The big idea: A single core runs one instruction at a time. Multitasking is the OS switching between processes so fast that all of them appear to run together.
What looks like simultaneity is rapid switching — and every switch costs real work.
| Concurrency | Parallelism | |
|---|---|---|
| Means | Several tasks in progress | Several tasks executing |
| Needs | One core is enough | More than one core |
| Achieved by | Switching between them | Genuinely at the same instant |
| A single core gives | Concurrency only | Never parallelism |
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Switching is not free: Every switch spends time saving and restoring state, and the new process arrives to find the cache full of the previous one's data — so its first memory accesses are slow.
Switch too often and the system spends more time switching than working. That is thrashing.
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What the OS is handing out
- CPU time — divided into slices by the scheduler
- Memory — each process given its own space, and kept out of everyone else's
- Devices — one process at a time may hold a printer or a file
- Files and locks — access serialised so two writers cannot corrupt one file
Deadlock, and its four conditions: Process A holds the printer and wants the scanner; B holds the scanner and wants the printer. Neither can continue.
It needs all four of: mutual exclusion, hold-and-wait, no pre-emption, and circular wait. Break any one and deadlock becomes impossible — which is how operating systems prevent it.
Starvation is not deadlock: Deadlock: nobody can proceed, ever. Starvation: the system is running fine, but one low-priority process never gets a turn because higher-priority work keeps arriving.
Starvation is fixed by ageing — raising priority the longer a process waits.
How this is tested — you must explain apparent simultaneity as switching, and name what it costs. It comes up two ways:
Paper 1 Section A
- Explain the role of the OS in multitasking, 3-4 marks
- Describe what happens in a context switch
- Distinguish concurrency from parallelism
Paper 1 Section B — case study
- Explain a deadlock in a scenario
- Justify why switching has a cost
The classic trap: Saying a single-core computer runs programs "at the same time". It runs one instruction at a time and switches rapidly — that is concurrency, not parallelism.
IB-style questionExplain[5 marks]
A single-core laptop appears to run a browser, a music player and a download at once. Explain how the operating system achieves this and what it costs.
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