Is memory address 0x0 usable?

Question

I was wondering... what if when you do a new, the address where the reservation starts is 0x0? I guess it is not possible, but why? is the new operator prepared for that? is that part of the first byte not usable? it is always reserved when the OS starts?

Thanks!

Answer 1

The null pointer is not necessarily address 0x0, so potentially an architecture could choose another address to represent the null pointer and you could get 0x0 from new as a valid address. (I don't think anyone does that, btw, it would break the logic behind tons of memset calls and its just harder to implement anyway).

Whether the null pointer is reserved by the Operative System or the C++ implementation is unspecified, but plain new will never return a null pointer, whatever its address is (nothrow new is a different beast). So, to answer your question:

Is memory address 0x0 usable?

Maybe, it depends on the particular implementation/architecture.

Answer 2

"Early" memory addresses are typically reserved for the operating system. The OS does not use early physical memory addresses to match to virtual memory addresses for use by user programs. Depending on the OS, many things can be there - the Interrupt Vector Table, Page table, etc.

Here is a non-specific graph of layout of physical and virtual memory in Linux; could vary sligthly from distro to distro and release to release:

http://etutorials.org/shared/images/tutorials/tutorial_101/bels_0206.gif

^Don't be confused by the graphic - the Bootloader IS NOT in physical memory... don't know why they included that... but otherwise it's accurate.