Defining The Fundamental Terms
Embedded real-time software is often referred to as the most difficult software to get right. These key terms were first defined in the 60's and their operational meanings haven't changed since then. Specifically:

Real-Time System: a system that controls an environment by receiving data, processing them, and taking action or returning results sufficiently quickly to affect the functioning of the environment at that time.

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[ref: James Martin, Design of Real-Time Computer Systems, Englewood Cliffs, NY: Prentice-Hall, 1967.]

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But we can take a more direct approach and define the term as it relates to what happens if such a system fails to respond "sufficiently quickly":

Real-Time System: A system that must satisfy explicit (bounded) response time constraints or it will fail.

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[ref: Phillip A. Laplante, Real-Time Systems Design and Analysis; An Engineer's Handbook, New York, NY: IEEE Computer Society Press, 1992.]

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What makes real-time systems so hard to get right? They typically have these "challenges" to the developer:

1. Modern real-time systems are usually embedded in devices or mechanisms for which the software most monitor data and issue controlling commands.

2. Many real-time systems interact with a "user" for whom they perform a needed function.

3. Real-time systems frequently require the concurrent processing of multiple inputs. Beyond the most trivial examples, real-time systems include both periodic (traditionally called "cyclic") processing at multiple frequencies and aperiodic (driven by sporadic events) processing.

4. Real-time systems are frequently constrained by physical limitations. These normally include processor resources (memory and throughput), response times, input/output channel bandwidths, storage device capacities and transfer rates, and so forth.

5. Real-time systems frequently have various sensor and intersystem interfaces that provide continuous or periodic input and actuator or intersystem interfaces that must be driven periodically. The sampling interval tolerances, event response intervals, and response precision are often critical. Permitted variances in responses are often required to be small as well.

6. The reliability of the software in such systems usually must be very high. This may be obvious when considering military systems, but the cost of correcting a software problem in a common appliance controller is overwhelming after a million units have been sold and distributed nationwide.

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[Derived from: John R. Ellis, Objectifying Real-Time Systems, New York, NY: SIGS Books, Inc., 1994.]

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Now let's look at a definition of the other key term in this phrase:

Embedded Computer System:

1. A computer system that is physically incorporated into a larger system whose primary function is not data processing.

2. It is integral to such a larger system from a design, procurement, or operations viewpoint.

3. Its outputs generally include information, computer control signals, and computer data.

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[Derived from: Subcommittee on Software Engineering Standards, Technical committee on Software Engineering, IEEE Computer Society: "Software Engineering Terminology - Draft of 23 March 1978"]

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So, when we combine these definitions, we find that an embedded real-time system is one in which the computer is a component of and usually providing a controlling function for some larger system. The software running in the embedded processor is expected to respond to data inputs, often derived from various sensors, within some predetermined time constraints. The bound on the response time constraint is often called a deadline.

Not yet convinced that developing such systems should be entrusted to only someone with considerable real-time system experience? Let's go on...

Learning More

Visit the following retiSoft pages to learn more about the various aspects of developing and embedded real-time system, and especially the software for such a system.

• An embedded real-time system development process: Describes a structured engineering process for developing embedded real-time systems.

• Embedded real-time software development issues: Describes some of the multitude of questions and problems that the developer must resolve to build an embedded real-time software system.

• Real-time software control : Describes the software components (kernels, executives, and operating systems) that control the execution of the elements of a real-time software system.