Windows CE programmers are familiar with File System Filter drivers. Starting with WEC7 the device manager supports filter drivers for Stream Interface Drivers. This is particulary useful if you want to modify how a device driver works. Suppose you have a driver in binary format -thus not source code to modify- and you want to add a custom IOCTL: you can create a filter driver which in its XXX_IOControl entry point implements the specific IOCTL, passing all the others to the original device driver. In another post we’ll see how to implement a filter driver.