Real-Time Operating Systems

Some real-time operating systems RTOS are applicable to a wide range of system configurations although others are geared to a particular wide or even microprocessor regardless of the surrounding electronic environment. RTOS get their capabilities by a combination of software features and increasingly a variety of micro-coded capabilities implemented in hardware.

Todays world Two broad classes of operating systems are used for real-time work

(1) dedicated RTOS designed completely for real-time applications

(2) general-purpose operating systems which have been enhanced to give real-time capability. Use of a real-time executive makes real-time performance- faster and more professionally than the general-purpose operating system.

Some operating systems must have a priority scheduling mechanism but RTOS must give a priority mechanism which allows high-priority interrupts to take precedence over less important ones. However, because interrupts happens in response to nonrecurring events, asynchronous, they must be serviced without 1st taking time to swap in a program from disk storage. Through consequently to guarantee the needed response time a real-time operating system must have a mechanism for memory locking- which is locking at least some programs in main memory so in which swapping overhead is avoided.

To determine that type of real-time operating system best matches an application; measures of RTOS excellence can be described and evaluated. Circumstance interrupt time and switching latency determine interrupt handling capability the very important aspect of a real-time system. Circumstance switching time is the time the operating system takes to store the state of the computer and the contents of the registers so which it can return to a processing task after servicing the interrupt.

The Interrupt latency and the maximum time lag previously the system gets around to switching a task occurs because in an operating system there are often non-reentrant or critical processing paths which must be completed before an interrupt can be processed.

Length of these paths the number of instructions before the system can service an interrupt indicates the worst-case time lags. If the time is too long, the system may miss data that are unrecoverable. That is very important that the designer know the time lag so that the system can compensate for it. The worst case occurs if a high-priority interrupts is generated immediately after the system enters a critical path among an interrupt and interrupt service.

Many operating systems perform concurrent processing [WOO90] or multitasking another main need for real-time systems but to be viable for real-time operation the system overhead must be low in terms of memory space used and switching time.