Principles of Computing and Information Technology Lecture 3 Software

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Principles of Computing and Information
Technology Lecture 3 - Software Operating
Systems

• Andy Dawson
• Department of Information Studies, UCL

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What were going to look at today

• Data representation
• How bits are given meaning
• Data structures
• How data is organised for retrieval
• Operating systems
• What are they?
• What do they do?
• A bit about system interfaces

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All software is layered

• Bit, Byte
• Field, Record
• File, Database
• Applications Program
• Different layers put stress on different aspects
  of representation.

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Data representation

• The Bit, or BInary digiT
• A bit has 2 states - 21 - 0,1
• 2 bits have 4 states - 2200, 01, 10, 11
• 3 bits have 8 states - 23000, 001, 010, 011,
  100, 101, 110, 111
• 8 bits 1 Byte
• 00000000 to 11111111
• 256 states - 28

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Data representation
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Data representation
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Data representation
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Base systems

• We work in Decimal - base 10
• Computers work in Binary - base 2
• Other handy bases for a computer are
• Octal (base 8)
• Hexadecimal (base 16)
• Decimal 225 2x100 2x10 5x1

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Base systems

• Binary 225 ?
• Octal 225 ?
• Hexadecimal 225 ?

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What can a byte represent?

• 1 byte 256 possibilities/states Which
  depend on context!

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What can a byte represent?

• 256 integers 0...255
• 128 signed integers -128...127
• 8 independent boolean statements (t/f)
• 2x4bit digits (Binary Coded Decimal)
• A set of 256 characters (e.g. EBCDIC)
• A set of 128 characters plus check bit (e.g.
  ASCII)
• A variable number of op codes/instructions
• 256 discrete locations/addresses

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Data representation

• How we assign meaning to data within the computer
• Conventionally (and arbitrarily) done
• There is no inherent meaning to data in the
  system - its all 0s and 1s until we interpret it
  in context
• Context is established at time of use
• We must know that context or we cannot use the
  data properly!

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Data Structure

• A grouping of data which is somehow connected it
  may include (or imply) operations performed on
  the data
• E.g.
• Shopping list
• Family tree
• Timetable
• Archival description or bibliographic record in
  card catalogue

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Conceptual file structure

• Field labels identifydata elements
• Field structure isconsistent
• Records have aunique identifier

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Record structure

• Records contain information relating to an
  individual item
• The information is comprised of data elements
• Each element is held in a separate field

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Example records

• C.Jamsa
• Simply DOS
• 1993
• E.Krol
• The Whole Internet again
• 2008

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Example record structure

• AU C.Jamsa 20
• TI Simply DOS
  30
• DA 1993 4
• AU E.Krol 20
• TI The Whole Internet again
  30
• DA 2008 4

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Fixed length field structure
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Variable length field structures
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What is an Operating System?

• A piece of software
• Between raw hardware and applications
• Allows user to communicate with the computer

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Aim of an Operating System

• To achieve maximum efficiency of processing by
• Resource sharing
• Eliminating intervention
• Recovering from errors
• Protecting separate jobs, processes, users
• from each other

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Types of operating system

• Single vs multi-user
• General purpose vs specialised
• Embedded systems
• Fit of operating systems to hardware and
  applications (and people!)

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Listers idealised model

• The onion skin model
• On top of hardware
• System nucleus
• Memory management
• I/O
• File access
• Scheduling and resource allocation
• User interface
• Lister, A. M Eager, R. Fundamentals of
  operating systems 5th ed.

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System Nucleus

• Major interface between HW and SW
• Set to match instruction set of HW
• Elements include
• FLIH
• Dispatcher
• We can treat this as a black box item

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Memory management

• Transformation of addresses
• relational to physical
• Movement between main memory storage
• Supervision of virtual memory

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Input/Output

• Critical to efficiency
• I/O messy and variegated
• different devices, speeds, units, permissible
  operations
• Queues and buffering
• Disk scheduling and formatting

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File access

• How files are organised on a disk
• The role of file management
• Naming
• Manipulating
• Organising
• Directory structures

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Directories and directory structure

• The concept of directory structure
• Whats in the skip
• Subdividing storage devices
• The Root directory
• Directories and subdirectories
• Trees grow upside down in computing
• Absolute and relative paths
• Some practical tips on naming and structuring
  files and directories

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Resource allocation and sharing

• Sharing limited set of resources between
  competing processes aiming for
• High level of resource utilisation
• Allowing all processes resources they need in
  reasonable time
• Need to avoid deadlock and livelock
• Mechanisms/scheduling algorithms
• first come first served, timesharing
  (equal/unequal), prioritisation

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User interface

• Raison detre?
• The command interpreter
• Primary requirements
• program execution
• I/O operations
• File system manipulation
• Error detection reporting

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Throughout - at every level

• Protection
• Prevent users doing things they shouldnt -
  accidentally or maliciously
• Security
• Preventing unauthorised use
• Reliability
• Predictability, error recovery stability

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User interface pt2

• The raison detre - but the weakest link?
• Command line interfaces
• Complexity of command languages
• The CCL myth
• Shells
• Environments

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The rise of the GUI WIMP

• Changes in the computing market
• The rise of the novice
• The soft machine concept
• Consistency
• Apple vs Microsoft
• 20 years of development unencumbered by
  progress
• Satisfying the advanced user

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The OS marketplace

• The early days IBM, Unix, CP/M, DOS, LISA,
  Windoze
• Win95/98/ME/2000, Win NT, Win XP, Vista
• Apple, Linux and the open source movement
• OS sizes and OS functionality
• Rational approaches to legacy systems

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Thats it for today...

• Any Questions?

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This weeks practicals

• Reference Manager
• Simple handout provided get full IS
  documentation online from
• http//www.ucl.ac.uk/isd/common/resources/bibliogr
  aphy