A virtual diagnosis model for computer systems is proposed

wherein a subsystem is considered to be a union of a task processor and a communication processor. The idea of preforming virtual tests and virtual diagnosis is presented

followed by the criteria for diagno-sability

both for centralized and distributed control. An optimal design for diagnosable systems is also presented. The results obtained show clearly that the virtual diagnosis can identify more faults than t-fault diagnosis under the same structural constraints.