NonStop is a series of server computers introduced to market in 1976 by Tandem Computers Inc., beginning with the NonStop product line. It was followed by the Tandem Integrity NonStop line of lock-step fault tolerant computers, now defunct (not to be confused with the later and much different Hewlett-Packard Integrity product line extension). The original NonStop product line is currently offered by Hewlett Packard Enterprise since Hewlett-Packard Company's split in 2015. Because NonStop systems are based on an integrated hardware/software stack, Tandem and later HPE also developed the NonStop OS operating system for them. NonStop systems are, to an extent, self-healing. To circumvent single points of failure, they are equipped with almost all redundant components. When a mainline component fails, the system automatically falls back to the backup. These systems are often used by banks, stock exchanges, payment applications, retail companies, energy and utility services, healthcare organizations, manufacturers, telecommunication providers, transportation and other enterprises requiring extremely high uptime. History Originally introduced in 1976 by Tandem Computers Inc., the line was later owned by Compaq (from 1997), Hewlett-Packard Company (from 2003) and Hewlett Packard Enterprise (since 2015). In 2005, the HP Integrity "NonStop i" (or TNS/E) servers, based on Intel Itanium microprocessors, was introduced. In 2014, the first systems "NonStop X" (or TNS/X), based on Intel x86-64 processors, were introduced. Sales of the Itanium-based systems ended in July 2020. Early NonStop applications had to be specifically coded for fault-tolerance. That requirement was removed in 1983 with the introduction of the Transaction Monitoring Facility (TMF), along with Pathway transaction management software and SCOBOL applications (or, later, NonStop Tuxedo transaction management software), which handles the various aspects of fault tolerance on the system level. Software NonStop OS is a message-based operating system designed for fault tolerance. It works with process pairs and ensures that backup processes on redundant CPUs take over in case of a process or CPU failure. Data integrity is maintained during those takeovers; no transactions or data are lost or corrupted. The operating system as a whole is branded NonStop OS and includes the Guardian layer, which is a low-level component of the operating system and the Open System Services (OSS) personality which runs atop this layer, which implements a Unix-like interface for other components of the OS to use. The operating system and application are both designed to support the fault tolerant hardware. The operating system continually monitors the