Windows: Nonpreemptive vs. Preemptive Scheduling

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

Preemptive scheduling, which Windows DOES NOT do, is defined in the following way:

   Between any two application instructions, N instructions may
   execute in another application's context, where N is greater than
   or equal to zero.

MORE INFORMATION

In nonpreemptive scheduling, an application is not forced out of context asynchronously (that is, it is not preempted). Instead, the application runs until it explicitly gives up control. Windows-aware applications give up control through various system calls. Although they are not aware of it, MS-DOS-based applications give up control whenever they attempt various I/O functions.

MS-DOS-based applications running under Windows version 3.0 are in fact preemptively multitasked. In contrast, all Windows-based applications are nonpreemptively multitasked. When the system is viewed from a Win386 perspective, Windows runs in the system virtual machine (VM) and that VM competes for time slices along with the rest of the MS-DOS-based applications running in other virtual machines. Keep in mind that unlike MS- DOS-based applications, all Windows-based applications run inside the system VM, and are not given their own virtual machine.

NOTE: An interrupt is not considered to be a form of preemption unless there is an application context switch during the interrupt. An interrupt takes the execution stream into the kernel, which returns back to the same place without running another application, much in the same way a call would.