Mainframes were historically core parts of business computing architecture – although they are less common than servers in modern data centres – and are targeted at performing large scale critical application or data processing. Mainframes are characterised by their: high reliability due to redundant internal engineering, strict backwards compatibility of older software, significant Input/Output capabilities and support for hot-swapping memory and processors. Reliability, Availability and Serviceability or RAS is a key design feature of mainframes, they are designed to run for very long periods of time – sometimes decades – continuously as they tend to run critical applications where downtime could be catastrophic to the business.