Cyclic Executive

Cyclic Executive is an alternative to real-time operating systems . It is a form of cooperative multitasking in which only one task is running. The single task is typically written as an infinite loop , e.g. B. in the programming language C in the main method.

Overview

Periodic processes are important software components in real-time computer systems. A periodic process consists of an action, such as a calculation, that is performed repeatedly in a periodic pattern.

A cyclic executive is a control structure or a program to explicitly nest the execution of several periodic processes on a single CPU. The nesting is done in a deterministic way so that the execution time is predictable. It can be seen as an implementation technique for a design methodology in which a real-time system mainly consists of a collection of periodic processes. The Prozessverschachtelung (s: process interleaving ) is (s: after a "cyclic schedule," cyclic schedule ) defined.

advantages

• Easy implementation
• Little effort
• Easy to plan

disadvantage

• No sporadic events treatable
• Everything has to work in step
• Code must be planned manually

example

The basic scheme circulates through a repetitive sequence of tasks on one frequency. An example is an embedded system that monitors a temperature sensor and updates an LCD . If the LCD should be updated ten times per second (every 100 milliseconds) and the temperature sensor for some other reason every 50 milliseconds, a loop could be constructed as follows:

int main(void)
{
   // initialization code here

   while (1)
   {
      currTemp = tempRead();
      lcdWrite(currTemp);

      // waste CPU cycles until 50 ms
      currTemp = tempRead();
      // do other stuff

      // waste CPU cycles until 100 ms
   }
}

The outer 100 millisecond cycle is defined as the major cycle . In this case there is also an inner minor cycle of 50 milliseconds.

See also

• Arduino Platform - A well-known example of using this technique
• Preemptive multitasking
• Real-time operating system

Web links

• Time-controlled scheduling strategies in real-time systems (PDF; 587 kB)

Individual evidence

1. ↑ The Cyclic Executive Model and Ada (PDF; 1.3 MB)