Int 2F Fn 1210  U - DOS 3+ Internal - Find Unreferenced Disk Buffer        [D]

   AX = 1210h
   DS:DI -> first disk buffer to check

Return: ZF clear if found
       DS:DI -> first unreferenced disk buffer
   ZF set if not found

Note:  in DOS 5+, this has become essentially a NOP, invoking the same code
     used by AX=1224h (SHARING DELAY)

See Also: AX=120Eh