Int 21H, AH=25H

1 and higher

SET INTERRUPT VECTOR

      AH = 25h
      AL = interrupt number
      DS:DX -> new interrupt handler

none

this function is preferred over direct modification of the interrupt
        vector table
      some DOS extenders place an API on this function, as it is not
        directly meaningful in protected mode
      under DR DOS 5.0-6.0, this function does not use any of the
        DOS-internal stacks and may thus be called at any time; however,
        under Novell DOS 7.0 - DR-DOS 7.02, this function was not reentrant.
        Since 1998/05/29, DR-DOS 7.03 no longer uses any internal stacks and
        tests for this function much earlier, to allow a minimal stack usage
        of just two words in addition to the IRET frame, allowing it to be
        called from INT 21h functions, specificially device drivers.  This
        fixes the MCS SMB client
      Novell NetWare (except the new DOS Requester) monitors the offset of
        any INT 24 set, and if equal to the value at startup, substitutes
        its own handler to allow handling of network errors; this introduces
        the potential bug that any program whose INT 24 handler offset
        happens to be the same as COMMAND.COM's will not have its INT 24
        handler installed
       

AX=2501h,AH=35h

Text based on Ralf Brown Interrupt List Release 61