# Proposal of an Adaptive Fault Tolerance Mechanism to Tolerate Intermittent Faults in RAM
## 4 Assessment of the Adaptive Fault Tolerance Mechanism
### Outcomes  
In Figures 7-9 we show the results of injecting some types of faults into Plasma and Plasma_ADAPTIVE cores under controlled conditions, as described above. Figure 7 shows the influence of faults in the Plasma core. It includes (in \(X\)-axis) the following experiments:  
* Single transient faults (labeled as \(1 xT\));
* Double random transient faults (\(2 xRTr\));
* Double adjacent transient faults (\(2 xATr\));
* Triple random transient faults (\(3 xRTr\));
* Triple adjacent transient faults (\(3 xATr\));
* Single intermittent faults. From all the experiments carried out (varying the fault model, \(\mathrm{L_{burst}}\), \(\mathrm{t_{A}}\) and \(\mathrm{t_{I}}\)), we have selected a subset:
* Intermittent stuck-at (ISA) fault model, with \(\mathrm{L_{burst}}\) in the range [2; 10], \(\mathrm{t_{A}}\) in the range [0.1T, T], and \(\mathrm{t_{I}}\) in the range [0.1T, T], and \(\mathrm{t_{I}}\) in the range [0.1T, T], where T is the clock cycle of the system (labeled as \(ISA\)_[2-10, 0.1T-T, 0.1T-T]_ in the graph). * ISA fault model, with \(\mathrm{L_{burst}}\) in the range [2; 10], \(\mathrm{t_{A}}\) in the range [T, 10T], and \(\mathrm{t_{I}}\) in the range [T, 10T] (\(ISA\)_[2-10, T-10T, T-10T]_). * ISA fault model, with \(\mathrm{L_{burst}}\) in the range [2; 10], \(\mathrm{t_{A}}\) in the range [10T, 100T], and \(\mathrm{t_{I}}\) in the range [10T, 100T] (\(ISA\)_[2-10, 10T-100T, 10T-100T]_). * Intermittent bit-flip (IBF) fault model, with \(\mathrm{L_{burst}}\) in the range [2; 10] (\(IBF\)_[2-10, 10]_). * ISA fault model, with \(\mathrm{L_{burst}}\) in the range [10; 20], \(\mathrm{t_{A}}\) in the range [0.1T, T], and \(\mathrm{t_{I}}\) in the range [0.1T, T] (\(ISA\)_[10-20, 0.1T-T, 0.1T-T]_). *  
Figure 7: Influence of faults in Plasma core. Transient and intermittent faults. For each experiment, two columns are presented, representing (in the \(Y\)-axis) the percentage of failures (\(\mathrm{P_{F}}\)) and the percentage of latent errors (\(\mathrm{P_{L}}\)) (see Section 4.1). From the figure, we can confirm that transient faults provoke high percentages of failures (\(\mathrm{P_{F}}\)) plus latent errors (\(\mathrm{P_{L}}\)) (about 85%). The remaining 15% of faults injected are masked by the system operation, usually due to a later write operation. As this model does not use any fault tolerance mechanism, fault multiplicity does not have any influence, and all transient faults have similar incidence. However, the effect of intermittent faults depends very much on the fault model, the fault timing, and the burst length. Indeed, only ISA faults with burst lengths in the range [10; 20] and \(\mathrm{t_{A}}\) in the range [T, 10T], ISA faults with \(\mathrm{t_{A}}\) in the range [10T, 100T], and IBF faults, provoke percentages of failures similar to or higher than transient faults. Nevertheless, only IBF faults cause latent errors, though much less than transient faults. The motive of these different figures caused by ISA and IBF faults lies in their different behavior: in IBF faults, the erroneous value persists after fault deactivation, whereas in ISA faults, it does not. On the other hand, these lower values of \(\mathrm{P_{F}}\) (in certain cases) and \(\mathrm{P_{L}}\) caused by intermittent faults are due to several factors:  
* When injecting ISA faults with \(\mathrm{t_{A}}\) in the range [0.1T, T], it is very unlikely to access a memory address affected by a fault. Moreover, this probability reduces for smaller numbers of fault activations. This implies that most fault activations do not affect the system. However, this probability grows as \(\mathrm{t_{A}}\) does, causing more failures than transient faults. * Something different occurs to IBF faults: if a fault activation does not affect the system, the next activation will cancel the effect of the previous one, thus reducing its damage. In Figure 8, we can see the effects of the same fault types in the behavior of the Plasma_ADAPTIVE core.