Operating Systems

The layer that organises raw storage into named files and directories, manages free space, and enforces permissions.

The system software that manages hardware and provides services and abstractions to all other programs.

A running instance of a program, with its own memory, file handles, and CPU time — the unit the operating system schedules and isolates.

The privileged core of an operating system — the only software that talks to hardware directly and the gatekeeper that protects every program from every other.

The part of the OS kernel that decides which thread runs on each CPU at any moment — balancing fairness, responsiveness, and throughput.

A single line of execution inside a process — the unit the CPU actually runs. A process can have many threads that share memory.

A technique that gives each process the illusion of a private, contiguous memory space — built from page tables that map virtual addresses to physical RAM.

The program that reads the commands you type, expands and parses them, and asks the OS to run them — the text interface between a user and the operating system.

The act of saving one thread's CPU state and loading another's so the same CPU core can run a different thread next.

A long-running background process with no controlling terminal, usually started at boot and supervised by the system, that quietly does work or waits for requests.

A state where two or more threads are each waiting for the other to release a lock — so none of them ever make progress.

The on-disk record a Unix-style file system keeps for each file — everything about it except its name and its contents.

A hardware signal that pauses the CPU mid-instruction so the OS can react to an event — a keystroke, a packet, a timer tick.

A synchronization primitive that ensures only one thread at a time can hold it — the basic tool for protecting shared state from data races.

The mechanism that moves fixed-size pages of memory between RAM and disk on demand — the engine inside virtual memory.

The mechanism a user-space program uses to ask the OS kernel to do something privileged — open a file, send a packet, allocate memory, fork a process.

A resource management optimisation where copying is deferred until modification — shared pages are kept as a single copy until one party writes, triggering a private copy only then.

Three dominant Unix file systems with different design philosophies — ext4 is stable and simple; ZFS is enterprise-grade with built-in integrity and pooled storage; Btrfs brings COW snapshots and checksums to Linux with a familiar interface.

The central design trade-off in operating system kernels — whether OS services run in privileged kernel space (monolithic) or as isolated user-space servers (microkernel), with different reliability, performance, and security implications.

An operating system that guarantees tasks complete within specified time bounds — critical for embedded systems, robotics, and safety-critical applications where missing a deadline is as bad as an incorrect result.

A signalling mechanism that controls access to shared resources by maintaining a counter — threads decrement it to acquire a resource and increment it to release, blocking when the count reaches zero.

Running multiple isolated operating systems simultaneously on a single physical machine by intercepting privileged operations and presenting each OS with a virtual hardware interface.

A security model where authority to access a resource is represented by an unforgeable token (a capability) that must be explicitly held and passed — removing the need for centralised access control tables and preventing ambient authority vulnerabilities.