Floppy Disk Drive (PDF)

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					                                  PPT/2K804/04
                    Revision no.: PPT/2K403/02




Floppy Disk Drive
                                                                                                                                                          Revision no.: PPT/2K804/04



Introduction


Common devices used

for most data storage

•   Magnetic tape

•   Floppy disk

•   Hard disk

•   Optical disk like CD

    ROMs.


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                                                                                                                                                           Revision no.: PPT/2K804/04



Floppy disk and its structure

• 3.5 Inches 1.44 MB floppy disk

• The disk is soft, thin and round made of Mylar or some other
  plastics material coated with a magnetic medium.

• It is this magnetic medium that stores information in the digital
  form (0’s and 1’s).

• The entire disk is covered by a hard plastic cover that protects
  it from external damage that could be in the form of moisture,
  dust or other magnetic fields.

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Floppy disk and its structure (contd.)


• This hard plastic cover has a small rectangular opening at one

  corner that has a small plastic tab fitted in it. This is called the

  "write protect" of a floppy.

• When the diskette is write protected a user cannot write or

  erase any information on the diskette. He can only read from it.

• The floppy disk is organized into concentric circles which are

  called tracks.


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Floppy disk and its structure (contd.)

• Each track is divided into pie shaped sections
  called sectors.

• Group of sectors are called as Block

• Group of Blocks are called as Clusters.


Boot sector, File allocation table
(FAT), Root directory structure




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Floppy disk and its structure (contd.)

• All sectors hold the same amount of data i.e. 512 bytes.

• For storing data space is allocated as needed in groups of
    sectors called clusters.

• A floppy disk, also called diskette's, has two sides.

•   Data can be written to or read from either side.

• The first side is identified as side 0, and the second as side 1.

• The mechanism that actually performs the reading and writing
    operation is called the read/write head.

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Floppy disk and its structure (contd.)


• The disk is logically divided into system area and data area

• Physically the system area starts from sector 1 track0 and

  continues on to some part of track1.

• The data area follows after the system area.

• The operating system uses the system area

• The data area is used to actually store the data.

• Data area is larger than the system area.


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Floppy disk and its structure (contd.)

• The system area is further sub-divided into the following

• Boot sector consist of
   – Bootstrap routine

   – BIOS parameter block (BPB).

• The bootstrap routine checks to see if the disk contains a copy
  of the two system files, IO.sys and Msdos.sys.

• The bootstrap reads these files into the memory and transfers
  the control to the IO.sys.

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Floppy disk and its structure (contd.)


• The BPB holds the information about the disk’s physical

    characteristics which is used by the device driver.

• FAT is the part of the system area that the DOS uses to keep

    the track of files stored in the disk.

• FAT is so important that DOS creates two copies of a FAT.

•   If one gets corrupted DOS uses the second copy.

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     Information contained in the BIOS parameter block
                     in the boot sector.




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                                                                                                                                                         Revision no.: PPT/2K804/04
Illustration of how FAT entries are
chained together

                                                              •          File A starts in cluster2 and then
                                                                         continues in cluster8.
                                                              •          The entry of cluster8 points to
                                                                         cluster11, which in turn points to
                                                                         cluster12, where the file ends.
                                                              •          This file is split-up into four clusters
                                                              •          Three of which are not in sequence.
                                                              •          File B is less fragmented, occupying
                                                                         clusters 3, 4, 5, 6, 7,9 and 10.
                                                              •          The entry for cluster7 points to cluster
                                                                         9, where the file continues. Cluster9
                                                                         points to 10 which contains the EOF
                                                                         value.
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Illustration of how FAT entries are
chained together (contd.)

Various entries which contain the following information about
files and subdirectories in the root directory follows after FAT.
File name                                                                         8 bytes
Attribute                                                                         1 byte
Time of creation                                                                  2 bytes
                                                                                                                    Note :
Starting FAT entry                                                                2 bytes
                                                                                                                    The Capacity of a
Extension                                                                         3 bytes                           floppy disk can be
Reserved                                                                          10 bytes                          calculated as :-
Date of creation                                                                  2 bytes                           No. of HEADS x
                                                                                                                    No. of TRACKS x
File size                                                                         4 bytes                           No. of Sector x
                                                                                                                    No. of Bytes/sector.

