Many list members were closely following the progress of Mars Pathfinder
when it first landed on Mars. You may recall that there were a number of
computer problems in the first few days of operation, which were fixed by
uploading new software.

An explanation of the problem appears in the October, 1997 issue of _Dr.
Dobb's Journal_. Writer Dierdre Blake quotes Pathfinder software engineer
Steve Stolper:

   The bug itself was a classic case of priority inversion. A
   high-priority task with hard, real-time deadlines was blocked
   by a low-priority task that was holding a shared resource.
   It was an insidious, non-repeatablle failure that only
   occurred when the low-priority task was preempted in a
   10-15 instruction window. The fact that our data rates
   are approximately eight times faster than anticipated, and
   the amount of science now being done on Mars is propor-
   tionally larger, exposed the bug... The configuration
   of the system was changed so that when the lower-priority
   task blocked, its priority was raised until it released
   the shared resource.

Stolper will be presenting a seminar entitled "Embedded Systems on Mars"
at the Embedded Systems Conference in San Jose, September 29 through
October 2. If you're in the area and might want to attend, more
conference information is available at <http://www.embedsyscon.com>.

-- Bruce

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