Glossary

UNIX

A popular multi-user, multitasking operating system originally designed to be a small, flexible system used exclusively by programmers. UNIX was one of the first operating systems to be written in a high-level programming language, namely C. This meant that it could be installed on virtually any computer for which a C compiler existed. Due to its portability, flexibility, and power, UNIX has become the leading operating system for workstations. Historically, it has been less popular in the personal computer market, but the emergence of a new version called Linux is revitalizing UNIX across all platforms.

upper level protocol

ULP. Protocols that map to and run on top of the Fibre Channel FC-4 layer. ULPs include Internet protocol (IP) and small computer system interface (SCSI).

UPS

See uninterruptable power supply.

user datagram protocol

UDP. A connectionless protocol that runs on top of Internet protocol (IP) networks. User datagram protocol/Internet protocol (UDP/IP) offers very few error recovery services, instead providing a direct way to send and receive datagrams over an IP network. UDP/IP is primarily used for broadcasting messages over an entire network. Contrast with transmission control protocol/Internet protocol.

V

VAC

See volts alternating current.

VDC

See volts direct current.

virtual machine

VM®. (1) A virtual data processing system that appears to be at the exclusive disposal of a single user, but whose functions are accomplished by sharing the resources of a real data processing system. (2) A functional simulation of a computer system and its associated devices, multiples of which can be controlled concurrently by one operating system (D, T).