Real Time Systems

A real-time system means that the system is subjected to real-time, i.e., the response should be guaranteed within a specified timing constraint or the system should meet the specified deadline. For example flight control systems, real-time monitors, etc. 

Types of real-time systems based on timing constraints:

1. Hard real-time system: This type of system can never miss its deadline. Missing the deadline may have disastrous consequences. The usefulness of results produced by a hard real-time system decreases abruptly and may become negative if tardiness increases. Tardiness means how late a real-time system completes its task with respect to its deadline. Example: Flight controller system. 
2. Soft real-time system: This type of system can miss its deadline occasionally with some acceptably low probability. Missing the deadline have no disastrous consequences. The usefulness of results produced by a soft real-time system decreases gradually with an increase in tardiness. Example: Telephone switches. 

Reference model of the real-time system:

Our reference model is characterized by three elements: 

1. A workload model: It specifies the application supported by the system. 
2. A resource model: It specifies the resources available to the application. 
3. Algorithms: It specifies how the application system will use resources. 

Terms related to real-time system:

1. Job: A job is a small piece of work that can be assigned to a processor and may or may not require resources. 
2. Task: A set of related jobs that jointly provide some system functionality. 
3. Release time of a job: It is the time at which the job becomes ready for execution. 
4. Execution time of a job: It is the time taken by the job to finish its execution. 
5. Deadline of a job: It is the time by which a job should finish its execution. Deadline is of two types: absolute deadline and relative deadline. 
6. Response time of a job: It is the length of time from the release time of a job to the instant when it finishes. 
7. The maximum allowable response time of a job is called its relative deadline. 
8. The absolute deadline of a job is equal to its relative deadline plus its release time. 
9. Processors are also known as active resources. They are essential for the execution of a job. A job must have one or more processors in order to execute and proceed towards completion. Example: computer, transmission links. 
10. Resources are also known as passive resources. A job may or may not require a resource during its execution. Example: memory, mutex 
11. Two resources are identical if they can be used interchangeably else they are heterogeneous. 
    

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