AH = 37h
        AL = subfunction
           0 Read switch character (returns current character in DL)
           1 Set switch character (specify new character in DL)
           2 (DOS 2.x only) Read device availability (as set by function AL=3)
           3 (DOS 2.x only) Set device availability, where:
             DL = 0 means /DEV/ must preceed device names
             DL <> 0 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=0FFh means the value in AL was not in the range 0-3.
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