> preventing this sort of backward compatiblity issue is the reason why AMD64 will trap if the unused bits of the address space are not all 1 or 0. This helps

> preventing this sort of backward compatiblity issue is the reason why AMD64 will trap if the unused bits of the address space are not all 1 or 0. This helps debugging, but it doesn't help future extensions of AMD64. Programs exist that store data in bits 48..63 of a pointer on AMD64, and expect to retrieve a pointer by doing "x << 16 >> 16" on the integer data. Increasing the address space is a pain no matter what.