Basic Real-Time Concepts --- Systems Concepts

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Systems Concepts

The hardware of the general-purpose computer solves problems by repeated execution of macroinstructions, collectively known as software. Software is traditionally divided into system programs and application programs.

System programs consist of software that interfaces with the underlying computer hardware, such as schedulers, device drivers, dispatchers, and programs that act as tools for the development of application programs. These tools include compilers, which translate high-order language programs into assembly code; assemblers, which translate the assembly language into a special binary format called object or machine code; and linkers, which prepare the object code for execution. An operating system is a specialized collection of system programs that manage the physical resources of the computer. As such, a real-time operating system is a systems program.

Application programs are programs written to solve specific problems, such as payroll preparation, inventory, and navigation. Certain design considerations play a role in the design of certain systems programs and application software intended to run in real-time environments.

The notion of a “system” is central to software engineering, and indeed to all engineering, and warrants formalization.

Definition: A system is a mapping of a set of inputs into a set of outputs.

When the internal details of the system are not of interest, the mapping function can be considered as a black box with one or more inputs entering and one or more outputs exiting the system (see Figure A).

Every real-world entity, whether synthetic or occurring naturally, can be modeled as a system. In computing systems, the inputs represent digital data from hardware devices and other software systems. The inputs are often associated with sensors, cameras, and other devices that provide analog inputs, which are converted to digital data, or provide direct digital input. The digital output of the computer system can be converted to analog outputs to control external hardware devices such as actuators and displays (Figure B).

Modeling a real-time system, as in Figure B, is somewhat different from the more traditional model of the real-time system as a sequence of jobs to be scheduled and performance to be predicted, which is very similar to that shown in Figure C. The latter view is simplistic in that it ignores the fact that the input sources and hardware under control are complex. Moreover, there are other, sweeping software engineering considerations that are hidden by the model shown in Figure C.

Look again at to the model of a real-time system shown in Figure 1.2. Note that in its realization there is some delay between presentation of the inputs (stimulus) and appearance of the outputs (response). This fact can be formalized as follows:

Definition: The time between the presentation of a set of inputs to a system (stimulus) and the realization of the required behavior (response), including the availability of all associated outputs, is called the response time of the system.

How fast the response time needs to be depends on the purpose of the system.