SOFTWARE ENGINEERING. Today - motivation: - Software Engineering: Why? - Software Engineering: What? We will start by examining some example cases. Agency sends 16,000 tax forms to one man / 1. Source: http://www.csc.calpoly.edu/~jdalbey/205/Resources/irs_bug.html
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SACRAMENTO (Scripps-McClatchy)--Somewhere in the San Diego area, there's a dentist who's probably still grinding his teeth over his latest brush with California’s tax collectors. During one week in September his office received an avalanche of tax forms in the mail -- 16,000 sets of forms in 16,000 individual envelopes.
"We did it," admitted Suzanne Schroeder of the state Employment Development Department. "It was a computer problem." The glitch occurred in a mailing of 1.4 million pieces that is sent out each quarter to employers, Schroeder explained.
The department was using new computer software for producing address labels which was provided by the U.S. Postal service, Schroeder said. The Postal Service software was designed to read the word "suite" abbreviated as "ste," she continued. But the addresses in the department's database abbreviate "suite" as "su". When the software couldn't read "su", it was supposed to jump to the previous line and read it again, Schroeder said.
But for this particular address, there was a foreign spelling on the previous line and the software couldn't read that either. That set off a series of other jumps, she added, until the computer began spitting out the same address over and over again. "We alerted the postal authorities and they corrected the problem with what they call a 'software patch,'" she said.
<Kenneth.Wood@prg.ox.ac.uk>Fri, 27 Aug 93 16:55:35 BST
The Feedback section of the latest New Scientist relates the following Computer Weekly story about an unfortunate programmer at an unnamed bank. Apparently, the bank wanted to target its wealthiest customers with a mailshot promoting various new services and the programmer in question wrote a program to select the 2000 wealthiest customers from the bank's records and to generate an appropriate letter for each.
In the process of testing the program, he made use of a fictitious customer named Rich Bastard. Unfortunately, as you may already have guessed, something went amiss and every single one of the bank's 2000 prize customers received a letter which began "Dear Rich Bastard, ..."
WASHINGTON (CNN) -- Failure to convert English measures to metric values was the root cause of the loss of the Mars Climate Orbiter, a spacecraft that smashed into the planet instead of reaching a safe orbit, a NASA investigation concluded Wednesday.
In a scathing report released Wednesday, an investigation board concluded that NASA engineers failed to convert English measures of rocket thrusts to newton, a metric system measuring rocket force.
One English pound of force equals 4.45 newtons. A small difference between the two values caused the spacecraft to approach Mars at too low an altitude and the craft is thought to have smashed into the planet's atmosphere and was destroyed.
The report cited other contributing causes to the September 23
The most likely cause of the lander’s failure, investigators decided, was that a spurious sensor signal associated with the craft’s legs falsely indicated that the craft had touched down when in fact it was some 130-feet (40 meters) above the surface. This caused the descent engines to shut down prematurely and the lander to free fall out of the martian sky.
(Story by James Gleick)
It took the European Space Agency 10 years and $7 billion to produce Ariane 5, a giant rocket capable of hurling a pair of three-ton satellites into orbit with each launch and intended to give Europe overwhelming supremacy in the commercial space business.
All it took to explode that rocket less than a minute into its maiden voyage last June, scattering fiery rubble across the mangrove swamps of French Guiana, was a small computer program trying to stuff a 64-bit number into a 16-bit space.
One bug, one crash. Of all the careless lines of code recorded in the annals of computer science, this one may stand as the most devastatingly efficient. From interviews with rocketry experts and an analysis prepared for the space agency, a clear path from an arithmetic error to total destruction emerges.
To play the tape backward:
At 39 seconds after launch, as the rocket reached an altitude of two and a half miles, a self-destruct mechanism finished off Ariane 5, along with its payload of four expensive and uninsured scientific satellites. Self-destruction was triggered automatically because aerodynamic forces were ripping the boosters from the rocket.
This disintegration had begun an instant before, when the spacecraft swerved off course under the pressure of the three powerful nozzles in its boosters and main engine. The rocket was making an abrupt course correction that was not needed, compensating for a wrong turn that had not taken place.
