COMP60611 Directed Reading1

Therac-25

DIRECTED READING:The results of directed reading sessions will not be marked. However, the material isexaminable. You are strongly advised, therefore, to write up a short report of two to three pages summarising your findings for your own later use. Directed reading is a group-based exercise.

Aim:

What is the very worst thing that might happen if we design or code a parallel program wrongly? We don’t often think about the consequences of bad design and implementation, but this is probably a mistake on our part. This directed reading exercise aims to start you thinking about such issues.

In the mid 1980s, a computer-controlled radiation therapy machine, called the Therac-25, by accident massively overdosed 6 patients, some of whom died soon afterwards. After investigation, it transpired that there were residual faults in the computer control software of machines in the field, some of which had contributed to the accidents. One of the software faults was essentially a “concurrency bug”.

These incidents were analysed and published by computer scientists Nancy Leveson and Clark Turner early in the 1990s, and an updated version of their paper can be found at This exercise requires you to (1)read this document carefully, (2) to summarise your thoughts about the whole scenario that it describes, and (3) to identify the nature of the problem with the “concurrency bug” (i.e., to analyse what mistakes were made by the designers/coders of the control software)as clearly as you can. You may also wish to consult related publications or other sources.

Outcomes:

  1. A three page report on your findings (for your own use). When referencing the literature, you should provide as full a reference as possible, including: Author(s); Title; Name of publisher; Date of publication. For journal or conference papers, you should include the name of the journal or conference and the page numbers of the paper.
  2. Up to three powerpoint slides summarising your group’s findings. This should include:
  • Brief bullet-point summaries of your key findings; and
  • A short list of references to any additional literature (research papers in journals and/or conferences, or books) that you consulted. Web sites may be cited, where pertinent.

Powerpoint slides should be emailed to . The deadline for submission is 9am on Monday 3rd October 2011.The slides will be used in the debrief session. Your group should be prepared to speak briefly to the presentation and to answer questions from the class. Each group will be allocated three or four minutes only.