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