Computers as weapons

Peter Turnbull wrote: case. Gah! The Therac case is positively frightening, and in a very real way could be called "death by keyboard interrupt service". The Therac could deliver E-beam or X-ray therapy; to generate X-rays it moved a metal target into the path of the beam and made appropriate changes to the beam current. The problems with the Therac were manifold, but the most fundamental one was that it was a hunk of cooperative real-time processes written by someone who didn't understand niceties like semaphores. As a consequence, there were failure modes in the code, one of which being that if keystrokes were entered while the treatment type (e-beam or x-ray) was being selected the metal target and the focusing rings would be positioned incorrectly with respect to the beam current -- generally, as I recall, the system would end up positioning the target and rings for X-ray and setting the beam current for e-beam -- which was a few orders of magnitude greater than the maximum permitted for x-ray. That created the problem -- but it wasn't the only one. When the machine misconfigured itself and the operator hit "treat", the machine would run for a fraction of a second and then shutdown when the dosimeters decided that the patient had been exposed to too much radiation. However, the dosimeter interlock was a soft one; when the machine tripped off a message was displayed to the operator indicating that the treatment plan had been interrupted. If the operator hit enter (which they almost always did) the system ignored the dosimeters and proceeded to irradiate the hell out of the patient. Who then usually died a painful and unpleasant death. -- Chris Kennedy chris(a)mainecoon.com http://www.mainecoon.com PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97