Real Time Systems | Operating System - Computer Science Engineering (CSE)

Real time system means that the system is subjected to real time, i.e., response should be guaranteed within a specified timing constraint or system should meet the specified deadline. For example: flight control system, 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 result 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 result produced by a soft real time system decreases gradually with increase in tardiness.
   Example: Telephone switches.

Reference model of real time system: Our reference model is characterized by three elements:

1. A workload model: It specifies the application supported by 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 job becomes ready for execution.
4. Execution time of a job: It is the time taken by 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 release time of a job to the instant when it finishes.
7. Maximum allowable response time of a job is called its relative deadline.
8. 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 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.