Nicolas Besson - Microsoft Windows Embedded CE 6.0 Exam Preparation Kit

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Located at the core of the operating system, the OAL is a perfect choice for mechanisms to measure the performance of the system and support performance optimization. As discussed in Chapter 3, "Performing System Programming," you can use the Interrupt Latency Timing (ILTiming) tool to measure the time it takes to invoke an interrupt service routine (ISR) after an interrupt occurred (ISR latency) and the time between when the ISR exits and the interrupt service thread (IST) actually starts (IST latency). However, this tool requires a system hardware tick timer or alternative high-resolution timer that is not available on all hardware platforms. If the new hardware platform supports a high-resolution hardware timer, you can support ILTiming and similar tools by implementing the functions listed in Table 5-7.<\/p>

Table 5-7 Profile timer support functions<\/strong> <\/p>

Function<\/th> Description<\/th> <\/tr>
OEMProfileTimerEnable<\/td> Enables a profiler timer.<\/td> <\/tr>
OEMProfileTimerDisable<\/td> Disables a profiler timer.<\/td> <\/tr> <\/table>
NOTE<\/div>
Thread scheduling and interrupt handling<\/div>

The OAL must also support interrupt handling and the kernel scheduler. The scheduler is independent of the processor type, yet interrupt handling must be optimized for different types of processors.<\/p> <\/cite>

Integrating New Device Drivers<\/p> <\/div>

Apart from the core system functions, the BSP also contains device drivers for peripherals. These peripheral devices can be components on the processor chip or external components. Even when separate from the processor, they remain an integral part of the hardware platform.<\/p>

Device Driver Code Locations<\/p> <\/div>

Table 5-8 lists the source code locations for device drivers according to the PQOAL model. If your BSP is based on the same processor as the reference BSP, then the adaptation of device drivers mainly requires modification to the source code in the %TGTPLATROOT% folder. It is also possible to add new drivers to the BSP if the new platform includes peripherals that are not present in the reference platform. For more information about developing device drivers, see Chapter 6, "Developing Device Drivers."<\/p>

Table 5-8 Source code folders for device drivers<\/strong> <\/p>

Folder<\/th> Description<\/th> <\/tr>
%_WINCEROOT%\Platform\%_TGTPLAT%<\/td> Contains platform dependent drivers.<\/td> <\/tr>
%_WINCEROOT%\Platform\Common\Src\Soc<\/td> Contains drivers for processor-native peripherals.<\/td> <\/tr>
%_WINCEROOT%\Public\Common\Oak\Drivers<\/td> Contains drivers for non-native peripherals that include external controllers.<\/td> <\/tr> <\/table>

Modifying Configuration Files<\/p> <\/div>

If you cloned your BSP from an existing BSP, all configuration files are already in place. However, it is important that you review the memory layout in the Config.bib file, as explained in detail in Lesson 2. The other configuration files require modifications only if you added new drivers or modified components in the BSP, as explained in Chapter 2, "Building and Deploying a Run-Time Image."<\/p>

Lesson Summary<\/p> <\/div>

It is advantageous to start the BSP development process by cloning an appropriate reference BSP. Ideally, this BSP should be based on the same or similar hardware platform because this makes the most of tested and proven production features. Windows Embedded CE features a PQOAL architecture and Platform Builder tools that facilitate the cloning process. The goal is to create a bootable system with minimum customizations and then add additional features and support for peripheral devices as necessary.<\/p>

The first component of a BSP that you might have to adapt is the boot loader, which is responsible for initializing the hardware platform and passing execution to the kernel. The second component is the OAL, which contains the platform-specific code that the kernel needs for hardware initialization, interrupt handling, and timer handling for the thread scheduler, KITL, and kernel debug output. The third part of the BSP you must adapt is the device drivers for peripheral devices. The fourth part of the BSP requiring adaptation is the configuration files that control the build process, determine the memory layout, and specify system configuration settings. If the BSP adaption is based on a reference BSP for the same processor architecture, then most of the CPU-related and memory controller-related BSP code can remain unchanged. You only need to address platform-specific code portions that focus on bringing up the hardware, rather than creating the necessary setup for the BSP.<\/p>

Lesson 2: Configuring Memory Mapping of a BSP<\/p> <\/div>

Memory management in Windows Embedded CE has changed significantly from previous versions. In past versions, all processes shared the same 4 GB address space. With CE 6.0, each process has its own unique address space. The new system of managing virtual memory enables CE 6.0 to run up to 32,000 processes in contrast to the previous 32 processes limitation. This lesson covers the details of the new memory architecture and management, so that you can map virtual memory regions to correct physical memory addresses on the platform.<\/p>

After this lesson, you will be able to:<\/strong> <\/p>

■ Describe how Windows Embedded CE manages virtual memory.<\/p>

■ Configure static memory mappings for a hardware platform.<\/p>

■ Map noncontiguous physical memory to virtual memory on the system.<\/p>

■ Share resources between OAL and device drivers.<\/p>

Estimated lesson time: 15 minutes.<\/strong> <\/p> <\/cite>

System Memory Mapping<\/p> <\/div>

Windows Embedded CE uses a paged virtual memory management system with a 32-bit virtual address space, mapped to physical memory by using the MMU. With 32 bits, the system can address a total of 4 GB of virtual memory, which CE 6.0 divides into the following two areas (see Figure 5-5):<\/p>

Kernel space<\/strong>Located in the upper 2 GB of virtual memory and shared between all application processes running on the target device.<\/p>

User space<\/strong>Located in the lower 2 GB of virtual memory and used exclusively by each individual process. Each process has its own unique address space. The kernel manages this mapping of the process address space when a process switch occurs. Processes cannot access the kernel address space directly.<\/p>

Kernel Address Space<\/p> <\/div>

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