Introduction to Operating System

1.1 What is Operating System? An operating system is a program that manages the computer hardware. It also provides a basis for application programs and acts as an intermediary between the computer user and computer hardware.

Types of Operating System Single Program OS A Single program to execute at a time Concurrent OS More than one program concurrently Time Sharing OS Multiple Users can access a single Computer System Multiprocessing OS Multiple program executed by multiple processors Virtual Storage OS Secondary Storage device as an extension of primary storage Real Time OS Computer to control other machines or process

What do Operating System do? The computer system can be divided roughly into four components: 1. The hardware 2. The operating system 3. The application software 4. The Users

USER View of OS Ease to use (Home PCs) Efficient resource utilization (Work stations)

SYSTEM view of OS OS is a resource allocator Manages all resources (CPU time, memory space, storage space, I/O and so on) 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

Computer Startup Bootstrap program is loaded at power up or reboot Typically stored in ROM, generally known as firmware Initialize all aspects of system Loads operating system kernel and starts execution

Process management A process is a program in execution. It is a unit of work within the system. Process needs resources to accomplish its task CPU, Memory, files, I/O devices

Process management The operating system is responsible for the following activities in connection with process management: Creating and deleting both user and system processes Suspending and resuming processes Providing mechanisms for process synchronization Providing mechanisms for process communication Providing mechanisms for deadlock handling

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

Storage Management OS provides uniform, logical view of information storage file Each medium is controlled by device (i.e., disk drive, tape drive)

Storage Management File-System management Files usually organized into directories Access control on most systems to determine who can access what OS activities include 1. Creating and deleting files and directories 2. Primitives to manipulate files and dirs 3. Mapping files onto secondary storage 4. Backup files onto stable (non-volatile) storage media

Mass-Storage Management Usually disks used to store data that does not fit in main memory or data that must be kept for a “long” period of time. OS activities 1. Free-space management 2. Storage allocation 3. Disk scheduling 4. Tertiary storage includes optical storage, magnetic tape 5. Varies between WORM (write-once, read-many- times) and RW (read-write)

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 even when protection is activated (including denial-of-service, worms, viruses, identity theft, theft of service)

Protection and Security Systems generally first distinguish among users, to determine who can do what User identities (user IDs, security IDs) include name and associated number, one per user User ID then associated with all files, processes of that user to determine access control Group identifier (group ID) allows set of users to be defined and controls managed, then also associated with each process, file Privilege escalation allows user to change to effective ID with more rights

Operating System Services User interface Program execution File-system manipulation Communication Error detection Resource allocation Accounting Protection and security