Humble Pi: When Math Goes Wrong in the Real World

by Matt Parker

The most dangerous 256 error I have found so far occurred in the Therac-25 medical radiation machine. This was designed to treat cancer patients with bursts of either an electron beam or intense X-rays. It was able to achieve both types of radiation from the one machine by either producing a low-current electron beam, which the patient was directly exposed to, or a high-current electron beam, which was aimed at a metal plate to produce X-rays. The danger was that the beam of electrons required to produce X-rays was so powerful that it could do severe damage to patients if it hit them directly. So if the electron beam’s power was increased, it was vital to make sure that the metal target and a collimator (a filter to shape the X-ray beam) had been placed in between the electron beam and the patient. For this, and a host of other safety reasons, the Therac-25 looped through a piece of setup code, and only if all the systems were verified as being in the correct settings could the beam be turned on. The software had a number stored with the catchy name of Class3 (that’s just how creative programmers can be when naming their variables). Only after the Therac-25 machine had verified that everything was safe would it set Class3 = 0. To make sure that it was checked every time, the setup loop code would add one to Class3 at the beginning of each loop so it started at non-zero. A subroutine with the slightly better name of Chkcol would activate whenever Class3 was not zero and the...