Reserving COM port(s) used by serial GPS devices - a quick guide

After successfully getting Windows 7 Pro 32bit on my CF-19 MK2 (CF-19FHGCFNS), I noticed that when installing devices that required virtual serial ports they would all be assigned to COM3 first, which I already knew at that point was the interface to the built-in GPS device present with my model.

For a while (due to time constraints), I just put up with the issue by altering the COM ports used by the newly installed devices via Device Manager to a truly free COM port.
I have been hooking up to lots of diagnostic equipment to this machine, and a few USB modems, so this was becoming quite a regular occurrence.

Finally, thanks to these links;

http://www.rosseeld.be/DRO/PIC/NextSerialPortNo.htm

http://forum.gpsgate.com/topic.asp?TOPIC_ID=8062

...I figured out how to force windows to ignore COM3 when installing devices.

Sure enough, after altering the bitmask to include the 3rd bit, COM3 was no longer available for use by other programs.
It's easy to check this if you look at the Advanced Port Settings dialogue in Device Manager for an existing device (e.g. one of the built-in modems). The COM port you have masked should now show as 'COM x (in use)' in the dropdown list. The same information is used during device installation.

Hopefully others will find this useful and perform this tweak when applying the GPS registry fix, before installing any HSDPA modems and such.
Let me know if this breaks anything so I can avoid the same fate!