Operating-Systems-Notes

Operating Systems Overview

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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 predifined policies.
• In addition, it ensures that applications are isolated and protected from one another.

Operating System examples:

Desktop	Embedded devices
Microsoft Windows	Android OS
MAC OS X (BSD)	iOS
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 Flowcart

• 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

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