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Illustration of how FAT entries are
chained together (contd.)
                       Different types of diskettes used in PCs.




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Mechanical parts of a floppy disk drive

• Read/Write Head

• The read/write is built around a core of soft iron.

• A coil of wire is wrapped around this core.

• This coil is actually tapped in the center, so that it behaves as
  if it were two separate coils.

• When the control circuits want to write something on the
  diskettes, they send a current through these coils.

• This creates electro-magnetic lines of force in the soft iron
  material.
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Mechanical parts of a floppy disk drive (contd.)


• The “air gap” at the bottom of the core acts to focus this force

  on the oxide layer on the surface of the diskette.

• This creates a magnetized spot on the oxide layer.

• When the control circuits want to read from the disk, the

  process is reversed.

• As the magnetized spots pass under the head they induce

  lines of force in the soft iron core, which then create tiny

  current in the coils of wire.
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Mechanical parts of a floppy disk drive (contd.)

                                      Head Loading Mechanism




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The 3. 5 inch drive clamping mechanism

• Hold the disk assembly firmly in position.

• First, slides the disk onto the drive.

• A metal plate is suspended above the disk assembly.

• When the disk is inserted, this plate is lowered, clamping the
  hub of the disk against the top of the drive motor.

• The motor-control circuit turns the motor on and off.

• As the motor turns, a sensor detects its speed and reports
  back to the control circuit.

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The 3. 5 inch drive clamping mechanism (contd.)

                                                          Disk clamping




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The Drive Control Circuitry

• Designed around a large IC called a Floppy Disk controller IC.
    (IC -765)
• The FDC is connected to the data bus of the main computer.
•   When the computer wants to write something onto the disk, it
    places the data onto the data bus and strobes the Write
    Enable line.
• The FDC converts the 8-bit parallel data to stream of serial bits
    on the Write Data line.
• The computer strobes the Read Enable line when it wants to
    read from the disk.

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The Drive Control Circuitry (contd.)

    Signal description for FDC to FDD interface.




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The Drive Control Circuitry (contd.)

• Sensors

• Write-Protect
   – Prevents the drive from writing to a disk when a disk is Write-
       protected .

   – Usually, the drive includes a switch which can sense the status of
       a Write-protect opening on the case of the disk.

• Track 0 detector
   – It is tripped when the head is reading the outermost track on the
       disk.
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The Drive Control Circuitry (contd.)

• Index sensor
   – Generates a pulse each time the disk completes a revolution.


• Disk change sensor

   – Used to tell the control circuits when a disk has been inserted into
      the disk drive.




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Floppy Disk Drive Cable

• All floppy disk drives are connected to the motherboard's
  external data bus by a 34-lead ribbon cable.
• Early PC BIOS logic was developed to recognize one or two
  floppy disk drives.
• Floppy disk drive is installed on the end connector (near the
  twist) is logically designated as the first or primary or A drive
  by the BIOS.and
• The drive attached in the middle of the cable is always the
  secondary or B drive. The BIOS will not recognize a B drive
  unless an A drive is physically installed.

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Floppy Disk Drive Cable (contd.)

• If you ever run into a situation with a two-floppy system where
  you want to turn the A: drive into the B: drive or vice versa,
  you can accomplish this easily through CMOS settings.




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Troubleshooting Floppy Disk Drive

• To resolve a problem when a floppy disk drive stops working,
  the steps are
   – Check the floppy disk.

   – Check for data errors on the disk.

   – Check the CMOS setting.

   – Check the floppy controller.

   – Check the cable.

   – Replace the floppy drive.


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Creation of basic boot disk in Windows 98

• One can create the boot disk graphically or by using the
  command prompt.

• Using the command prompt
   – Insert a floppy disk into the drive.

   – Give the command
       FORMAT A: /S

• Graphically it can be done in Windows 98 as
   – Open the Control Panel, click Add/Remove programs, choose
       Startup Disk tab, and select the Create Disk Option.

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DOS Errors




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