HP 3000 Manuals HP 3000 Manuals
Definitions (S-Z)
savepoint A place within a DBCore transaction to which you
can rollback (partially undo) then continue with
same transaction.
scaling To divide the graphics area into units convenient
for your application.
scanner/parser A routine that subdivides an instruction into
components that the operating system (or another
subsystem) can more easily understand and use.
scratch a volume To make data unavailable on a volume so that the
volume can be reinitialized. In effect, to delete
data on a volume.
scratch tapes Used tapes containing information that is no longer
needed.
SCRATCH volume A volume whose data is no longer needed that has
been marked as available for a new volume or volume
set. The SCRATCHVOL command marks the volume. The
UNSCRATCHVOL unmarks the volume without losing any
data or label information, provided the disk has
not been written to.
scrolling The act of adding a new line of data to a video
terminal's screen by adding it to the bottom of the
screen and shifting all previous lines upward.
search path An MPE/iX mechanism that controls which file is
opened once a command is determined not to be a UDC
nor an MPE command.
sector A portion of a track on a disk, and the smallest
addressable piece of the disk. MPE-formatted disks
use 128-word sectors (256 bytes).
security 1) The provisions that prevent unauthorized users
from entering the system, accessing data, or using
resources, programs, or capabilities.
2) The provisions included in MPE to protect the
system from unauthorized use. MPE offers several
means to create a secure environment. The most
basic level of security includes organizing files
into groups and users into accounts, either of
which may be assigned a password. Security also
refers to the ability to read, write, append, lock,
and execute files, optionally assigned to accounts
by the system manager and to groups and users by
the account manager.
segmented library A file containing code segments that are shareable,
(SL) general-use MPE, utility, and subsystem procedures
not unique to a particular process. The three
levels of segmented libraries are: LIB=G (group
library), LIB=P (public library), LIB=S (system
library). G, P, and S refer to the location of the
CM program being run. If program.group.account is
run with LIB=G, then SL.group.account is used. If
program.group.account is run with LIB=P, then
SL.PUB.account is used. Otherwise, SL.PUB.SYS
(LIB=S) is used. The SL resolves external
procedure calls not contained in the program
itself. The group SL is available to any user who
can access the group; the public SL is available to
all account users; and the system SL is available
to all system users.
segmenter A subsystem of the MPE V/E operating system that
performs all execution. Its primary function is to
gather and link into segments most of the resources
needed to form an executable program file.
self-clocking data The transmission of data when the clock information
is part of the data.
separator A symbol that separates the parameters of an
instruction. Some examples of separators are
commas, spaces, and semicolons.
sequential The order (ascending or descending) in which data
items are physically stored and accessed.
serial See sequential.
serial disk A serially-accessed disk that is configured as a
magnetic tape. Flexible diskettes, disk packs, and
cartridge tape (MPE V/E) may all be used as serial
disks. They are designed to store system data
(backups), load subsystems, and to perform
standalone CPU and non-CPU diagnostics.
serial interface A single data line that transfers data bytes
sequentially between devices.
server A node unit of a network that provides a specific
service to network users.
session A mode in which the HP 3000 is used interactively
by entering commands and data through a terminal's
keyboard and receiving immediate responses to
input. A session is initiated with the HELLO
command. A session is ended with the BYE command,
or a second HELLO command that logs the user off
the first session and onto another session.
session layer Layer five of the Open System Interconnection (OSI)
network model. The session layer provides the
means for cooperative presentation entities to
organize and synchronize their dialog and manage
their data exchange.
signal The software interrupt sent to processes, informing
them of special situations or events.
simplex The operation of a channel only in one direction,
with no reverse capability.
single-cycle Simple, hard-wired instructions that are
execution efficiently pipelined to allow RISC architecture to
execute an instruction on virtually every machine
cycle.
single file Single file consistency is the preservation of the
consistency internal state of a file. The backup system
ensures that a file is in a logically consistent
state before it stores the file.
single-user mode A DBEnvironment startup mode that allows only one
DBE session to be active at a time.
single-user state A condition of the MPE/iX operating system in which
the system console provides the only communication
mechanism between the system and its user.
single-user system A system that can communicate with only one fixed
terminal.
