Can a 32-bit processor really address 2^32 memory locations?

Question

I feel this might be a weird/stupid question, but here goes...

In the question Is NULL in C required/defined to be zero?, it has been established that the NULL

Answer 1

First, let's note the difference between the linear address (AKA the value of the pointer) and the physical address. While the linear address space is, indeed, 32 bits (AKA 2^32 different bytes), the physical address that goes to the memory chip is not the same. Parts ("pages") of the linear address space might be mapped to physical memory, or to a page file, or to an arbitrary file, or marked as inaccessible and not backed by anything. The zeroth page happens to be the latter. The mapping mechanism is implemented on the CPU level and maintained by the OS.

That said, the zero address being unaddressable memory is just a C convention that's enforced by every protected-mode OS since the first Unices. In MS-DOS-era real-mode operaring systems, null far pointer (0000:0000) was perfectly addressable; however, writing there would ruin system data structures and bring nothing but trouble. Null near pointer (DS:0000) was also perfectly accessible, but the run-time library would typically reserve some space around zero to protect from accidental null pointer dereferencing. Also, in real mode (like in DOS) the address space was not a flat 32-bit one, it was effectively 20-bit.