"hot-standby" to the master, rather than in lockstep. Since there is no advantage in having the slave unit actively process each step, a common method of working is for the master to copy its state at the end of each step's processing to the slave. Should the master fail at some point, the slave is ready to continue from the previous known good step. While either the lockstep or the DMR approach (when combined with some means of detecting errors in the master) can provide redundancy against hardware failure in the master, they do not protect against software error. If the master fails because of a software error, it is highly likely that the slave - in attempting to repeat the execution of the step which failed - will simply repeat the same error and fail in the same way, an example of a common mode failure. Triple modular redundancy Where the computing systems are triplicated, it becomes possible to treat them as "voting" systems. If one unit's output disagrees with the other two, it is detected as having failed. The matched output from the other two is treated as correct. See also NonStop (server computers) Stratus VOS VAXft References External links Enabling Memory Reliability, Availability, and Serviceability Features on Dell PowerEdge Servers, 2005 Chipkill correct memory architecture, August 2000, by David Locklear Classes of computers Fault-tolerant computer systems