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Windows NT as Real-Time OS?

More and more companies are trying to use Windows NT as a standard Operating System (OS) at all levels of the industrial hierarchy. The use as server and workstation is obvious, but some people want to use it also on the factory floor. These factory floor applications demand real-time system behaviour. Can Windows NT be a component to fulfil this need? First, we will define what a real-time system is and the OS characteristics we need to allow developers to build such a real-time system. The distinction will be made between hard and soft real-time systems. In the second part, we demonstrate how and why Windows NT cannot fulfil the requirements of a hard real-time system. We show, however, that for some simple soft real-time applications, Windows NT could be used under certain circumstances.

INTRODUCTION

Windows NT was not designed with the requi-rements of a Real-Time Operating System (RTOS) in mind: it has been designed as a General Purpose OS (GPOS) or, to be more precise, as a Network OS (NOS). Nevertheless, because Win-dows NT was created by developers of the VMS Operating System, some characteristics from the real-time world have been introduced. For example Micro-soft introduced the notion of real-time class processes. They are scheduled in the same way, as it would be in an RTOS. The ISR (Interrupt Service Routine) has been designed in a very efficient way [1]. However, do these elements allow for a classification of Windows NT as a RTOS?

WHAT IS A REAL-TIME SYSTEM?

Definition

A Real-Time System responds in a timely predictable way to unpredictable external stimuli arrivals.

To fulfil this, some basic requirements are needed:

Meet deadlines. After an event occurred an action has to be taken within a predetermined time limit. Missing a deadline is considered a (severe) software fault. On the contrary, it is not considered as a software fault when a text editor reacts slowly and so enervating the user. This lack of response is catalogued as a performance problem — which can probably be solved by putting in a faster processor. It can be demonstrated that using a faster processor will not necessarily solve the problem of missing dead-lines [2].

Simultaneity or simultaneous processing: even if more than one event happens simultaneously, all deadlines for all these events should be met. This means that a real-time system needs inherent parallelism. This is achieved by using more than one processor in the system and/or by adopting a multi-task approach.

Hard and Soft Real-Time Systems

A classification can be made into hard and soft real-time systems based on their properties.

The properties of a hard real-time system are:

No lateness is accepted under any circumstances

Useless results if late

Catastrophic failure if deadline missed

Cost of missing deadline is infinitely high

A good example of a hard real-time system is the fly-by wire control system of an aircraft.

A soft real-time system is characterised by:

Rising cost for lateness of results

Acceptance of lower performance for lateness

Examples are a vending machine and a network interface subsystem. In the latter you can recover from a missed packet by using one or another network protocol asking to resend the missed packet. Of course, by doing so, you accept system performance degradation.

Other real-time systems examples are nuclear power plant control, industrial manufacturing control, me-dical monitoring, weapon delivery systems, space na-vigation and guidance, reconnaissance systems, laboratory experiments control, automobile engines control, robotics, telemetry control systems, printer controllers, anti-lock breaking, burglar alarms — the list is endless.

The difference between a hard and a soft real-time system depends on the system requirements: it is called hard if the requirement is "the system shall not miss a deadline" and soft if "the system should not miss a deadline".

There are a lot of discussions going on about the exact meaning of a hard and soft real-time system. One could even argue that a soft real-time system is not a real-time system, as the first requirement: meet deadlines, is not met. Indeed, the term "real-time" is often misused to indicate a fast system. And fast can then be seen as "should meet timing deadlines", thus meaning a soft real-time system. Therefore, we define a RTOS as an OS that can be used to build a hard real-time system.

Hard or Soft RTOS do not exist!

People often confuse the notion of real-time systems with real-time operating systems (RTOS). From time to time people, even misuse hard and soft attributes. They say this RTOS is a hard RTOS or this one is a soft one. There is no hard RTOS or soft RTOS. A