Secondary Storage: Floppy Drives Chapter 7

1
Secondary Storage Floppy DrivesChapter 7

• David Turton
• Conestoga College
• Institute of Technology and Advanced Learning
• http//www.conestogac.on.ca/dturton
• Doon 1D17, x3610

2
Secondary Storage

• Secondary storage (disk drives)
• Much slower than memory
• Do not lose data when power is turned off
• physical organisation
• how data is physically written and organised on
  the disk
• Head (platter side), sector track
• logical organisation
• how the OS and BIOS view the data
• Partitions, FAT, directories

3
Floppy Drive
Note Cable - red wire pin 1 - after twist A
drive
4
Formatting into sectors tracks

• High-density
• 80 tracks
• 18 sectors/track
• Each sector is 512 bytes
• Same for all disks
• Smallest allocation unit
• 3-byte file uses 512-byte sector
• 80 18 512
• 1,474,560 bytes
• 1,440 KB
• (1 KB 1024 bytes)

5
Read/Write Head- two heads on actuator arm- one
above, one below

• Each bit is a magnetised rectangle
• Non-magnetised space between each bit and each
  track
• 0 and 1 have different magnetic orientation
• when reading, bit induces a voltage in the
  read/write head

6
Floppy Physical Organisation

• Disk is mylar, with cobalt or iron oxide
• may have Teflon coating
• Disk has no data when first manufactured
• formatting writes F6
• division symbol ?
• bits 1111 0110

• 1st sector on each track has designated code
• high density floppy
• bits closer together
• twice magnetic strength
• not as sensitive to magnetic field
• double-density
• Could format as high
• bits will eventually affect each other

7
Logical vs Physical Organisationsectors vs
clusters

• Sector is single segment on one track
• contains 512 bytes
• BIOS manages by physical sectors
• double density
• 2 sectors/cluster
• top bottom heads
• high density
• 1 sector/cluster

• OS sees data as list of fixed-sized clusters
• smallest unit that can be read or written
• also called a file allocation unit
• list of clusters stored in file allocation table
  (FAT)

8
Formatting a floppy creates

• Tracks sectors
• writes series of ? on each track
• writes the beginning sector mark
• Boot Record (aka MBR)
• Info on disk organisation
• Bootstrap loader to load O/S, if disk bootable
• 2 copies of FAT
• 12-bit slot for each cluster
• Zeros cluster empty
• All 1's End of file
• Other subsequent cluster

• root directory
• Fixed number of entries (224)
• File name/extension
• Time/date last update
• Attributes
• Starting cluster
• quick format
• format A /q
• recreates root directory FAT
• does not write rest of disk

9
Boot Record (MBR)

• track 0, sector 1
• contains basic information
• number of sectors, sectors/cluster
• bits in each FAT record
• version of DOS/Windows used

• on diskette, MBR and boot sector are the same
• different on hard drives
• bootable disk
• MBR contains name of file to load OS

10
Root (or Main) Directory

• Contains files starting cluster
• long file names
• 1 entry for each 11 characters
• 1 entry for 8.3 DOS name
• subdirectories carry same info
• but are not limited like the root is

11
Locating a File on disknote fragmented files
have clusters which are not contiguous
12
Problems with Floppy Drives

• Inexpensive (C14.00)
• impractical to repair
• alignment symptom
• disk written by one drive cannot be read by
  another

13
Diskette Errors

• invalid or missing Command.com
• 2 hidden files ok..
• Incorrect DOS version
• format command mismatch
• not ready reading drive A
• disk not readable or incorrectly inserted
• track 0 bad, disk not usable
• formatting a disk of wrong type
• write-protect error writing drive A
• close write-protect window

• POST 600-series errors
• general fault reading drive A abort retry..
• faulty drive
• unformatted disk
• non-system disk or disk error
• not a bootable disk
• invalid drive specification
• cant find drive because it failed POST

14
Replacing a Diskette
Notes -coloured edge of cable is pin 1 -drive A
is after twist