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  Function 25H (decimal 37) sets an interrupt vector. Before you call
  function 25H, place the segmented address of an interrupt handler in DS:DX
  and an interrupt number in AL. DOS stores the segment and offset of your
  interrupt handler in the proper interrupt vector.

  When updating an interrupt vector, you should use function 25H instead of
  simply computing the address of the vector and updating it directly. Not
  only is it simpler to call this function than to do the work yourself, but
  this function gives the operating system the chance to detect when an
  important interrupt vector is modified.

  To examine the contents of an interrupt vector, see function 35H in the
  next chapter.