slave device A peripheral device not directly connected to the
HP 3000. A slave device does not have its own
device controller, but instead is controlled
through a master device.
small computer A SCSI is an interface designed to connect small
system interface computer systems to devices such as disk storage,
(SCSI) printers, and other peripherals.
softcopy The display on a video terminal. The opposite of
hardcopy.
soft reset A reset that initializes a variety of terminal
functions but does not reset the memory. Refer to
hard reset.
software A set of programs, instructions, rules, and
procedures concerned with the operation of a
computer. The opposite of hardware.
software dump A facility that gives the system operator the
facility (SDF) capability of writing all main memory to a serial
storage medium. It operates in a standalone
environment (without MPE/iX), and is used following
a system failure or a system halt.
Software Status A Hewlett-Packard publication supplying customers
Bulletin with information on Hewlett-Packard software
enhancements.
source code One or more files containing the language used by
programmers to write a program. It must be
compiled into machine-readable data (object code)
before it can be executed by the computer.
span To spread related data across volume sets.
spoolfile A file awaiting printing. This file can reside on
disk or can be moved to tape. A spoolfile may be
either OPEN, ACTIVE, READY, or LOCKED. These states
describe different stages of the spooling process
depending upon whether the file is an input or
output spoolfile.
spool SPOOL is an acronym for "simultaneous peripheral
operations online". Spooling allows many processes
to simultaneously write output for a single
printer.
spreadsheet A program that allows the user to enter data into
row and column positions (known as "cells"). A
spreadsheet program provides mathematical
manipulation of the values in the cells.
stack A data structure in which items are added at the
end of a sequential list and can only be retrieved
from the same end.
standard input A system-defined file that provides input for
programs. This file can be a device (for example,
a keyboard or printer), or an actual data file.
The formal file designator for standard input is
$STDIN or $STDINX.
standard output A system-defined file that holds output from a
batch job (including the job statements and error
messages). This file can be a device (for example,
a terminal screen or printer), or an actual file.
The formal file designator for standard output is
$STDLIST.
star network A point-to-point network topology. The star
topology is often used for centralized data
collection or supervisory control. It is also used
when a central node has a large database or control
program that is accessed by the other nodes. In a
star network, there is at most one intervening node
between any two nodes. Star networks are
vulnerable to failure of the central node. If the
central node fails, no network communication is
possible.
START An initial system loader (ISL) utility, and its
options, used to start the system from disk,
building the system data structures. This utility
is used to recover from a hang or failure or to
reboot the system after scheduled downtime. The
START RECOVERY option is equivalent to the MPE V/E
WARMSTART procedure, and the START NORECOVERY
option is equivalent to the MPE V/E COOLSTART
procedure.
static backup Backup is static when the files and structures are
inaccessible during the time that they are being
stored. That is, they are locked exclusively by
the backup subsystem.
status bits or words Bits (or words, if status words are used) that
indicate the condition of a device. The status
bits or word can be checked by a program or
microcode and, based upon the value, a specific
sequence of instructions executed.
$STDIN A system-defined file name referring to the
standard input "file" (which can be an actual file
or a device). $STDIN often refers to the keyboard
for interactive sessions and a file for batch jobs
or programs. $STDIN treats a colon (:) appearing
in the first column of input data as an
end-of-file. See standard input.
$STDINX Same as $STDIN. However, unlike $STDIN, $STDINX
treats the colon (:) prompt appearing in the first
column of input data as part of the data file,
rather than an end-of-file indicator.
$STDLIST A system-defined file name referring to the
standard output "file" (which can be file or a
device). $SDTLIST often refers to the terminal for
interactive sessions and the printer for batch
jobs. See standard output.
stop bits When two computers communicate information over an
asynchronous connection, the sending computer adds
one or more stop bits to the end of each byte that
it sends. The stop bits tell the receiving
computer that an entire byte has been sent and that
a separate byte is on its way.
storage device A device (such as a disk pack, a disk cartridge, a
flexible disk, magnetic tape, or cartridge tape)
onto which data can be stored and subsequently
retrieved.
STORE 1) The process of saving HP 3000 files to tape or
serial disk. Storing is executed by using the
STORE command.
