It also includes the device driver test kit DDTK to test the native drivers extensively. Real-time considerations. Windows CE v. Device drivers are not immune to those changes and can, in fact, diminish real-time performance if not properly implemented.
As mentioned earlier, nested interrupts are supported based on their priorities. Note that some architectures do not support nesting interrupts on their priorities x86 is one , so this concept does not always apply. If it does, ISRs must be written accordingly.
For instance, critical sections might require turning off all interrupts. This is even more true if, for performance consideration, the interrupt is directly processed in the ISR instead of the IST. In such a case, turning off interrupts directly impacts the systems' interrupt latency, and may degrade the overall system performance if not carefully fine-tuned.
Interrupt service threads are subject to premption by the scheduler based on their priority, a situation that may not be acceptable in some cases. Under 3. Real-time processing can be achieved by raising a given IST's priority, but again, the impact of doing so must be reviewed against the other ISTs in the system. It is a good practice to store a driver's priority in the registry where it can easily be modified, as opposed to hard-coding it in the driver.
This naturally requires the driver to load the priority value from the registry and adjusts the IST's priority accordingly once created.
An IST can also set its quantum the time it executes before the scheduler kicks in to a value other than the default which is set in the OAL. In particular, an IST can set its quantum to 0, which means run to completion. However, whatever the quantum value is, interrupts are still processed. Should a higher priority thread become ready to run as the result of processing an interrupt, this thread will preempt the current thread. Therefore, run-to-completion is useful if the driver doesn't want to be interrupted by another thread of the same priority in a multi-threaded driver for instance.
Running to completion has little impact on the overall system performance, since higher-priority threads can still preempt as needed.
There is no doubt that developing drivers for Windows CE 3. However, it is important to note that custom driver development requires substantial work, ranging from a few weeks to a few months. Engineers new to CE typically require three to six months to go through the steep learning curve, although training class will probably reduce the period of time.
No matter what approach you take, it is important to allocate ample resources and time for Windows CE device driver development.
Since , he's been involved in the development of Windows CE devices, from platform adaptation to application development. Contact him at. You must Sign in or Register to post a comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. You must verify your email address before signing in. Check your email for your verification email, or enter your email address in the form below to resend the email.
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Thank you for verifiying your email address. We didn't recognize that password reset code. We've sent you an email with instructions to create a new password. Skip to content Search for:. Native drivers Windows CE has been designed to directly use built-in devices. Stream interface drivers Stream interface drivers all share a common interface. Device driver implementation Adapting Windows CE to a specific platform is mostly a matter of developing the drivers for the particular devices the board features.
Driver architecture As mentioned above, built-in devices can be controlled by native drivers, which are linked with the GWES, and stream interface drivers, which areloaded by the device manager. Tools Microsoft provides a driver development tool called Platform Builder. Real-time considerations Windows CE v. Previous Mobile IP. You may have missed. January 13, Nitin Dahad. January 12, Nitin Dahad. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits.
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Software in such a scenario might play a more active role in the operation of the conveyor belt, perhaps sending a signal to a motor controller to increase or decrease the speed when the current value is out of some predefined range. A distinction is sometimes made between hard real-time systems and soft real-time systems. A hard real-time system doesn't tolerate missed deadlines.
Missing even a single deadline results in a catastrophe. Consider a high-speed, high-volume color printer that might be used to print the paper edition of a magazine like the one you are reading now. Failure to place each one exactly right results in blurry images and upset readers. A soft real-time system tolerates some amount of failure. In a data collection system, for example, recovery from missing data points can sometimes be extrapolated from known values.
One example of a soft real-time system is a multimedia application in which there is a relatively high tolerance for missing deadlines related to decompressing sound or video. Such failures might result in temporary degradation of output quality, but the presentation itself remains largely intact. A serious real-time system implementor uses the terms milliseconds and microseconds to define specific time requirements.
With the Sleep function, it is the amount of time a thread should yield to the scheduler. The WaitForSingleObject function uses its millisecond parameter to specify a timeout for waiting on a blocked synchronization object.
In the past, dedicated hardware was needed to support real-time requirements that require microsecond sensitivity. As CPUs have gotten faster, microsecond-level real-time requirements can be met with a heavier reliance on software than was possible in the past. Another important term is interrupt latency, the delay between a hardware interrupt and the start of the actual processing of the interrupt. For some, this is a key benchmark since this sets a limit on the responsiveness of a device to outside events.
A core goal for Windows CE 3. While this goal has been met, this number is probably not very meaningful unless you happen to want to use the exact hardware configuration that was used in the testing. Otherwise, it's best if you do your own testing on your own hardware.
You'll find details of the performance testing support that Microsoft provides in the Windows CE 3. When you evaluate an operating system for time-critical support, you start by identifying those features that can affect real-time performance.
For my purposes, I am going to focus on these elements: interrupt handling, processes, threads, synchronization objects, and memory. Note: this section covers features that are part of every version of Windows CE. If you're primarily interested in the real-time enhancements that are specific to Windows CE 3.
Interrupt handling in Windows CE is a multistage process, as you can see in Figure 1. Device driver writers provide two pieces that play a key role in interrupt handling: a tiny, kernel-mode Interrupt Service Routine ISR to decide how to handle an interrupt, and a user-mode Interrupt Service Thread IST to provide processing when any substantial work must be done. Figure 1Interrupt Handling. Device driver writers call two separate Win32 functions at driver initialization time to glue the pieces together.
