[daniel02216]

This is one reason why OS X enforces a hard page-zero on 64-bit programs - it is a hard error to map or allocate anything in the lowest 4 GB of memory, so programmers will immediately discover pointer truncation bugs when porting 32-bit code to 64-bit.

[cokernel_hacker]

What if you have a load command in your mach-o which asks for an address below the 32-bit boundary? Will loading the image fail?