1. The first step is to find the source code. The source code may be in different places depending on the version of Windows CE that you are targeting and the BSP that you are using. The following are some locations to look:
Windows CE 4.2 and earlier:
%_WINCEROOT%\Platform\<BSP Name>\eboot
Windows CE 5.0 and 6.0:
%_WINCEROOT%\Platform\<BSP Name>\src\Bootloader
If you have received your BSP from a third party, the location could be different.