Embedded Software for SoC - Grupo de Mecatrônica EESC/USP

Detecting Soft Errors by a Purely Software Approach 41
2.1. Transformation rules
The studied approach exploits several code transformation rules. The rules are
classified in three basic groups presented in the following.
2.1.1. Errors affecting data
This group of rules aims at detecting those faults affecting the data. The idea
is to determine the interdependence relationships between the variables of
the program and to classify them in two categories according to their purpose
in the program:
intermediary variables: they are used for the calculation of other variables;
final variables: they do not take part in calculation of any other variable.
Once the variables relationships are drawn up‚ all the variables in the
program are duplicated. For each operation carried out on an original variable‚
the operation is repeated for its replica‚ that we will call duplicated variable.
Thus‚ the interdependence relationships between the duplicated variables are
the same with those between the original variables. After each write operation
on the final variables‚ a consistency check between the values of the
two variables (original and duplicated) is introduced. An error is signaled if
there is a difference between the value of the original variable and that of the
duplicated variable.
The proposed rules are:
Identification of the relationships between the variables;
Classification of the variables according to their purpose in the program:
intermediary variable and final variable;
Every variable x must be duplicated: let x1 and x2 be the names of the
two copies;
Every operation performed on x must be performed on x1 and x2;
After each write operation on the final variables‚ the two copies x1 and
x2 must be checked for consistency‚ and an error detection procedure is
activated if an inconsistency is detected.
Figure 4-1 illustrates the application of these rules to a simple instruction
sequence consisting of two arithmetical operations performed on four variables
(Figure 4-1a). The interdependence relationships between the variables
are: a =f(b‚ c) and d =f(a = f(b‚ c)‚ b). In this case only d is considered as
a final variable while a‚ b and c are intermediary variables. Figure 4-1b shows
the transformations issued from the set of rules presented.
2.1.2. Errors affecting basic instructions
This group of rules aims at detecting those faults modifying the code provoking
the execution of incorrect jumps (for instance by modification of the