Importance of Formal specification

1. Why Formal
Specification
Eng A Kajavatan BSc Eng (Mora),MSc (Soton)

2. Therac – 25 Case Study

3. Six accidents due to software error
Source: https://www.youtube.com/watch?v=uEvu2PlDhO0

4. Therac-25
• Provide radiation treatment to cancer patients.
• Developed by the company Atomic Energy Canada Limited (AECL) and a
French company CGR
• Most computerized and sophisticated radiation therapy machine of its time.
• Sold eleven Therac-25 machines that were used in the United States and
Canada beginning in 1982

5. Why recalled?
• Six accidents involving significant overdoses of radiation to patients resulting
in death occurred between 1985 and 1987.
• Patients reported being “burned by the machine” which some technicians
reported, but the company thought was impossible.
• A bug – Race condition
• Recalled in 1987.

6. Other major failures

7. AT & T Long distance breakdown - 1990
• On January 15, 1990, AT & Ts long-distance telephone switching system
crashed
• Estimated loss USD 60 million (in 1990)
• Estimated loss of 200 000 airline reservation

8. Patriot Missile Failure Gulf war
• On February 25, 1991, Gulf War, an American Patriot Missile battery in
Dharan, Saudi Arabia, failed to track and intercept an incoming Iraqi Scud
missile
• Killed 28 soldiers
• 100+ injured

9. Ariane 5 Explosion - 1996
• A European space agency project (7 billion) exploded within a minute.
• 10+ years of research work wasted
• Estimated loss USD 370 million

10. Reference
• Case Study - Therac-25: https://ethicsunwrapped.utexas.edu/wp-
content/uploads/2020/01/Therac-25-1.pdf
• Safety-critical system: https://en.wikipedia.org/wiki/Safety-critical_system
• Using Formal Methods for Engineering Embedded Systems :
https://www.techbriefs.com/component/content/article/tb/features/articles/27307
• What happened? Episode 4: Therac 25 accidents :
https://www.youtube.com/watch?v=uEvu2PlDhO0
• Lecture# 01 "Formal Methods in Software Engineering” :
https://www.youtube.com/watch?v=D1frcipfi50