Note: This API call is for DOS personality only. Use Family API for portability.

2018/09/07 07:04 · prokushev

Int 21H, AH=25H


1 and higher



Family API


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




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

See also



Process manager INT 20H, INT 21H: 00H, 25H, 26H, 31H, 34H, 35H, 4CH, INT 22H, INT 27H, INT 28H
File manager INT 25H, INT 26H, INT 21H: 0DH, 0EH, 0FH, 10H, 11H, 12H, 13H, 14H, 15H, 16H, 17H, 19H, 1AH, 1BH, 1CH, 21H, 22H, 23H, 24H, 27H, 28H, 29H, 2EH, 2FH, 32H, 3305H, 36H, 39H, 3AH, 3BH
Character Device I/O INT 29H, INT 21H: 01H, 02H, 03H, 04H, 05H, 06H, 07H, 08H, 09H, 0AH, 0BH, 0AH, 0CH
Signals INT 23H, INT 24H, INT 21H: 3300H, 3301H, 3302H
Memory manager
Date and Time INT 21H: 2AH, 2BH, 2CH, 2DH
Misc INT 21H: 30H, 3306H, 3700H, 3701H, 3702H, 3703H
NLS INT 21H: 3303H, 3304H, 3800H, 3801H
2018/09/04 19:23 · prokushev

en/docs/dos/api/int21/25.txt · Last modified: 2020/12/09 02:54 by prokushev