The default 4GiB setup is done to very quickly catch poorly written / ported 32-bit code that promotes a 32-bit integer to a pointer. In 64bit mode, such a promotion will create a pointer with the top bits set to zero. Any de-reference of this sort of promotion will cause a SEGFAULT. This might happen if some sort of mixed mode compilation happened for a 64bit process where some of the code was still compiled and linked assuming 32bit mode. (mixed mode Intel code mostly)