Operation

The block diagram illustrates the general operation of an AMX system. Execution begins in the user domain providing the opportunity for hardware specific and application dependent setup prior to the initialization of the AMX system. Once AMX has initialized all of its internal variables and structures, it executes a sequence of user provided Restart Procedures. These procedures can invoke AMX services to start tasks and initialize interval timers. It is also possible to pre-define specific tasks and timers which will be automatically created by AMX during its initialization phase.

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Once all of the application Restart Procedures have been executed, control is transferred to the AMX Task Scheduler which selects the highest priority task which is ready for execution and gives it control of the processor.

The task ends by returning to the AMX Task Scheduler which again finds the next highest priority task ready to execute and gives it control of the processor. A task, once executing, is free to call any of the AMX task services. For instance, a task can send a message to another task, wait for an event or wait for a timed interval. If the task wishes to wait for an event, the AMX service procedure will suspend the task and request the AMX Task Scheduler to force execution of the next highest priority task ready for execution.

When a hardware interrupt occurs, the processor automatically vectors control to a user provided Interrupt Service Procedure. This procedure services the hardware device to clear the source of interrupt. If the procedure wishes to invoke any of the AMX services, it must inform the AMX Interrupt Supervisor that the interrupt has occurred. It is then free to invoke the AMX services to send messages to tasks, signal events or wake tasks. The procedure ends by informing the AMX Interrupt Supervisor that the interrupt service is complete.

The AMX Interrupt Supervisor will either return directly to the interrupted task, or, if as a result of servicing the interrupt, a higher priority task is ready for execution, will request the AMX Task Scheduler to suspend the interrupted task at its point of interruption and begin or resume execution of the higher priority task.

Whenever a clock interrupt occurs, the application Interrupt Service Procedure dismisses the hardware clock interrupt and calls the AMX Clock Handler to trigger the AMX Kernel Task if required. The Kernel Task is triggered at the user defined system tick interval if, and only if, there is any outstanding timing activity required in the system. In this case, the interrupted task is suspended and the AMX Kernel Task begins execution.

The AMX Kernel Task monitors all tasks which are in a timed wait state. If a task's timer expires, the AMX Kernel Task primes the task to resume execution with a timeout indication.

The AMX Kernel Task also services all active user timers. Whenever a user timer expires, the corresponding user Timer Procedure is executed. This procedure can invoke a subset of the AMX services to send messages to tasks, signal events or wake tasks. If the timer is defined to be periodic, the AMX Kernel Task automatically restarts it with the pre-defined period.

This brief description of the AMX Multitasking Executive is provided to show the simplicity of the operating system. The rules are simple. Restart Procedures kick the system off the ground. Tasks execute in response to requests from other tasks, from Interrupt Service Procedures or from Timer Procedures. Tasks can create other tasks. Interrupt Service Procedures handle the specifics of the hardware configuration, service the device in question and send messages to tasks to handle the more complex processing associated with the event. Timer Procedures provide precise interval measurement and can be used to provide periodic requests for task execution.