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:

1. 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].
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.