How and when a process is swapped in and out of the CPU, thus enabling multi-tasking

Three states

1. Running
2. Ready to run
3. Blocked

Process

• Processing is controlled by a scheduler
• Currently active programs held in the READY QUEUE
• Each program allocated a time slice (amount of time that it will spend in the CPU) when it reaches the front of the RUNNING QUEUE
• If time slice exceeded, moved to the back of the process/ready queue
• If I/O required whilst in the CPU, the task is temporarily moved into the blocked queue

Polling

• OS periodically checks the progress of jobs suspended in the spooling system/availability of I/O devices
• Allows jobs to re-join the runnable queue when I/O is completed

Within the processor queue, you have a main and fast queue for larger and smaller amount of tasks (think of the quick checkouts for people with few items at supermarkets).

Threading

<aside> 👉 When a process is split up into smaller processes known as threads which can be executed separately across multiple cores in the CPU

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