The original processor that went into service at Baynard house, London, was known as the MK2 BL processor. It was replaced in 1980 by the POPUS1 (Post Office Processor Utility Subsystem). POPUS1 processors were later installed in Lancaster House in Liverpool and also, in Cambridge. Later, these too were replaced with a much smaller system known as R2PU or Release 2 Processor Utility. This was the four CPU per cluster and up to 8-cluster system, as described above. Over time, as the system was developed, additional "CCP / Performance 3" clusters were added (clusters 5, 6, 7 and 8) using more modern hardware, akin to late-1990s computer technology, while the original processing clusters 0 to 3 were upgraded with, for example, larger stores (more RAM). There were many very advanced features with this fault-tolerant system which helps explain why these are still in use today – like self fault detection and recovery, battery-backed RAM, mirrored disk storage, auto replacement of a failed memory unit, the ability to trial new software (and roll back, if necessary) to the previous version. In recent times, the hard disks on the CCP clusters have been replaced by with solid-state drives to improve reliability.