KOKUSAI BALEXT-SMP

Brand: KOKUSAI 

Type: BALEXT-SMP

Origin: the United States

Warranty: 365 days

Colour: new/used

Shipping method: Courier delivery

Fault Tolerance Principle The principle of fault tolerance in the redundant processor units is based on continuous updating of the backup unit to the same status as the primary unit. This enables the backup unit to assume control without affecting surrounding systems in a bumpless manner. This principle involves dynamic division of the program execution into execution units and the creation of rollback points at which the processor unit's status is completely defined. In this context, the processor unit's total status is defined as the processor unit's internal status, that is, the contents of the processor registers, plus the contents of the data memory. The backup unit's status is updated each time the primary unit passes a rollback point, enabling the backup unit to resume program execution from the last rollback point passed, should the primary unit fail due to error. In order to minimize the amount of information involved in the update, the backup unit is updated only with the changes taking place since the latest rollback point. Between rollback points, these changes that writes in the data memory, are stored in a log buffer in the backup unit. At a rollback point, the processor's total register contents are also written into the data memory, so that this information is also logged. Once the rollback point is established, the logged write operations are transferred to the backup unit's data memory.

If the primary unit fails because of an error, the backup unit resumes execution from the last rollback point, which means the last execution unit is partially re-executed by the backup unit. In order to re-execute a portion of the execution unit without affecting the peripheral units (communication units on the CEX-Bus), the peripheral units' references are also logged between rollback points. During re-execution, the results of the peripheral units' references, which have already been executed, are used, rather than re-executing them. The results of read operations are retrieved from the log, and write operations pass without execution, since they have already been executed. The peripheral units' statuses, then, are not affected by the reexecution in any way, except for the time delay which occurs.