ID Number: Q11248
2.00 3.00
MS-DOS
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.
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, DOS applications give up control whenever
they attempt various I/O functions.
DOS applications running under Windows 3.0 are in fact preemptively
multitasked. In contrast, all WINDOWS 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 DOS applications running in
other virtual machines. Keep in mind that unlike DOS applications, all
windows 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.