$08000000-$0fffffff is reserved for CPU accelerator board RAM expansion. (also used by some A1200 boards too, it is not limited to A3000/A4000)
Check makes sure address $08000000 exists, for example $08000000 becomes $00000000 if CPU only has 24-bit address space (detecting between 68020 and 68EC020) or if not all address lines are connected. You don't want to overwrite execbase and exception vectors when testing memory
