What is an Operating System?

An Operating System (OS) is system software that sits between user applications and the computer's hardware. It manages hardware resources (CPU, memory, I/O devices, storage) and provides an environment in which user programs can run conveniently and efficiently.

Formally, an OS is a resource manager and an extended/virtual machine:

• Resource manager: it decides which process gets the CPU, how memory is allocated, and which I/O request goes first.
• Extended machine: it hides messy hardware details (disk geometry, device registers) behind clean abstractions like files, processes and sockets.

Functions of an Operating System

1. Process Management — creates, schedules, suspends and terminates processes; provides synchronization and inter-process communication.
2. Memory Management — keeps track of which parts of memory are in use, allocates space to processes, and reclaims memory when freed.
3. File Management — organizes data on disk into files and directories, controls access and provides a uniform interface (open, read, write).
4. Device / I/O Management — provides device drivers, buffers, spools and schedules I/O requests so user programs need not know hardware details.
5. Security & Protection — authenticates users, controls access via permissions, isolates processes from each other.
6. Networking — implements protocol stacks (TCP/IP) so machines can communicate.
7. User Interface — supplies a CLI (shell) and/or GUI for user interaction.
8. Error Detection & Recovery — monitors hardware and software faults and takes corrective action.
9. Resource Accounting — keeps statistics of resource usage for billing or tuning.

Goals of an OS

• Convenience — make the computer easier to use.
• Efficiency — use hardware effectively.
• Ability to evolve — allow new features without disturbing existing ones.

User View vs System View

Summary

• An OS abstracts hardware and manages resources.
• Its core functions are process, memory, file, device, security, and network management.
• It exposes services through the system call interface which user programs invoke via library wrappers.