In the normal memory area we would count from 0x0000 and upwards... with this method we will define an area of memory for the 680x0 so we'll start from the end of the chunk of memory reserved. The 68k emu will think that the end of the chunk of memory reserved for him is 0x0000 instead of being 0xffff (for example).
We would have something like this:
Real addresses:
0x0000.....(x86 memory)........0x8000...(680x0 memory)...0xffff
The logic address the 68k programs would see will be:
0xffff.....(x86 mem).......0x8000....(68k memory)...0x0000
The data from the x86 memory area would look like this:
(start of memory)...12345678THIS_IS_MY_TEXT...(end of mem)
and after copying it to the 68k area it would always be stored in reverse order and the real aspect would be this:
(start of memory)...TXET_YM_SI_SIHT87654321...(end of mem)
But logically as the 68k addresses will start from the end of the chunk they would look as the x86 memory area
(start of memory)...12345678THIS_IS_MY_TEXT...(end of mem)
If we want to modify the numbers "3456" (4 bytes from the 3rd position) and change them by "ASDF" we would look the variable that stores the real ending address of the 68k memory chunk. Let's say it's 100. Our 4 bytes start in the 3rd byte, so we'll make 100 minus 2(the address)+the size of the data to transfer, the real starting address will be 94, and the real ending address will be 98. We simply copy the bytes byte per byte (using the method I have explained to get the real address) if we want to make a byte per byte copy. If the transfer involves copying a 32bit number, for example 12|34|56|78 in big endian, in the x86 memory it will look like 78|56|34|12, so copying it to the 68k ram will not require swapping the bytes because the memory is already reversed.
Have I explained my idea more clearly?
The thing that should be changed is the 68k emu, to use the addresses in the opposite order. The author of Executor did this previously, but I guess that using memory in the opossite order may require some important changes in UAE's 68k emulator. Once we are inside the emulator there shouldn't exist many problems in changing the addresses... it would be transparent for the programs.