1. How long was the Therac-25 in operation before the ?rst documented accident? How much longer did it take for the system to be declared unsafe?

2. What is a race condition in software? Why are race conditions dif?cult to debug?

3. The following reasons have been given for the failure of computerized systems:

I. A system designed for one purpose was used for another purpose.
II. Software was reused without adequate testing.
III. There was an error in storing or converting a data value.
IV. A line of code became a single point of failure.
V. The overall system was too complicated to analyze.
VI. There was a software race condition.
VII. There was another software error (not listed above).

For each of the systems listed below, select the principal reason or reasons why it failed to operate as speci?ed.

a. Patriot missile
b. Ariane 5
c. AT&T long-distance network
d. Mars Climate Orbiter
e. Mars Polar Lander
f. Denver International Airport baggage system
g. Tokyo Stock Exchange
h. Direct recording electronic voting machines
i. Therac-25