Operating Systems - Computing System - Lecture Slides, Slides of Computer Science

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CT101: Computing Systems

Introduction to Operating Systems

Contents

• The History of Operating Systems
• Operating System Architecture
• Coordinating the Machine’s Activities
• Handling Competition Among Processes
• Security

Evolution of Shared Computing

• Batch processing
• Interactive processing
  • Requires real-time processing
• Time-sharing/Multitasking
  • Implemented by Multiprogramming
• Multiprocessor machines

Time Sharing / Multitasking

• Users seeking services from same machine at the same time – time sharing - Implemented using a technique called multiprogramming (time is divided into multiple intervals, execution of one job is limited to a single time interval)
• Multiple terminals connected to same machine
  • Driven by the fact that in the past computers were very expensive
• When multiprogramming is applied to single-user

environments is usually called multitasking

Embedded Operating Systems

• Used in hand held devices (PDAs), mobile

phones, cars, etc…

• Limited data storage and power

conservation are the big challenges

• Examples: VxWorks, Windows CE (Pocket

PC), Palm OS, Symbinan, ThredX,

RomDOS, etc…

Types of Software

• Application software
  • Performs specific tasks for users: spreadsheets, database systems, desktop publishing, program development, games, etc…
• System software
  • Provides infrastructure for application software
  • Consists of operating system and utility software

File Manager

• Role – coordinate the use of machine’s

mass storage facilities

• Hierarchical organization
  • Directory (or Folder ): A user-created bundle of files and other directories (subdirectories)
  • Directory Path: A sequence of directories within directories
• Access/operations to files is provided by

file manager via a file descriptor

Device Manager

• Part of OS presented as a collection of device drivers – specialized software that communicate with the controllers to carry out operations on peripheral devices connected to the computer
• Each driver is specifically designed for its type of device (e.g. printer, monitor, etc..) and translates generic requests into device specific sequence of operations

Processes

• Process: The activity of executing a program (NOT THE SAME THING AS A PROGRAM!!!) - Program – static set of directions (instructions) - Process – dynamic entity whose properties change as time progresses. It is an instance in execution of a program.
• Process State: Current status of the activity
  • Program counter
  • General purpose registers
  • Related portion of main memory

Process Manager

• Scheduler – the part of kernel in charge with the strategy for allocation/de-allocation of the CPU to each competing process - Maintains a record of all processes in the OS (via a process table ), introduces new processes to this pool and removes the ones that completed
• Dispatcher is the component of the kernel that overseas the execution of the scheduled processes - Achieved by multiprogramming

Dispatcher

• Dispatcher: Controls the allocation of CPU (of time slices) to the processes in the process table - The end of a time slice is signaled by an interrupt. - Each process is allowed to execute for one time slice
• It performs “ process switch ” – procedure to change from one process to another - ProcessA Dispatcher ProcessB

Handling Competition for

Resources

• Important task of OS is to allocate resources to the processes
• Semaphore: A “control flag”
• Critical Region: A group of instructions that should be executed by only one process at a time
• Mutual exclusion: Requirement for proper implementation of a critical region so that only one process at a time will execute the sequence of instructions part of a critical region

Security

• One of the role of OS is to provide security
• Attacks from outside
  • Problems
    • Insecure passwords
    • Sniffing software
  • Counter measures
    • Auditing software
  • Example:
    • SW that would impersonate the Operating System’s user login screen

Security (continued)

• Attacks from within
  • Problem: Unruly processes
  • Counter measures: Control process activities via privileged modes and privileged instructions
  • Examples on attacker SW:
    • Alters the timer of OS – extend its own time slice and dominate the machine
    • Access to peripheral devices directly – access to files that otherwise access would have been denied
    • Access memory cells outside its allowed area, it can read and alter data from other processes