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During this interval, it
appears that the key input
microcomputer is haled.
Key input
Task
Remote control
Task
Program execution
interrupt
Program execution
resumed
Figure 3.5 Task Execution Interruption and Resumption
In the state shown in Figure 3.5, it appears to the programmer that the key input task or its microcomputer is halted while
anot
her task assumes execution control.
Task execution restarts at the point of last interruption as the register contents prevailing at the time of the last interruption
are recovered. In other words, task switching refers to the action performed to save the currently executed task register
contents into the associated task management memory area and recover the register contents for the task to switch to.
To establish the kernel, therefore, it is only necessary to manage the register for each task and change the register contents
upon each task switching so that it looks as if a number of microcomputers exist (See Figure 3.6).
Remote control
Task
A
ctual
Register
R0
R1
PC
Kernel
Register Register
Key input
Task
R0
R1
PC
R0
R1
PC
Figure 3.6 Task Switching
The example presented in Figure 3.7
5
indicates how the individual task registers are managed. In reality, it is necessary
to provide not only a register but also a stack area for each task.
5
It is figure where all the stack areas of the task were arranged in the same section.