Operating System (Reference : OS[Silberschatz] + Norton 6e book slides)

What is an Operating System? No universally accepted definition. A program that acts as an intermediary between a user of a computer and the computer hardware. A program that controls the execution of application programs, hiding from them details of the hardware. Operating system goals: – Control execution of application programs – Provide services to application programs – Control the use of system resources by application programs

Four Components of a Computer System

What is an Operating System? OS is a resource allocator – Manages all resources – Decides between conflicting requests for efficient and fair resource use OS is a control program – Controls execution of programs to prevent errors and improper use of the computer

Types of Operating Systems Real-time operating system – Very fast small OS – Built into a device – Respond quickly to user input An application's process priority level may exceed that of a system process real-time OS is more frequently dedicated to a narrow set of applications – MP3 players, Medical devices

Types of Operating Systems Single user/Single tasking OS – One user works on the system – Performs one task at a time – MS-DOS and Palm OS – Take up little space on disk – Run on inexpensive computers

Types of Operating Systems Single user/Multitasking OS – User performs many tasks at once – Most common form of OS – Windows XP and OS X – Require expensive computers – Tend to be complex

Types of Operating Systems Multi user/Multitasking OS – Many users connect to one computer – Each user has a unique session – UNIX, Linux – Maintenance can be easy – Requires a powerful computer

Multi user/Multi tasking OS

Providing a User Interface User interface – How a user interacts with a computer – Require different skill sets Graphical user interface (GUI) Command Line interface (CLI)

Computer Startup bootstrap program is loaded at power-up or reboot – Typically stored in ROM, generally known as firmware – Initializes all aspects of system – Loads operating system kernel and starts execution Kernel: – “The program running at all times on the computer” – central component of an OS

Functions of Operating System Process Management Memory Management Storage Management I/O Management Protection and Security

Process Management Process – A program in execution – Instance of a program running on a computer – Something that can be assigned to and executed on a processor Process needs resources to accomplish its task – CPU – Memory – Devices Process termination requires reclaim of any reusable resources Why process management? – Many processes running at the same time – Resources are limited and valuable – I/O operations are relatively very slow

Process Management OS has to switch between different processes running at the same time OS ensures: – each process gets a share of CPU time at specific intervals – state of each process is maintained while switching Process management activities – Creating and deleting processes – Suspending and resuming processes – Providing mechanisms for process synchronization Process synchronization or serialization, strictly defined, is the application of particular mechanisms to ensure that two concurrently- executing processes do not execute specific portions of a program at the same time. If one process has begun to execute a serialized portion of the program, any other process trying to execute this portion must wait until the first process finishes.

Process Management – Providing mechanisms for process communication – Providing mechanisms for deadlock handling a set of processes is blocked waiting for requirements that cannot be satisfied

Memory Management All data and instructions should be in memory in order to execute a program Memory management activities – Keeping track of which parts of memory are currently being used and by whom – Deciding which processes and data to move into and out of memory – Allocating and deallocating memory space as needed – Protection and access control OS ensures each process: – must have enough memory to execute – can neither run into the memory space of another process, nor be run into by another process

Storage Management OS provides uniform, logical view of information storage – Abstracts physical properties to logical storage unit - file File-System management – Files usually organized into directories – Access control on most systems to determine who can access what – OS activities include Creating and deleting files and directories Mapping files onto secondary storage Backup files onto stable (non-volatile) storage media

I/O Management One purpose of OS is to hide peculiarities of hardware devices from the user I/O manager responsible for – Memory management of I/O including buffering - storing data temporarily while it is being transferred caching - storing parts of data in faster storage for performance spooling - placing jobs in a temporary working area from where the data is sent to/from the device – The most common spooling application is print spooling: documents formatted for printing are stored onto a buffer (usually an area on a disk) by a fast processor and retrieved and printed by a relatively slower printer at its own rate.documentsbuffer printer – General device-driver interface Device Driver: Program that tells the OS how to communicate with the device – Drivers for specific hardware devices

Protection and Security Protection – any mechanism for controlling access of processes or users to resources defined by the OS Security – defense of the system against internal and external attacks – Huge range, including denial-of-service, worms, viruses, identity theft, theft of service Ensures – Data integrity Data and its structure – Confidentiality – Authenticity Original data, unmodified