Computing Concepts

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Computing Concepts

• File Organisation and MS-DOS

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What we are going to do today

• Remember how files are organised into directories
• Features of operating systems
• How the operating system manages files on disc

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A reminder the file directory

• Directories are called folders in Windows

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A reminder

• Hierarchical directory structure
• Folders within folders
• Tree data structure

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Path a files full name

• Write down the full path of POOLS
• \LETTERS\BEGGING\POOLS

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MS Windows hierarchical directory structure
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Operating systems

• File organisation is a task for the operating
  system
• Operating system is software installed on the
  computer to make it usable without a degree in
  electronic engineering
• Early PC operating system was MS-DOS
• (Microsoft Disc Operating System)
• Disc Operating System means file organisation

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Operating systems

• Microsoft Windows family of operating systems
• Windows 95
• Windows 98
• Windows 2000
• Windows XP

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Operating systems

• Microsoft Windows has two streams
• Graphical front end to MS-DOS
• Windows 95
• Windows 98
• Not based on MS-DOS
• Windows 2000
• Windows XP
• Streams look the same but completely different
  inside

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Operating Systems

• Linux operating systems
• Red Hat Linux
• SuSE Linux
• Mandrakesoft Linux
• Competitor to Microsoft
• Based on Unix operating systems
• Recent developments provide graphical front ends

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What an operating system does

• Just like mother
• Teenager does not know how to operate toaster etc
  ..
• but mother does

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What a mother does

• Teenager makes service call
• Mother responds by controlling the device needed
• When device done it interrupts mother
• Mother responds by delivering output to teenager

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What an operating system does

• Alternative would be to have the teenager
  understand how to work all the hardware
• Not regarded by teenager as efficient!
• Computers are invented by teenagers
• Can you see where this is heading?

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What an operating system does

• Who is mother
• Operating system
• Who is the teenager
• Application program

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Tasks carried out by an operating system

• Running programs
• Managing files
• Managing memory
• Controlling peripherals
• Utility programs

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Tasks carried out by an operating system

• Running programs
• OS finds file on disc, loads it into main memory
  and launches it
• Managing files
• OS organises files on disc
• Managing memory
• OS enables programs to share memory as they run
  simultaneously

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Tasks carried out by an operating system

• Controlling peripherals
• OS knows how to work all the printers, keyboards,
  scanners etc attached to the PC
• If it doesnt then a driver is added from
  peripherals web site or CD ROM OS grows!
• Utility programs
• Games, clocks, calculators etc

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Disc Storage

• Hard disc structure
• How files are organised using MS-DOS or Windows
  FAT

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Hard disc drive

• taking the lid off
• hard aluminium discs
• coated with magnetizable material

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Diagram of hard disc

• Arms carry read/write heads

• When arm stationary, heads can read/write on
  circular band around disc

Track under read/write head

• This band is called a track

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Tracks on a disc side

• This is for an old 720kB DD disc

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Disc formats

• 1.44 MB floppy disc
• 80 tracks per side
• 18 sectors per track
• 100MB zip disc
• 3065 tracks per side
• 32 sectors per track

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File Allocation

• Operating system contains file system to save
  files on disc
• MS Windows uses two kinds of file system
• FAT
• Came in with MS-DOS
• Compatible with Linux on same computer
• NTFS
• Came in with Windows NT
• More robust and secure
• Not compatible with Linux on same computer

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FAT File Allocation

• Basic unit of disc allocation for a file is a
  cluster
• 1 cluster 2 sectors normally
• A cluster can only hold one file
• Cannot be shared by other files
• A file will occupy one or more clusters
• Uses linked list data structure

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FAT File Allocation

• Two data structures used
• Directory is a table data structure
• Table sequence of records
• One record per file or subdirectory
• File Allocation Table (FAT) is an array of
  integers
• Each element corresponds to a cluster on the disc

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• What clusters contain the file named DATA?

• 6, 2, 8
• EOF End of File
• FREE currently unoccupied
• BAD not to be used

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Adding a file

• Find first free cluster
• Linear search looking for FREE
• Fill it with file data
• If not enough room
• Get the next free cluster
• Fill it with data
• Keep going until all file data is stored