Do CPUs make mistakes?

Question

Imagine that a regular computer intensively works for 5 years non-stop. The CPU always works at 100% and is constantly reading and writing to memory. Is it true that the computer will not make a single mistake?

Answer 1

Even in the absence of any errors caused by the CPU, storage elements are subject to bit flips (known as Single Event Upsets) from cosmic radiation. More information on that in Compiling an application for use in highly radioactive environments.

Radiation effects are more severe at higher altitudes, where the atmosphere provides less protection, so computers in Denver experience more bit flips than computers in Miami or Los Angeles. And similarly if you are designing equipment for use in a hospital near an X-ray machine.

Unless your hypothetical computer has an extremely small amount of memory, it is unlikely to work without any mistake for 5 years. Note however that some of the bit flips may occur in parts of the memory that you are not using, in which case they won't affect you.

You may find it interesting to read How to Kill a Supercomputer. Typical ECC (Error Correcting Code) memory can correct any single bit flip in a word, and can detect but not correct any two bit flips in a word. Note also that in some cases radiation can permanently damage memory cells, and those cells will never recover even after a cold start.