What is an OS; its functions

An operating system (OS) is system software that manages the computer hardware and provides a convenient, efficient environment in which users and application programs can run. It acts as an interface between the user and the hardware and as the manager of all resources (CPU, memory, files, I/O devices).

Functions of an operating system

1. Process management: creates and deletes processes, schedules the CPU, suspends and resumes processes, and provides process synchronization and communication.
2. Memory management: keeps track of which memory parts are in use and by whom, allocates and de-allocates memory, and handles swapping and virtual memory.
3. File management: creates, deletes, reads, writes and protects files and directories, and maps files onto secondary storage.
4. Device (I/O) management: hides hardware details through device drivers, buffering, caching and spooling, and schedules I/O requests.
5. Secondary-storage management: free-space management, disk allocation and disk scheduling.
6. Protection and security: controls access of programs and users to resources, authenticates users and guards against unauthorised use.
7. User interface / command interpretation: provides a shell (CLI) or a GUI so the user can give commands.
8. Error detection and handling: detects errors in the CPU, memory, I/O devices or user programs and takes suitable action.
9. Resource allocation and accounting: decides who gets which resource and records usage for billing and statistics.
10. Networking: lets machines communicate and share resources.

Overall, the OS works as an extended machine (it hides the ugly hardware and gives simple abstractions such as files and processes) and as a resource manager (it shares the hardware fairly and safely among competing programs).