Appeal 2007-2127
Reexamination Control No. 90/006,621
i. Task or context switching
Task or context switching is described in Nguyen, Advanced
Programmer's Guide to OS/2, pages 10-11:
The implementation of multitasking requires that the operating system
switch rapidly between executing programs or threads. . . . Switching
between programs or threads when implementing multitasking is
called task or context switching.
In order to understand task switching one has to be able to
imagine what goes on within the CPU when a program or thread is
executing. As a program executes, the contents of the registers in the
machine are constantly changing in response to the instructions which
the program issues. If one could take a "snapshot" of all the CPU
registers one would have a perfect representation of the state of a
program at any given moment. This includes all the segment
registers, the stack registers and the LDTR [Local Descriptor Table
Register]. In order to switch among multiple programs OS/2 saves a
"picture" of the CPU in memory every time it interrupts the execution
of a program or thread to execute another one. Switching to the next
task to be executed entails loading its "picture" from memory into the
CPU and continuing to execute it from the point at which it was
interrupted.
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