Lockstep processing works along the following lines. CPMs contain two Advanced Micro Devices 29050 microprocessors (AIMS-1) or two HI-29KII chips (AIMS-2) connected in a way that allows all processor inputs and outputs–address, data, instructions–to be immediately compared. If these comparisons yield different results, the processing is flawed. If they yield identical answers–the normal case–the processing result is valid. If an error is detected, "all processing results are halted, an error is logged, and recovery is attempted," says Tsikalas. "Outputs, or results, are not allowed to escape from the containment zone until such time that the processor environment can be verified–by power-up or runtime BITE [built-in test equipment]–to be good." These approaches, combined with the redundant system architecture, help to explain Boeing’s approximately 99.3 percent schedule reliability rate in 2002 for the 777.