NAME
        SumKickData -- compute the checksum for the Kickstart delta list

   SYNOPSIS
        checksum = SumKickData()
        D0

        ULONG SumKickData(void);

   FUNCTION
        The Amiga system has some ROM (or Kickstart) resident code that
        provides the basic functions for the machine.  This code is
        unchangeable by the system software.  This function is part of a
        support system to modify parts of the ROM.

        The ROM code is linked together at run time via ROMTags (also known
        as resident structures, defined in exec/resident.h).  these tags tell
        Exec's low level boot code what subsystems exist in which regions of
        memory.  The current list of ROMTags is contained in the ResModules
        field of execbase.  by default this list contains any romtags found
        in the address ranges $F80000-$FFFFFF and $F00000-$F7FFFF.

        There is also a facility to selectively add or replace modules to the
        ROMTag list.  These modules can exist in RAM, and the memory they
        occupy will be deleted from the memory free list during the boot
        process.  SumKickData() plays an important role in this run-time
        modification of the ROMTag array.

        Three variables in execbase are used in changing the romtag array:
        KickMemPtr, KickTagPtr, and KickCheckSum. KickMemPtr points to a
        linked list of mementry structures. the memory that these mementry
        structures reference will be allocated (via AllocAbs) at boot time.
        The mementry structure itself must also be in the list.

        KickTagPtr points to a long-word array of the same format as the
        ResModules array.  The array has a series of pointers to ROMTag
        structures.  The array is either NULL terminated, or will have an
        entry with the most significant bit (bit 31) set.  The most
        significant bit being set says that this is a link to another
        long-word array of ROMTag entries.  This new array's address can be
        found by clearing bit 31.

        KickCheckSum has the result of SumKickData().  It is the checksum of
        both the KickMemPtr structure and the KickTagPtr arrays.  If the
        checksum does not compute correctly then both KickMemPtr and
        KickTagPtr will be ignored.

        If all the memory referenced by KickMemPtr can't be allocated then
        KickTagPtr will be ignored.

        There is one more important caveat about adding ROMTags. All this
        ROMTag magic is run very early on in the system -- before expansion
        memory is added to the system. Therefore any memory in this
        additional ROMTag area must be addressable at this time. This means
        that your ROMTag code, mementry structures, and resident arrays
        cannot be in expansion memory.  There are two regions of memory that
        are acceptable:  one is chip memory, and the other is "Ranger" memory
        (memory in the range between $C00000-$D80000).

        remember that changing an existing romtag entry falls into the
        "heavy magic" category -- be very careful when doing it.  The odd are
        that you will blow yourself out of the water.

   NOTE
        SumKickData was introduced in the 1.2 release

   RESULT
        Value to be stuffed into ExecBase->KickCheckSum.

   WARNING
        After writing to KickCheckSum, you should push the data cache.
        This prevents potential problems with large copyback style caches.
        A call to cacheclearu will do fine.

   SEE ALSO
        initresident(), findresident()