Embedded Systems Design, 2nd Edition

Real-time operating systems 227

time needed to pre-empt and resume tasks. The processor utilisa-

tion can also be calculated and thus give some idea of the perform-

ance needed or how much performance is spare. Typically, the

higher the utilisation (>80%), the more chance that the priority

assignment is wrong and that the system will fail. If the utilisation

is below this, the chances are that the system will run correctly.

This is one of the problems with theoretical analysis of such

systems in that the actual design may have to break some of the

assumptions on which the analysis is based. Most embedded

systems will have asynchronous events and tasks ...