INT 21 - DOS 1+ - SELECT DEFAULT DRIVE
        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
SeeAlso: AH=3Bh,AH=DBh