IEEE Technology and Society Magazine - December 2016 - 81

Random corruptions are not enough. The throttle
angle variable must be overwritten with a large
value, and the bit in question must be zeroed.
■ The number of corruptions resulting from the initial
corruption must be limited (e.g., pinpointing those
two locations but sparing all or most others) such
that much of the software continued to function.
■ The theory requires particular timing and ordering of the two hypothetical corruptions (more
detail in testimony below).
Can we completely rule out the theoretical possibility
that under one of the two assumptions above, these two
hypothetical memory corruptions might occur together,
as improbable as that may be? Maybe not. But the mere
possibility that this could theoretically happen is not
enough to conclude "to a reasonable degree of engineering certainty" that it is likely to have occurred at the
time of the accident.
Furthermore, many scenarios covered by this alternative theory also require that the Brake Echo Check
not work correctly. For reasons already discussed,
that reduces the credibility of the alternative theory
even further.
■

FigUrE 2. Slide 21.

Example Testimony Relevant
to the Two Theories
Bit Flip Hypothesis
The expert presented the slide in Figure 1. (See also
Slide 37, and Testimony, PDF pages 70-72.)
He identified what he said were a number of problems in the software that could possibly cause an
unspecified memory corruption under some circumstances. For example, see the slide in Figure 2. (This
slide appears to incorrectly represent that certain
defects were found that caused memory corruption. For
example, the expert testified that pointers were not
checked before being dereferenced [Testimony, PDF
pages 91-92]. This means that if an invalid pointer value
were to occur due to a bug or hardware error, the software would not detect the invalid value and prevent it
from being used. It does not mean that the expert found
an actual occurrence of an invalid pointer value. This
slide appears to represent instead that he found an
actual occurrence of an invalid pointer value that
caused memory corruption. As another example, the
expert testified that there was significantly less safety
margin in the stack sizing than Toyota thought, resulting
in an increased potential for stack overflow [Testimony,
PDF pages 105-108, 111-112, and Slides 25-27]. This
slide appears to represent instead that the expert found
an actual occurrence of stack overflow.)
The expert said that memory corruption could potentially cause task death:
DECember 2016

FigUrE 1. Slide 19.

∕

Well memory corruption is so significant because
it's a memory corruption that can cause a task
death and task death can cause in a general
sense unpredictable results, but in a specific
sense, as with task X, cause loss of throttle control and also a disablement of a number of the fail
safes. [Testimony PDF page 96]

He identified what he said were a
number of problems in the software
that could possibly cause an
unspecified memory corruption under
some circumstances.
Nowhere did he identify specific evidence of the
bit flip (or any memory corruption) having occurred,
nor did he identify any specific bug that caused this
bit to flip.

Open Throttle Hypothesis
The facts of the accident appear to contradict this
hypothesis. As confirmed by the expert's answers
below, the driver was slowing the vehicle on an exit

IEEE Technology and Society Magazine

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