<Li> Strict backward compatibility with older software </Li> <Li> High hardware and computational utilization rates through virtualization to support massive throughput . </Li> <P> Their high stability and reliability enable these machines to run uninterrupted for decades . </P> <P> Mainframes have high availability, one of the primary reasons for their longevity, since they are typically used in applications where downtime would be costly or catastrophic . The term reliability, availability and serviceability (RAS) is a defining characteristic of mainframe computers . Proper planning and implementation is required to realize these features . In addition, mainframes are more secure than other computer types: the NIST vulnerabilities database, US - CERT, rates traditional mainframes such as IBM Z (previously called z Systems, System z and zSeries), Unisys Dorado and Unisys Libra as among the most secure with vulnerabilities in the low single digits as compared with thousands for Windows, UNIX, and Linux . Software upgrades usually require setting up the operating system or portions thereof, and are non-disruptive only when using virtualizing facilities such as IBM z / OS and Parallel Sysplex, or Unisys XPCL, which support workload sharing so that one system can take over another's application while it is being refreshed . </P>

Thousands of computers working in parallel to deliver services is called a​