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Search icon An illustration of a magnifying glass. User icon An illustration of a person's head and chest. The kernel's event handler is the first to catch a raised interrupt. The interrupt, as well as lower priority interrupts, are disabled, but all other interrupts of higher priority remain enabled provided the underlying architecture supports nesting interrupts based on their priorities.
The related interrupt service routine ISR is then called. The sole purpose of the ISR is to return a logical identifier to the interrupt service handler ISH in the kernel, but it can implement more functionality if performance is an issue. Since Windows CE 3. For instance, ISRs that implement critical code that can't be preempted must temporarily disable all interrupts. However, this should be done only to execute a few instructions, since keeping higher-priority interrupts disabled may interfere with the real-time nature of the system more about this later.
The kernel uses the returned logical identifier to unblock the related interrupt service thread IST. The IST resides in the driver and implements the core driver's functionality interrupt processing by accessing the hardware as required.
Once the interrupt is processed, the IST signals the end of interrupt and the kernel re-enables the interrupt. Developers can assign one handler per interrupt or one handler for multiple interrupts. Listing 2 shows parts of the serial communication interrupt handling on an x86 platform. The code is adapted from Platform Builder 3. Listing 3 shows parts of the IST of a serial communication driver for an x86 platform. This thread waits for the event object to be set by the kernel when the ISR returns the related interrupt identifier , then handles the interrupt.
Once the interrupt is processed, the thread calls Interrupt-Done to signal the interrupt processing completion to the kernel. 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. Drivers linked with the GWES must adhere to a predefined interface called the device driver interface DDI , whereas stream interface drivers are required to implement the standard stream functions.
Drivers for built-in devices can have a layered or monolithic architecture, as illustrated in Figure 2. A layered driver relies on a piece of code that can be reused across platforms to simplify and shorten the development time. This code, provided by Microsoft, is called the module device driver MDD and implements the core functionality of the driver. It doesn't access the hardware directly, but relies on another piece of code, which is hardware-dependent, called the platform-dependent driver PDD.
When porting a layered driver from one platform to another, only the PDD has to be rewritten, not the entire driver. If performance is an issue, one might prefer to develop a given driver as a monolithic piece of code, as opposed to a layered one. The development time is increased, and the code cannot easily be ported to another platform, but a tighter integration-which is what a monolithic driver provides-might be the only approach to meet certain performance requirements.
Whichever architecture is retained, the driver must always conform to the DDI for the device it controls. Most drivers for built-in devices follow a layered architecture because Microsoft provides layered samples for them. Nonetheless, an OEM can re-implement those drivers to better fit its devices. The only drawbacks are that the modified MDD might not be as portable as the original of course, this might not be an issue , and changes might be required in the PDD.
Microsoft provides a driver development tool called Platform Builder. Actually, Platform Builder allows building, downloading, and debugging complete Windows CE images including device drivers for various processors. It integrates an editor, cross-compilers for all supported processors, a linker, an integrated debugger, an image loader, and a flock of additional tools.
New to Platform Builder v. 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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