Multithreading in Windows NT
Threads can be defined as "code sequences that run multitasked on individual stacks," "something that behaves absolutely unpredictably and differently each time I try to debug it," "a programming tool that models concurrency," or "a concept that helps designing modular and interleaved applications".
What this tells us is that there are many different aspects to multitasking. Microsoft® Windows™ NT™ responds totally differently to user interaction than Windows version 3.1—for example, while one application starts up, you can switch to another application and work with it (because the two applications execute different threads). This is because threads within the foreground application might utilize the machine so heavily that other processes or threads in the application are locked out. By assigning the task manager a higher priority than most applications, Windows NT ensures that it gets to preempt applications' threads if necessary.
This strategy to share the CPU is called preemptive multitasking and is different from the multitasking that Windows 3.1 performs between applications. The latter variation is called non preemptive multitasking and relies on an application voluntarily relinquishing control to the operating system before letting another application execute. In a non preemptive multitasking scheme, the amount of time a task is allowed to run is determined by the task, whereas in a preemptive scheme, the time is determined by the operating system.
