DOS(2) getting assigned drive letters

Hai,

I'm in the middle of developing some file wrapper stuff, don't ask. I can't get my head around how to get assigned drives information. You know, which drives are registered in DOS(2) at the moment? Drive A:, B:,H: ?

Couldn't find anything like that in the DOS specs other than http://map.grauw.nl/resources/dos2_functioncalls.php#_SELDSK function. But that only tells me how many, not which drive letters are taken.

So, anybody?