The driver is written as a basic streams interface, however it only uses the Init and Deinit functions. These are the required entry points needed by DEVICE.EXE in order to load as a device driver.

The driver reads the registry to get which COM port to use via the function

GetCommPort().  Example registry: 
IF CEPC_SERMOUSE 
[HKEY_LOCAL_MACHINE\Drivers\BuiltIn\SerMouse] 
   "Index"=dword:1 
   "Dll"="SerMouse.dll" 
   "Prefix"="SRM" 
   "Order"=dword:1 
   "Port"=dword:1 
ENDIF 

Implemenation note: You could make a Mouse Set Speed utility by allowing an app to do a CreateFile on "SRM1:" and making IOCTL calls. Thus you'd also have to implement SRM_Open, SRM_Close, SRM_IOControl.