=along ~
 !    oo ~      oo    !
 !      oo    oo      ! <-head=opposite
 !        oooo        !      side of disk
 #====================#

 

[Bertv100]

The names are logically assigned, so head refers to the read/write head and for a floppy there are two of those (because a floppy has two sides). A cylinder is a tube, in this case the edge of it: if you place 80 tight fitting cylinders in each other and then take a slice out, you have a disc consisting of 80 cylinders i.e. something like a floppy disk. A sector is a centerpoint angle of a disc. If a disc has 18 sectors, each sector has an angle of 20°.

Regards,
Bert Vingerhoets
vingerhoetsbert@hotmail.com

http://student.vub.ac.be/~bvingerh/

Don't worry what people think about you. They're too busy wondering what you think about them.

 

[Bertv100]

In conclusion: you were right about sector and head, but I think not about cylinder.
Visualization using the ASCII Art Standard: ;-)

 #====================#
 !        oooo        !
 !      oo  : oo      !
 !    oo   ~:   oo    ! <-sector=between..
 !  oo   ~  : ~   oo  !
 !  o  ~    :......o  !
 !  o  ~        ~  o  !
 !  oo   ~    ~   oo  ! <-cylinder=along ~
 !    oo   ~~   oo    !
 !      oo    oo      ! <-head=opposite
 !        oooo        !      side of disk
 #====================#

Regards,
Bert Vingerhoets
vingerhoetsbert@hotmail.com

http://student.vub.ac.be/~bvingerh/

Don't worry what people think about you. They're too busy wondering what you think about them.

 

[adholioshake]