A GENERAL VIEW - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 7

Figure 1.1 Both the mouse (a) and the turtle (b) behave in real time with respect to their
natural habitat. Nevertheless, the survival of fast animals such as a mouse or a fly can be
jeopardized by events (c and d) quicker than their reactive capabilities.
One value was too large to be converted and the program was not explicitly designed to
handle the integer overflow error, so the Inertial Reference System halted, as specified
in other requirements, leaving the luncher without inertial guidance. The conversion
error occurred because the control software was reused from the Ariane 4 vehicle,
whose dynamics was different from that of the Ariane 5. In particular, the variable
containing the horizontal velocity of the rocket went out of range (since larger than
the maximum value planned for the Ariane 4), thus generating the error that caused
the loss of guidance.
The examples considered above indicate that the environment is always an essential
component of any real-time system. Figure 1.2 shows a block diagram of a typical
real-time architecture for controlling a physical system.
Some people erroneously believe that it is not worth investing in real-time research
because advances in computer hardware will take care of any real-time requirements.
Although advances in computer hardware technology will improve system throughput
and will increase the computational speed in terms of millions of instructions per sec-
ond (MIPS), this does not mean that the timing constraints of an application will be
met automatically.