Figure 1.1. WinDriver Architecture

For hardware access, your application calls one of the WinDriver user-mode functions. The user-mode function calls the WinDriver kernel, which accesses the hardware for you through the native calls of the operating system.

WinDriver's design minimizes performance hits on your code, even though it is running in user mode. However, some hardware drivers have high performance requirements that cannot be achieved in user mode. This is where WinDriver's edge sharpens. After easily creating and debugging your code in user mode, you may drop the performance-critical modules of your code (such as a hardware interrupt handler) into the WinDriver Kernel PlugIn without changing them at all. Now, the WinDriver kernel calls this module from kernel mode, thereby achieving maximal performance. This allows you to program and debug in user mode, and still achieve kernel performance where needed. For a detailed overview of the Kernel PlugIn feature, see Chapter 11.
Kernel PlugIn is not implemented under Windows CE. In this operating system there is no separation between kernel mode and user mode, therefore top performance can be achieved without using the Kernel PlugIn. To improve the interrupt handling rate on Windows CE, follow the instructions in Section 9.2.8.1 of the manual.