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Q11248: Windows: Nonpreemptive vs. Preemptive Scheduling

Article: Q11248
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
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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.

A nonpreemptive system, such as Windows, will guarantee that this number N will
always be 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.

Additional query words: 3.00 no32bit 3.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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