HookInterrupt connects specific interrupts to the ISR. This function gets called at system bootup. Note that system bootup happens for a cold-start, or when the reset button is pushed; the boot sequence does not occur when the system wakes from the sleep state caused, for example, by hitting the power button on a handheld PC or on a Pocket PC. It waits until the ISR tells the kernel to set the event that allows it to run. Once everything is in place, the only thing that remains is for an interrupt to occur.
When an interrupt occurs, the Windows CE kernel responds by saving the state of currently executing user-mode code. The kernel calls the appropriate ISR to ask it to quickly decide how to handle the interrupt. An ISR is a function in the device driver that might buffer incoming bytes, but whose primary job is to decide how an interrupt should be handled.
It lets the kernel know the results of its decision by means of the return value it passes back to the kernel. The kernel uses this identifier to scan its table for the event handle of the associated IST. The kernel can then trigger the IST to start running by putting its event into a signaled state with a call to SetEvent. Of course, an IST doesn't have to be triggered for every interrupt.
For example, the GPS driver mentioned earlier stores up location information until its buffers are full. Only when its buffers are full does the driver tell the kernel to trip the event and cause its IST to run and handle the buffered data.
There are several reasons that ISRs delegate most of the work of interrupt handling. Another constraint is the fact that there is a very limited set of system services that are available to the ISR. Perhaps the most important consideration is the impact on other parts of the system, including other interrupts and even high-priority threads. When an ISR runs, everything else that's in the system is asleep.
If an ISR spends too much time handling a particular interrupt, it might affect the proper operation of other time-critical operations. For all of these reasons, an ISR delegates most of the actual work of handling the interrupt. An IST is a thread created by the device driver at driver startup, using the same Win32 function that anyone can use: CreateThread. Once started, an IST starts its workday by going to sleep not a bad job if you can get it!
It goes to sleep by calling the WaitForSingleObject function. As you may have noticed, the Win32 threading functions play a significant role in all of the interrupt code. This is in stark contrast to the separate driver interfaces that device driver writers may have worked with in the past. Figure 2Processes in the Remote Process Viewer. A process in Windows CE is like a typical process found everywhere else in that it represents a unit of ownership. The most important thing that a process owns is its private address space.
A process in Windows CE has a smaller address space than processes on desktop systems. Separate address spaces protect each process from careless code in other processes, contributing to the overall robustness of the system and of each application.
An important aspect of the Windows CE context switching mechanism is that a context switch was designed to be very fast. This is due in part to the way that process address spaces are set up. Unlike in Windows , which gives each process its own set of page tables, on Windows CE all processes share a single set of page tables. The benefits include saved memory usage and faster context switch time.
A process switch is much faster on Windows CE than on Windows This has important implications for developers of time-critical systems. It means you are free to use multiple processes that have all the benefits of separate processes but minimal interprocess context switch delays. Figure 3Thread Priorities. Figure 3 shows how the thread priorities for a Windows CE 1. The range of available priorities for the Windows CE 1. The importance of this change depends on how important thread priorities are to your application.
If thread priorities are important, and if you have an application written to run on Windows CE 1. This will allow you access to the broader range of Windows CE 3. Threads are important for real-time programming because they give you a way to separate time-critical work from work that is not time-critical. The scheduler allows threads with a higher priority to run before threads with a lower priority, which you assign by calling SetThreadPriority you can access the wider range of priorities on Windows CE 3.
So if you have a task or set of tasks that are time-critical, you can create a separate thread and assign it responsibility for that work. Students in my classes frequently want to know when to use threads. If you think of threads as being like people, you can start to see how creating threads is almost like hiring a new person to join your team.
Some tasks, like driving a car, are meant to be done by a single person. It's just not as much fun trying to do it alone. In the same way, some things are meant to be handled by more than a single thread. The most obvious involves the asynchronous nature of outside devices. A second obvious example has to do with communicating with the outside world.
A program's user interface should have a dedicated user interface thread, which doesn't have many other responsibilities beyond responding to the needs of the user. And when you have time-critical work to do, this should also get delegated to a thread that does little else besides executing your critical code. It should avoid user interface work, file system work, and interacting with random communications devices unless it is an integral part of its time-critical operation.
A specialist real-time thread is what the doctor especially Dr. GUI orders when you are serious about any time-critical operations.
Of course, you know that adding a new person to a team has its complications, and the same is true with threads. Adding a new person to a baseball team involves deciding what position that person can play. It also takes time to coordinate a player's style of fielding with other players on the field. The moment you add a second thread to a process, you introduce the possibility that the two threads will run into each other. Like two baseball players who collide while trying to field a ball, two threads can collide when accessing any of the myriad things that they share.
To establish boundaries between the operations of different threads, the Win32 API provides synchronization objects. Synchronizing the activity of threads is so important that Windows CE has always had most of the Win32 synchronization objects. All versions of Windows CE support critical sections, mutexes, and events.
Support for a fourth type of synchronization object, semaphores, was added in Windows CE 3. This means the latest release of Windows CE has the same set of synchronization objects that current desktop versions of Windows have. If you've never worked in a multithreaded environment, it might not be exactly clear what synchronization is and why it is important.
A simple rule is that any time two threads share anything, you must prevent conflicts using synchronization objects. So what does it mean for something to be shared? How can there be a conflict, and how do you guard against it? Figure 4 will shed some light on this. It shows BadThrd. When it starts running, this program creates two threads, with ThreadMainA and ThreadMainB as the entry point for each. The two threads share global variable x. So what exactly happens when this program runs?
There are many possibilities. Perhaps the two threads will get into a deadly embrace and lock up over the variable, or perhaps each thread will be able to run through each of the loops without ever locking up. Perhaps you could ship this code and never encounter a problem.
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