Embedded Software for SoC - Grupo de Mecatrônica EESC/USP

42 Chapter 4
operand of an existing jump‚ or by transforming an instruction into a jump
instruction) producing thus a faulty execution flow. To detect this type of error‚
we associate to each basic block in the program a boolean flag called
status_block. This flag takes the value “0” if the basic block is active and
“1” for the inactive state. At both the beginning and the end of each basic
block‚ the value of the status_block is incremented modulo 2. In this way
the value of the status_block is always “1” at the beginning of each basic
block and “0” at the end of each block. If an error provokes a jump to the
beginning of a wrong block (including an erroneous restarting of the currently
executed block) the status_block will take the value “0” for an active block
and “1” for an inactive state block. This abnormal situation is detected at the
first check preformed on gef flag due to the inappropriate values taken by
the control flow flags. The application of these rules is illustrated in Figure
4-2.