What is a process and how does context-switching work?

A process is the OS's abstraction of a running program; rapidly saving one process's state and loading another's (a context switch) fakes parallelism on one core.

* A context switch saves the running process's state to its control block and restores another's, an operation made costly by TLB flushes and lost cache warmth. *

Each process gets its own address space and the illusion of owning the machine. On a single core the OS interleaves them so quickly that they appear to run at once, switching whenever a timer fires or a process blocks waiting on I/O.

Process = running program with:

• Its own address space
• Illusion of exclusive hardware access

Concurrency types:

• Multi-core: True parallelism (multiple CPUs)
• Single-core: Apparent parallelism via context-switching

Context = everything needed to resume:

• Program Counter (PC)
• Register file contents
• Memory contents
• Open files

Context switch triggers:

• Periodically (timer interrupt)
• When process waits (I/O, sleep)

Threads: Lightweight processes sharing address space - easier data sharing, more efficient scheduling.

Go deeper:

• Context switch (Wikipedia) — saving one process/thread's state and restoring another's, plus what triggers it.
• Process (computing) (Wikipedia) — what a process is and its running/ready/blocked lifecycle.
• Thread (computing) (Wikipedia) — threads share a process's address space, so switching between them is cheaper.