Multitasking and Context Switching Explained — GCSE Computer Science

What is a process?

A process is a program in the act of running. When you open a web browser, a media player, and a word processor at the same time, the operating system creates three separate processes. Each process has its own:

• section of memory (code, data, stack),
• set of CPU register values (the current state of computation),
• process ID (PID) — a unique number assigned by the OS,
• current state: running, ready, or waiting (blocked).

A process is not the same as the program file on disk — the file is static instructions; the process is the live, executing instance of those instructions.

What is context switching?

A context switch is the mechanism by which the CPU moves from executing one process to another. The CPU can only truly execute one process at a time on a single core, so the operating system's scheduler decides which process runs and for how long.

When it is time to switch:

1. The OS saves the current process's state — all register values, the program counter, and other data — into a data structure called the Process Control Block (PCB).
2. The scheduler selects the next process to run from the ready queue.
3. The OS loads the saved state from that process's PCB into the CPU registers.
4. Execution resumes from exactly where that process was last paused.

This save-and-restore cycle is a context switch.

What is the Process Control Block?

The Process Control Block (PCB) is a record maintained by the OS for every active process. It holds everything needed to pause and later resume a process:

Think of the PCB as a bookmark — it records exactly where a book was left open (the program counter) together with notes on everything the reader had in mind (register values), so the book can be picked up and continued perfectly later.

What is time slicing?

Time slicing (also called time-sharing) is the technique of dividing CPU time into small fixed intervals called time quanta (typically 10–100 milliseconds). The scheduler gives each ready process a time quantum; when the quantum expires, a context switch occurs and the next process gets its turn.

Because each switch takes only microseconds and the human eye cannot distinguish changes faster than about 50 ms, all three programs appear to run simultaneously — that is the illusion of multitasking.

What is the difference between multitasking and parallel processing?

A modern computer does both: each core multitasks (running many processes per core through time slicing) and multiple cores genuinely run different processes in parallel. At GCSE, multitasking and context switching refer to the single-core concurrent model.

Why does context switching have a cost?

Saving and loading the PCB takes CPU cycles that could otherwise be doing useful work — this is called overhead. If time quanta are very short, the CPU spends a disproportionate fraction of its time context-switching rather than running processes. Conversely, if quanta are too long, interactive programmes feel sluggish because a user's keystroke might wait a full second before the relevant process gets CPU time. The OS scheduler is designed to balance these competing demands.

Frequently asked questions

How does the operating system decide which process runs next?

The scheduler uses a scheduling algorithm to pick the next process from the ready queue. Common algorithms include:

• Round Robin — processes take turns in a fixed cycle (related to time slicing above),
• Priority scheduling — higher-priority processes run first,
• Shortest Job First (SJF) — the process estimated to take the least time runs next. At GCSE, round-robin and priority scheduling are the most commonly examined examples.

What is the difference between a process and a thread?

A thread is a lightweight unit of execution within a single process. One process can contain multiple threads that share the same memory space. Context switching between threads of the same process is faster than between different processes because no memory address space needs to change. Multi-threading is used in applications such as web browsers where one thread handles downloads while another handles the user interface.

Can context switching cause bugs?

Yes. In multi-threaded programs, two threads may try to modify the same variable simultaneously — a problem called a race condition. The result depends on which thread wins the context-switch lottery, making the bug non-deterministic and difficult to reproduce. Operating systems provide tools such as mutexes and semaphores to lock shared resources and prevent race conditions. At GCSE, knowing that concurrency introduces synchronisation challenges is sufficient.

How is multitasking relevant to the GCSE exam?

GCSE questions on operating systems frequently ask students to explain how an OS manages multiple processes. Expect to describe the scheduler, explain what the PCB stores and why it is needed, and describe context switching in the correct sequence: save state → select next process → load state → resume. Drawing the save/load sequence step by step usually earns full marks.

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