Programming Embedded Systems in C and C++

Glossary 161

real-time system

Any computer system, embedded or otherwise, that has deadlines. The follow-

ing question can be used to identify real-time systems: is a late answer as bad

as, or even worse than, a wrong answer? In other words, what happens if the

computation doesn’t finish in time? If nothing bad happens, it’s not a real-time

system. If someone dies or the mission fails, it’s generally considered “hard”

real-time, which is meant to imply that the system has “hard” deadlines. Every-

thing in between is “soft” real-time.

recursive

Refers to software that calls itself. Recursion should generally be avoided in an

embedded system because it frequently requires a large stack.

reentrant

Refers to software that can be executed multiple times simultaneously. A reen-

trant function can be safely called recursively or from multiple tasks. The key

to making code reentrant is to ensure mutual exclusion whenever accessing

global variables or shared registers.

register

A memory location that is part of a processor or a peripheral. In other words,

it’s not normal memory. Generally, each bit or set of bits within the register

controls some behavior of the larger device.

relocatable

A file containing object code that is almost ready for execution on the target.

The final step is to use a locator to fix the remaining relocatable addresses

within the code. The result of that process is an executable.

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