Small systems of low complexity are typically designed as foreground/background systems or super-loops. An application consists of an infinite loop that calls modules (i.e., tasks) to perform the desired operations (background). Interrupt Service Routines (ISRs) 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.