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

AH=35h

Text based on Ralf Brown Interrupt List Release 61