There doesn't seem to be a complete enum.  From http://mono-project.com/FAQ%3a_Technical#Mono_Platforms:

"The newer framework 2.0 added Unix to the PlatformID enum but, sadly, with a different value: 4 and newer versions of .NET distinguished between Unix and MacOS X, introducing yet another value 6 for MacOS X."

I will still need p = 128.  Will that be OK?