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:

• Job – A job is a small piece of work that can be assigned to a processor and may or may not require resources.
• Task – A set of related jobs that jointly provide some system functionality.
• Release time of a job – It is the time at which job becomes ready for execution.
• Execution time of a job – It is the time taken by job to finish its execution.
• 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.
• Response time of a job – It is the length of time from release time of a job to the instant when it finishes.
• Maximum allowable response time of a job is called its relative deadline.
• Absolute deadline of a job is equal to its relative deadline plus its release time.
• 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.
• Resources are also known as passive resources. A job may or may not require a resource during its execution. Example: memory, mutex
• Two resources are identical if they can be used interchangeably else they are heterogeneous.