Steering was controlled by the on-board computer, which mistakenly thought the rocket needed a course change because of numbers coming from the inertial guidance system. That device uses gyroscopes and accelerometers to track motion. The numbers looked like flight data -- bizarre and impossible flight data -- but were actually a diagnostic error message. The guidance system had in fact shut down.
This shutdown occurred 36.7 seconds after launch, when the guidance system's own computer tried to convert one piece of data -- the sideways velocity of the rocket -- from a 64-bit format to a 16-bit format. The number was too big, and an overflow error resulted. When the guidance system shut down, it passed control to an identical, redundant unit, which was there to provide backup in case of just such a failure. But the second unit had failed in the identical manner a few milliseconds before. And why not? It was running the same software.
This bug belongs to a species that has existed since the first computer programmers realized they could store numbers as sequences of bits, atoms of data, ones and zeroes: 1001010001101001. . . . A bug like this might crash a spreadsheet or word processor on a bad day. Ordinarily, though, when a program converts data from one form to another, the conversions are protected by extra lines of code that watch for errors and recover gracefully. Indeed, many of the data conversions in the guidance system's programming included such protection.
But in this case, the programmers had decided that this particular velocity figure would never be large enough to cause trouble. After all, it never had been before. Unluckily, Ariane 5 was a faster rocket than Ariane 4. One extra absurdity: the calculation containing the bug, which shut down the guidance system, which confused the on-board computer, which forced the rocket off course, actually served no purpose once the rocket was in the air. Its only function was to align the system before launch. So it should have been turned off. But engineers chose long ago, in an earlier version of the Ariane, to leave this function running for the first 40 seconds of flight -- a "special feature" meant to make it easy to restart the system in the event of a brief hold in the countdown.
Reference: http://www.uoguelph.ca/~meby/Story by Mark Eby
Therac 25 was engineered by Atomic Energy Canada Limited (AECL) in conjunction with a French company CGR. It was an advancement in the fight against cancer. The million dollar, dual-mode linear accelerator was first developed in 1976 and the commercial version was available in 1982. There were eleven installed altogether, 5 in the USA and 6 in Canada.
The machine precisely aimed a beam of radiation at a patient to treat tumors caused by cancer. The x-rays produced were used to reach deeper tissue in the human body. The machine had two settings, a low energy, 200-rad mode, and a x-ray mode of 25 million electron volt capacity. The low setting could be directly aimed at the patient whereas the high-energy mode had to aim at the patient through a thick tungsten shield. It was controlled through a terminal hooked up to an old Vax mainframe so that a technician could run it from another room.
In almost every case treatment went fine with no complications and it provided the necessary radiation to cure the cancerous tumors.
In six of the cases of people being treated something went wrong. Human error, along with a bug in the software caused the treatment to malfunction. Normally a patient is treated with low-energy doses of electrons from Therac 25. It is the increased, high-energy x-rays that caused a problem. In each case that Therac 25 malfunctioned, the technician entered the wrong dosage and then corrected it.
The two modes, electron mode "e" and x-ray mode "x" were controlled form the Vax terminal. In Texas, the technician entered mode "x" instead of the proper mode "e". Upon realization of the error the technician scrolled up to "Edit", corrected the mistake, hit "e" and then hit "Enter". The total time that it took for the sequence of events to occur was less then eight seconds.
The technician believed that everything was all right and pressed "D" when the "Beam Ready" prompt came up. At the completion of the inputs an error message showed, so the technician reset the computer and did the sequence again. This time an error message showed and the system stopped. Meanwhile, the man being treated was being burnt by the radiation so bad that he got off the table he was on and found the technician. He complained about the pain he was feeling in his shoulder but the technician had no reason for the cause of the pain.
It was believed that the proper mode was in place and that the proper dosage was being given. It was not until three weeks later when the same events occurred that the problem was discovered. When the technician reset the computer the arm with the tungsten withdrew but the beam did not switch. The man was bombarded with 25 000-rads with 25 million electron volts, 125x the normal dose. The man died four months later.