This is a case where the abstractions of the C programming language start to break down. 0 is indeed a null pointer constant in C, but the hardware sees 0 as a memory address like any other. Some systems have nothing valid there, but some other systems (like this one) put exception vectors at this location. When certain types of hardware interrupts or exceptions happen, the CPU jumps to certain fixed addresses around address 0 (the specific types of exceptions and the specific addresses are a property of the CPU architecture).
I somehow managed to survive this transition. But it was painful for me, and tremendously constraining for the culture.，更多细节参见新收录的资料
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