Real-time Operating Systems - Design Philosophies

Design Philosophies

The most common designs are:

• Event-driven which switches tasks only when an event of higher priority needs servicing, called preemptive priority, or priority scheduling.
• Time-sharing designs switch tasks on a regular clocked interrupt, and on events, called round robin.

Time-sharing designs switch tasks more often than strictly needed, but give smoother multitasking, giving the illusion that a process or user has sole use of a machine.

Early CPU designs needed many cycles to switch tasks, during which the CPU could do nothing else useful. For example, with a 20 MHz 68000 processor (typical of late 1980s), task switch times are roughly 20 microseconds. (In contrast, a 100 MHz ARM CPU (from 2008) switches in less than 3 microseconds.) Because of this, early OSes tried to minimize wasting CPU time by avoiding unnecessary task switching.