Int 21 Fn 3305  - Dos 4.0+ - Get Boot Drive                                [D]

   AX = 3305h

Return: DL = boot drive (1=A:,...)

Notes: fully reentrant
   NEC 9800-series PCs always call the boot drive A: and assign the other
     drive letters sequentially to the other drives in the system
   this call is supported by OS/2 Warp 3.0, but not earlier versions of
     OS/2; it is also supported by Novell DOS 7