Class B software package

AN3181

12/31

Doc ID 17286 Rev 2

3.1.3

Class B flow control

A specific method is used to test and check program execution flow. Unique user defined

numbers are assigned for every software execution block. These unique numbers are stored

in two complementary counters. When a block is executed, a symmetrical four step change

of the counter pair content is done (add or subtract the label values). The first two steps

check if the block is correctly called from main flow level (processed just before calling and

just after return from the called procedure). The next two steps check if the block is correctly

completed (processed just after enter and just before return from the procedure). An

example is given in Figure 2 where a routine performing a component test is called in the

control flow sequence and the four step checking service is shown. This method decreases

CPU load as the check is done by counting one member of the complementary counter pair

only. As there is always the same number of call/return and entry/exit points, the value

stored in the counter pair must be always complementary after any block is processed.

Several flow check points are placed subsequently in the code flow. Counter integrity is

always checked here and any unexpected value results in calling the FailSafe() routine.

Figure 2.

Control flow four steps check routine

1.

For this example, the unique number for Component test 1 is “5” and for the procedure itself it is defined as

“7”. The counters are initialized to 0x0000 and 0xFFFF. The table in the upper right corner in Figure 2

shows how the counters are changed in four steps. Also shown is their complementary value after the last

step (return from procedure) is done.