Operating Systems Overview

Operating Systems :

  • Direct operational resources [CPU, memory, devices]
  • Enforces working policies [Resource usage, access]
  • Mitigates difficulty of complex tasks [abstract hardware details (using system calls)]

What is an Operating System?

  • Intermediate between Hardware and Software applications
  • Hides hardware complexity (Read/write file storage, send/receive socket network)
  • Handles resource management (CPU scheduling, Memory management)
  • Provide isolation and protection (allocate different parts of memory to different applications so that applications don't overwrite other memory locations)

Operating System definition:

An Operating System is a layer of systems software that:

  • directly has privileged access to the underlying hardware;
  • hides the hardware complexity;
  • manages hardware on behalf of one or more application according to some predefined policies.
  • In addition, it ensures that applications are isolated and protected from one another.

Operating System examples:

Desktop Embedded devices
Microsoft Windows Android OS
LINUX Symbian
... ...

OS Elements

  • Abstractions (corresponds to applications that OS executes)
    • process, thread, file, socket, memory page
  • Mechanisms (on top of Abstractions)
    • create, schedule, open, write, allocate
  • Policies (how mechanisms are used to manage underlying hardware)
    • Least Recently Used (LRU) , Earliest Deadline First (EDF), etc.

Example :

Memory Management:

  • Abstractions: Memory page
  • Mechanisms: Allocate, map to a process
  • Policies: LRU

OS Design Principles

  • Seperation of mechanism and policy
    • implement flexible mechanisms to support many policies
    • e.g. LRU, LFU, random
  • Optimize for common case
    • Where will the OS be used?
    • What will the user want to execute on that machine?
    • What are the workload requirements?

User/ Kernel Protection Boundary

  • user-level => applications [underprivileged mode]
  • kernel-level => OS Kernel [privileged access, hardware access]


  • User-Kernel switch is supported by hardware.
    • using trap instructions
    • system calls like:
      • open (file)
      • send (socket)
      • malloc (memory)
    • signals

System call Flowchart


  • To make a system call, an application must:
    • write arguments
    • save relevant data ast well defined location
    • make system calls using system call number
  • In synchronous mode : wait until system call completes.

Basic OS services

  • process management
  • file management
  • device management
  • memory management
  • storage management
  • security

Linux System Calls

Task Commands
Process Control fork (); exit(); wait();
File Manipulation open(); read(); write();
Device Manipulation ioctl(); read(); write();
Information Maintenance getpid(); alarm(); sleep();
Communication pipe(); shmget(); mmap();
Protection chmod(); umask(); chown();

Linux Architecture