The interrupt descriptor table in real mode on x86 is stored at the zero page. It is valid to dereference a pointer with value zero under DOS. Modern OS's explicitly avoid mapping at page zero in order to trap NULL pointer dereferences. That does not mean you cannot access memory mapped at zero, just that the system might have mechanisms to catch errant programs. It is not the responsibility of the C compiler to make the distinction of whether or not accessing memory at zero is errant, but rather the architecture and the system which determine how to respond to a process that attempts to do so.