Switchar/availdev (INT 21h, 37h)

        Entry:
          AH = 37h
          AL = subfunction
                 0 Read switch character (returns current character in DL)
                 1 Set switch character (specify new character in DL)
                 2 Read device availability (as set by AL=03h)
                   Return: DL = device availability
                 3 Set device availability, where:
                     DL = 00h means \DEV\ must preceed device names
                     DL <> 00h means \DEV\ need not preceed device names

        Return:
          DL = Switch character (if AL=0 or 1)
               Device availability flag (if AL=2 or 3)
          AL = AL = FFh if invalid subfunction.