What happens if the computer tries to access a memory address beyond the actual memory of the machine? A special kind of hardware error results that must be handled gracefully by the software without crashing the system, though the offending program must be squished and killed. In the CSC-1, it is impossible to ask to read or write a memory location beyond the beginning or end of memory, since there are 4096 words of memory, whose addresses go from 0 to 4095. The MAR is exactly 12 bits long, so the legal bit patterns are

000000000000        (0)
000000000001        (1)
    ...
111111111111     (4095)

In the CRC-1 computer, it is impossible to cause this kind of memory fault. Some UNIX systems call it a bus error, because an illegal address went out over the memory bus.