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Concurrent system-level error detection techniques using a watchdog processor are surveyed. A watchdog processor is a small and simple coprocessor that detects errors by monitoring the behavior of a system. Like replication, it does not depend on any fault model for error detection. However, it requires less hardware than replication. It is shown that a large number of errors can be detected by monitoring the control flow and memory-access behavior. Two techniques for control-flow checking are discussed and compared with current error-detection techniques. A scheme for memory-access checking based on capability-based addressing is described. The design of a watchdog for performing reasonable checks on the output of a main processor by executing assertions is discussed.