The need for reliable complex systems motivates the development of techniques by which acceptable service can be maintained, even in the presence of residual errors. Recovery blocks allow a software designer to include tests on the acceptability of the various phases of a system's operation, and to specify alternative actions should the acceptance tests fail. This approach relies on certain architectural features, ideally implemented in hardware, by which control and data structures can be retrieved after errors.
A brief account is presented of the recovery block scheme, together with a description of a new implementation of the underlying cache mechanism. The salient features of a proposed computer architecture are described, which incorporates this implementation and also provides a high level of detection for errors such as the corruption of code and data. A prototype system has been constructed to test the viability of these techniques by executing programs containing recovery blocks on an emulator for the proposed architecture. Experiences in running this system are recounted with respect to the execution of programs based on erroneous algorithms and also with respect to errors introduced by deliberate attempts to corrupt the system.