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specify the interfaces and hardware rather than come up with standards that may have to be bent when new technology becomes available or may be interpreted incorrectly by other developers. To make matters worse for the PC, there are literally millions of programmers working on the Windows operating system, drivers, and applications. With the more code that is written, the greater is the chance that incompatibilities between the system, drivers, and applications will occur, making it more likely that there will be problems with everything working properly together. In a small microcontroller, the team of people developing the application and the interface software is very small, and the chances for incompatible application software and hardware interface code not being detected and being passed along to the end user is much more remote. To illustrate how a multitasking RTOS works, I am going to present a web surfer s PC and show how the different features and software interconnect to provide a method of connecting to the Internet. Instead of having multiple computers, each one providing one function; the PC makes functions (known as resources) available to the different applications, as shown in Fig. 13.1. In a real system, the different tasks (which are represented by different boxes in Fig. 13.1) would be given priorities. This re ects the importance of messaging and operation of the speci c tasks relative to the others. This means that if high- and medium-priority tasks are waiting to execute, the high-priority task will run rst. In most PCs, the Modem task would be given a speci c priority because it is the resource that is the most constrained by the speed at which it can operate and the demands by the different applications for the available bandwidth. In periods of inactivity (between the surfer s keystrokes with no web pages being currently downloaded), low- or medium-priority tasks are executed because the highpriority tasks are not required. Low-priority tasks normally are interactions with the operator because if a task takes a long time to execute (in computer terms), a human working with it probably won t notice any delays in the computer s operation.
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Figure 13.1 The major tasks and hardware interfaces that are needed to implement a web-sur ng application.
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All the tasks can be arranged in a diagram to show how they communicate via messages. In Fig. 13.1, the arrows represent the directions messages move in. An important concept about messaging in an RTOS is that each message is initiated by a task. Normally, each task is waiting (blocking) on a message, waiting to respond to it and execute the request that is part of the message. After the task has executed its function, it then may respond to the conditions by sending messages to other tasks. For example, to send e-mail, the Modem task could execute the code
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ModemSend() { char * Packet; while ( 1 == 1 ) { while (GetMsg() == 0); while (ModemBusy != 0); Packet = ReadMsg(); SendMsg(Packet); AckMsg(); } } // elihw // Modem Data Packet Send Routine
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Packet to be Sent Loop forever Wait for a Message to Send Out
// Wait for the Modem to be Available // Read the Sent Packet
// Send the Packet over the Internet // Acknowledge the Original Message
// End ModemSend
This task will wait for a packet to be passed to it that will have to be sent to the Internet. If the modem is already being used (i.e., a previous packet is being sent or a packet is being received), it will wait for the modem to become free before sending the packet. Once the packet is in the queue to be sent by the modem, the ModemSend task acknowledges the original request to send the packet and waits for the next request to come in. This is a grossly oversimpli ed example of what actually happens, but it should give you the idea that tasks send request messages to other tasks to request that a function be performed. The receiving tasks only process the request when they are able to and acknowledge that they have done so when the resources to do so are free. I should point out one aspect of tasks and processes in RTOSs that may be not be readily apparent. Tasks should access only one hardware device. Thus the SendModem task in the preceding example only accesses the modem output functions. To access other hardware in the system, the RTOS application will send messages to the appropriate tasks that control the different hardware. This is an important philosophical point about access and RTOSs because, obviously, applications could be written that accessed multiple hardware interfaces. This
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