Page 36 - DCAP608_REAL TIME SYSTEMS

Unit 3: Hard Versus Soft Real-time System

Processors have attributes (pre-emptivity, context switch time, speed). Two processors are of the Notes
same type, if they are functionally identical and can be used interchangeably.
According to the model the basic component of any real-time application are jobs. The operating
system treats jobs as a unit of work and allocates processors and resources.

Did u know? A job J is characterized by many parameters:
i
 execution time
 deadlines

 pre-emptivity
 resource requirements
 soft or hard real-time

3.2 Release Time

If all jobs are released when the system begins execution, then there is said to be no release time.
The release time r of a job is the instant of time at which the job becomes available for execution.
i
Release time may be jittery so that r is in the interval [r -, r +]. Jobs can be scheduled and executed
i i i
at any time after its release time, whenever its data and control dependency conditions are met.

Did u know? The response time is the length of time from the release time of the job to the
time it completes.

Self Assessment

Fill in the blanks:

1. A job is a unit of work that is scheduled by the ………………
2. A job executes on a processor and may depend on some ………………
3. The basic component of any real-time application are ………………

4. The ……………… system treats jobs as a unit of work and allocates processors and resources.
5. If all jobs are released when the system begins execution, then there is said to be
……………… release time.

6. The release time r of a job is the instant of time at which the job becomes available for
i
………………
3.3 Deadline and Timing Constraint

Deadline is defined as an instant of time a job’s execution is required to be completed. If
deadline is infinity, then job has no deadline. Absolute deadline is equal to release time plus
relative deadline. Timing constraint is a constraint imposed on timing behaviour of a job: hard
or soft.

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