Floppy Disk Drives

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Floppy Disk Drives
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Cylinders

• The set of tracks on a disk that are on each side
  of all the disk platters in a stack and are the
  same distance from the center of the disk

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Sectors

• A section of one track defined with
  identification markings and an identification
  number. Most sectors hold 512 bytes of data

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Track

• One of the many concentric circles that holds
  data on a disk surface. Consists of a single
  line of magnetic flux changes and is divided into
  some number of 512-byte sectors.

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Floppy Disks

• Floppy disks organize information in tracks and
  sectors, just like the hard drive

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Formatting the floppy disk

• Hard drive and floppy formatted
• Formatting the disk is like adding lines to paper
  so you can write straight

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• Formatting the disk writes the information that
  the operating system needs to maintain a
  directory and file table of contents
• On a floppy, there is no distinction between
  high-level and low-level and you dont have to
  create any partitions such as on a hard drive

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When you format a floppy

• The operating system reserves the track nearest
  to the outside edge of a disk (track 0) almost
  entirely for its purposes
• Track 0, Side 0, Sector 1 contains the DOS Boot
  Record (DBR) or Boot Sector, that the system
  needs to begin operation

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• The next few sectors contain the FATs, which
  keeps records of which clusters or allocation
  units on the disk contains information and which
  are empty
• Finally, the next few sectors contain the root
  directory - where the operating system stores
  information about the names and starting
  locations of the files on the disk

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What is a cluster or allocation unit?

• Page 781
• Refer to FAT 16 and FAT 32 chart in workbook
• Table 12.12 shows Default Cluster Allocation
  unit sizes

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Cluster

• Allocation unit in DOS 4.0 or higher
• A single cluster is the smallest unit of the disk
  that DOS can allocate when it writes a file
• Contains one or more sectors

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Cluster

• Having more than one sector per cluster reduces
  the FAT size and enables DOS to run faster
  because it has fewer individual clusters to
  manage
• Trade off wasted disk space

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How does the drive know if the disk has been
changed?

• Diskette Changeline
• The standard PC floppy controller drive use a
  special signal on pin 34 called Diskette
  Changeline to determine whether the disk has
  beden changed, or more accurately, whether the
  same disk loaded during the previous disk access
  is still in the drive.

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Disk change

• Pulsed signal that changes a status register in
  the controller to let the system know that a disk
  has been either inserted or ejected

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Why understand this?

• If the disk change signal is not received before
  the next access, the system can assume that the
  same disk is still in the drive. Any information
  read in to memory during the previous access can
  therefore be reused without rereading the disk.

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Types of floppy drives

• Older examples - not covered
• 3 1/2 inch 1.44 MB most popular
• First appeared from IBM in the PS/2 product line
  introduced in 1987
• Holds 1.2 times as much data as the 5 1/4 inch
  floppy

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Controller

• Can support 3 1/2 and 5 1/4 inch drivews running
  at the 500 KHz rate
• The data rates used by both of these HD drives
  are identical and compatible with the same
  controllers

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• 1.44MB does no account for the area that FAT file
  system reserves for file management. Actual
  file-storage area is 1.42 MB

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Floppy disk construction

• Refer to the diagram on page 788-789

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5 1/4 inch

• Large, round hole in the center
• When you close the disk drive door, a cone-shaped
  clamp grabs centers the disk through the center
  hole
• Index hole - small hole in the disk itself. This
  is the starting point for all the sectors on the
  disk
• Rite/enable notch

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3 1/2 inch

• Rigid, plastic case helps stablize the magnetic
  medium inside
• allows for greater amounts of data densities
• Metal shutter is removable covers the magnetic
  surface access
• Uses metal center hub with an alignment hole

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• The drive grasps the metal hub and the hole in
  the hub enables the drive to position the disk
  properly
• Write-protect/enable slider. Depending on the
  position of the slider, you can write to the disk
  or protect it from being written on.
• Troubleshooting
• Viruses

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Caring for and Handling Floppy Disks - Drive
Installation Procedures

• Read page 790 - 793