System information

Glossary
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Glossary
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) (D).
UPS See uninterruptable power supply.
URL See uniform resource locator.
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).
virtual storage VS. (1) Storage space that may be regarded as addressable main
storage by the user of a computer system in which virtual addresses
are mapped to real addresses. The size of virtual storage is limited by
the addressing scheme of the computer system and by the amount of
auxiliary storage available, not by the number of main storage
locations. (2) Addressable space that is apparent to the user as
processor storage space, from which the instructions and the data are
mapped to the processor storage locations (A, D, I).