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Task status and transition 6-3
3.2
Task scheduling
Task scheduling determines the priority to be used in allocating tasks in the READY (execution
enabled) status to the CPU and executing them.
When there are two or more tasks which are put in the READY status, ready queues for CPU
allocation are used to determine the priority for executing the tasks. One of these READY status
tasks is then selected and executed (RUN status).
Only tasks with the same priority ranking are assigned to a given ready queue. Therefore, where
several tasks with differing priority rankings exist, a corresponding number of ready queues are
created. Tasks within a given ready queue are handled on a first come first serve (FCFS) basis. The
task where a READY status is first established has priority. The smaller the number, the higher the
task priority level.
Task 1
Task 3
Task 1
Task 4
Task 5
Task 2
Ready queue 1
Ready queue 2
Ready queue 3
Order in which tasks are put in READY status.
The head of the task with the highest priority
is put in RUN status.
High
Low
32
33
34
Priority level
Task scheduling
33602-R7-00
A RUN status task will be moved to the end of the ready queue if placed in a READY status by any
of the following causes:
1) A WAIT status command was executed.
2) The CPU occupation time exceeds a specified time.
3) A task with a higher priority level is put in READY status.
Task 1
RUN status
READY status
Moves to the end of the ready queue, and Task 3 is executed.
Task 3
Task 4
Execution sequence
1
Task 1
Task 3
Task 4
Task 1
2
Moves to the end of the ready queue, and Task 4 is executed.
Task 3
Task 4
Task 1
Task 3
3
Ready queue
33603-R7-00
NOTE
•
When the prescribed CPU
occupation time elapses,
the active command is
ended, and processing
moves to the next task.
However, if there are no
other tasks of the same
or higher priority (same
or higher ready queue),
the same task will be
executed again.
Содержание SCARA YRCX Series
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Страница 27: ...Chapter 2 Constants 1 1 Outline 2 1 2 2 Numeric constants 2 1 3 3 Character constants 2 2 ...
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Страница 489: ...Chapter 13 Appendix 1 1 Reserved word list 13 1 2 2 Changes from conventional models 13 3 ...
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