Small systems of low complexity are typically designed as foreground/background systems or super-loops. An application consists of an infinite loop (F1-1(1)) that calls modules (i.e., tasks) to perform the desired operations (background). Interrupt Service Routines (ISRs) shown in F1-1(3) handle asynchronous events (foreground). Foreground is also called interrupt level; background is called task level.
Critical operations that should be performed at the task level must unfortunately be handled by the ISRs to ensure that they are dealt with in a timely fashion. This causes ISRs to take longer than they should. Also, information for a background module that an ISR makes available is not processed until the background routine gets its turn to execute, which is called the task-level response. The worst-case task-level response time depends on how long a background loop takes to execute and, since the execution time of typical code is not constant, the time for successive passes through a portion of the loop is nondeterministic. Furthermore, if a code change is made, the timing of the loop is affected.
Most high-volume and low-cost microcontroller-based applications (e.g., microwave ovens, telephones, toys, etc.) are designed as foreground/background systems.