Renesas 70 3.1.2 Operating Principles of Real-time OS

Chapter 3 Introduction to MR308
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3.1.2 Operating Principles of Real-time OS
The real-time OS is the software that makes a one-microcomputer system look like operating a number of mi-
crocomputers. You should be wondering how the real-time OS makes a one-microcomputer system function like
a number of microcomputers.
As shown in Figure 3.4 the real-time OS runs a number of tasks according to the time-division system. That is, it
changes the task to execute at fixed time intervals so that a number of tasks appear to be executed simultane-
ously.
Time
Key input
Task
Monitor
Task
Mechanical
control
Task
Volume control
Task
LED
illumination
Task
Remote control
Task
Figure 3.4 Time-division Task Operation
As indicated above, the real-time OS changes the task to execute at fixed time intervals. This task switching
may also be referred to as dispatching (technical term specific to real-time operating systems). The factors
causing task switching (dispatching) are as follows.
• Task switching occurs upon request from a task.
• Task switching occurs due to an external factor such as interrupt.
When a certain task is to be executed again upon task switching, the system resumes its execution at the point
of last interruption (See Figure 3.5).
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