How to do several things at once

Concurrent tasks are tasks that are in progress (but not necessarily running) at the same time.  Concurrency requires an operating system that saves the state of the processor for each concurrent task.

In order to run concurrently, a task must request resources before using them, either exclusively (locking out other tasks) or non-exclusively (allowing other tasks to share) and release resources as soon as it is finished with them; an exclusive request for a resource that is currently allocated non-exclusively must wait for all other tasks to release the resource.

A task must return to the operating system while it is waiting for something to happen.