A real time operating system needs to be "event driven" and have the ability to perform certain tasks in a very timely manner.
Most true real-time operating systems use interrupts to handle events as they occur, such as a time-critical input message from a serial port or a digital I/O event from some peripheral hardware.
Next, there should be some kind of prioritization, meaning that one process may be time critical, such as controlling the rods at a nuclear reactor, but other processes might be a little less critical, such as dimming the hallway lights at the reactor facility after hours.
Third, there must be a mechanism in place for "multitasking", meaning that the computer must be able to switch between multiple active processes. Some operating systems create a process "stack" for each process, and if a high priority process is to be activated to handle an event, the operating system simply switches control to the higher priority process by loading the current data from the process stack. A true real time operating system can do that in just a couple clock ticks.
Most true real-time operating systems use interrupts to handle events as they occur, such as a time-critical input message from a serial port or a digital I/O event from some peripheral hardware.
Next, there should be some kind of prioritization, meaning that one process may be time critical, such as controlling the rods at a nuclear reactor, but other processes might be a little less critical, such as dimming the hallway lights at the reactor facility after hours.
Third, there must be a mechanism in place for "multitasking", meaning that the computer must be able to switch between multiple active processes. Some operating systems create a process "stack" for each process, and if a high priority process is to be activated to handle an event, the operating system simply switches control to the higher priority process by loading the current data from the process stack. A true real time operating system can do that in just a couple clock ticks.