if you're doing for a1200, you create the normal rom and extended roms (from address 0xe00000 as doobrey pointed and with romheader) apply the 1mb patch to your exec. dissect your roms wordwise(16 bit) into u6a and u6b pieces. then concatenate extended roms u6a with normal u6a (extended first) and extended u6b with normal u6b (again, extended first) burn then with byte swap and it should work.
