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In preemptive multitasking, the operating system kernel can also initiate a context switch to satisfy the scheduling policy's priority constraint, thus preempting the active task. In general, preemption means "prior seizure of". When the high-priority task at that instance seizes the currently running task, it is known as preemptive scheduling.
Cooperative multitasking was the primary scheduling scheme for 16-bit applications employed by Microsoft Windows before Windows 95 and Windows NT, and by the classic Mac OS. Windows 9x used non-preemptive multitasking for 16-bit legacy applications, and the PowerPC Versions of Mac OS X prior to Leopard used it for classic applications. [1]
Specifically, the scheduler is permitted to forcibly perform a context switch (on behalf of a runnable and higher-priority process) on a driver or other part of the kernel during its execution, rather than co-operatively waiting for the driver or kernel function (such as a system call) to complete its execution and return control of the processor to the scheduler when done.
The scheduler is the part of the kernel responsible for determining which task runs next. [12] Most real-time kernels are priority based. In a priority-based kernel, control of the CPU is always given to the highest priority task ready to run. Two types of priority-based kernels exist: non-preemptive and preemptive.
Very early MS-DOS and Microsoft Windows systems were non-multitasking, and as such did not feature a scheduler. Windows 3.1x used a non-preemptive scheduler, meaning that it did not interrupt programs. It relied on the program to end or tell the OS that it didn't need the processor so that it could move on to another process.
When the first program reached an instruction waiting for a peripheral, the context of this program was stored away, and the second program in memory was given a chance to run. The process continued until all programs finished running. [3]
A Round Robin preemptive scheduling example with quantum=3. Round-robin (RR) is one of the algorithms employed by process and network schedulers in computing. [1] [2] As the term is generally used, time slices (also known as time quanta) [3] are assigned to each process in equal portions and in circular order, handling all processes without priority (also known as cyclic executive).
Some preemptive multitasking scheduling systems behave as run-to-completion schedulers in regard to scheduling tasks at one particular process priority level, at the same time as those processes still preempt other lower priority tasks and are themselves preempted by higher priority tasks.