HP Vectra XU5 Reference Manual page 22

Data Integrity
The processor uses a number of techniques to maintain data integrity. It employs two
methods of error detection:
Data Parity Checking
This is supported on a byte-by-byte basis, generating parity bits for data addresses sent
out of the microprocessor. These parity bits can be checked by the external subsystems
that will be using the data.
Internally
The processor uses functional redundancy checking to provide maximum error detection
of the processor and its interface. When functional redundancy checking is used, a
second processor (the checker) operates in lock step with the master processor. The
checker samples the master's outputs and compares them with the values it computes
internally. An error signal is produced if a mismatch is detected.
Vectra XU Technical Reference System Overview 22