Introduction

A real-time system is characterized by the need to respond rapidly to events occurring asynchronously in time. A multitasking system is one in which a number of activities or processes must be performed simultaneously without interference with each other. A system in which several activities must operate simultaneously, each with time-critical precision, is called a real-time multitasking system. The AMX Multitasking Executive provides a simple solution to the complexities introduced by real-time multitasking. AMX supervises the orderly execution of a set of application program modules called tasks. Each task solves a particular problem and provides a specific functional capability within the system.

As we all know, the microprocessor can only do one thing at a time. Fortunately, it does all things very quickly. However, to get the effect that all activities are occurring simultaneously, it is necessary to rapidly switch back and forth from one process to another in a well controlled fashion. It is AMX, acting as a context switcher, which organizes and controls the use of the microprocessor to achieve this apparent concurrent execution of tasks. AMX employs a preemptive, priority-driven scheduling algorithm which ensures that the highest priority task which is ready to do useful work will always have control of the processor.

AMX is prompted to switch tasks by requests from three different sources: tasks, interrupts and timers. The simplest, of course, is a request from the executing task asking AMX to perform an operation which, of necessity, invokes a task of higher priority. For instance, the executing task may request AMX to start a higher priority task by sending a message to it. External events can generate hardware interrupts to which the processor responds. An application device dependent Interrupt Service Procedure is immediately executed to service the event. This procedure can request AMX to perform an operation affecting one or more of the tasks operating in the system. For instance, the service procedure might request AMX to wake some task known to be waiting for the particular event. If the task wakened is of higher priority than the task which was executing at the time of the interrupt, AMX will automatically do a task switch when the interrupt service is complete.

Most real-time systems require a hardware clock to provide precise interval measurement. This clock provides the third mechanism which can force a task switch to occur. When the hardware clock interrupt occurs, the application clock service procedure informs AMX that a hardware clock tick has occurred. The AMX Clock Handler triggers the AMX Kernel Task to provide timing services if so required.