What is "null pointer assignment error"?

I actually can not recall the source, but according to the source, this run time error is restricted to small and medium memory models being put into use by corresponding compiler. You see, as told before, the null pointer actually does not points to zero, in fact different compilers use different but fixed memory location to be used as null pointer.

Lets consider the case of TC compiler, this compiler places four zero bytes at the bottom of the data segment and TC copyright notice. TC also uses DS:0000 location, bottom of the data segment as null pointers location. So, assigning a value to this null pointer, would actully change the four bytes and probably, mess up the copyright notice.

Now, at the program termination, the four zeros and copyright banner are checked for any kind of alteration. If any alterations are found, it generates a Null Pointer Assignment error.

So, I think its not just the null pointer, any pointer that gets wild, if tries to access some key areas, you are greeted with Null Pointer Assignment Error.

http://www.faqs.org/qa/qa-3786.html

A NULL pointer assignment is a runtime error It occurs due to various reasons one is that your program has tried to access an illegal memory location. Illegal location means either the location is in the operating systems address space or in the other processes memory space. In stdio.h NULL is defined as 0 So whenever your program tries to access 0th location the operating system kills your program with runtime assignment error because the 0th location is in the operating systems address space and operating system doesn't allow access to its address space by user program .

Example code:

int* ptr = NULL;  
*ptr = 3;

Explanation:
On almost every system, address 0 is reserved. System won't allow you to write to that location. If you try, you will get a runtime exception (access violation, segmentation fault, etc.).