Int 21 Fn 0E  - DOS 1+ - Select Default Drive                              [D]

   AH = 0Eh
   DL = new default drive (00h = A:, 01h = B:, etc)

Return: AL = number of potentially valid drive letters

Notes: under Novell NetWare, the return value is always 32, the number of
     drives that NetWare supports
   under DOS 3+, the return value is the greatest of 5, the value of
     LASTDRIVE= in CONFIG.SYS, and the number of drives actually present
   on a DOS 1.x/2.x single-floppy system, AL returns 2 since the floppy
     may be accessed as either A: or B:
   otherwise, the return value is the highest drive actually present
   DOS 1.x supports a maximum of 16 drives, 2.x a maximum of 63 drives,
     and 3+ a maximum of 26 drives

See Also: AH=19h,AH=3Bh,AH=DBh