2) A machine instruction that tells the CPU to take
information from a register and put it in memory.
store-and-forward A method of forwarding messages in a network. In a
store-and-forward network, a node can send a
message to another node to which there is no direct
link. Intermediate nodes can forward the message
to the correct destination node. Messages can be
stored and forwarded between several nodes.
stream To execute a batch job by using the MPE STREAM
command.
stress testing To test a system by executing a maximum load, over
time, to verify that the system will continue to
operate given maximum levels of interactive and
batch processing.
string A point-to-point network topology. The string
topology requires fewer communication links than
there are computers in the network, and requires
the fewest number of links. For communication
between non-adjacent nodes, messages are stored and
forwarded by intervening nodes. If a link fails,
the nodes separated by the failure are not able to
communicate.
Structured Query The industry standard relational database language.
Language (SQL)
subdividing data A way of limiting access to data by grouping data
into volumes, volume classes, and volume sets.
subline A portion of the command line that occurs only if
the command was continued on the input line by
terminating it with an ampersand. Also referred to
as a continuation line.
subqueue priority The priority of a job, session, or system process,
with the five system queues (A, B, C, D, or E). The
subqueue priority determines the priority of CPU
usage. A lower subqueue priority indicates a
higher priority for CPU time.
subsystem That part of the operating system that performs
specific functions for applications (for example,
IMAGE database management system).
supervisor See control program.
swapping See disk swapping.
switch subsystem The MPE/iX switch subsystem determines whether code
is in native mode or compatibility mode, and
automatically switches between modes as needed
while an application is running.
synchronous A method of transmitting data using a clock signal
to coordinate timing. The opposite of
asynchronous.
synchronous data The protocol used in a systems network
link control (SDLC) architecture (SNA) network to transmit data over a
communications link. SDCL is a bit-oriented
protocol that transmits data in frames.
syntax The rules governing the structure of a language or
instruction.
syntax error An error in an instruction due to a misspelled
word, a missing character, or improper punctuation.
SYS account A special account on the HP 3000, included with the
system when it is first installed. It contains all
MPE files (stored in the segmented library),
supported subsystems, utility programs, and
compilers.
system A group of one or more CPUs that communicate
through buses without the use of data
communications software.
system asynchronous A method of performing I/O whereby a process
I/O informs a driver or subsystem that it wants to know
when data has arrived or when it is possible to
perform a write request. The driver or subsystem
maintains a set of buffers through which the
process performs I/O. System asynchronous I/O frees
the process's buffers upon return from the I/O
request.
system buffers Any buffer used by the system to send messages to
the console.
system call An operating system kernel function available to
the user through a high-level language (such as
COBOL, FORTRAN, or Pascal). Also referred to as an
intrinsic or a system intrinsic.
system catalog A file containing UDC information for each user and
accounts using UDCs on a particular system.
system configuration The process of defining for the operating system
the current physical layout and workload of a
particular installation. The configuration is
typically modified when new terminals, another line
printer, or a disk drive are added. The system
also may be reconfigured to assign new class names
to existing devices, change the size of the system
tables or virtual memory, modify system logging, or
alter any other configuration parameter.
system console The terminal, usually logical device 20, the system
operator uses to monitor system activity, respond
to resource requests, and send messages to users'
terminals. The console (and most of its associated
privileges and responsibilities) may be transferred
to another logical device with the CONSOLE command.
system control panel A panel on the computer, containing control
switches and status indicator lights.
system crash See crash.
system-defined files The files defined by MPE and made available to all
users to indicate standard input or output devices,
special temporary files, and files opened for
output that do not perform an actual write
operation.
system directory A directory maintained by MPE that records the
name, group, and account of each permanent file on
the system. The directory contains the size of
each file, its location on the disk, who may access
it, and other information.
system disk The disk volume, mounted as logical device 1. It
contains MPE, I/O configuration information, the
accounting structure and file directory, and
utilities and subsystems. It also contains an area
reserved for virtual memory and may be used to
store user files.
system domain See system volume.
system event log See system logging.
system failure An internally detected error from which recovery is
not possible. Rather than continue to operate,
risking data integrity, the operating system halts
the computer.
system file A directory maintained by MPE that records the
directory name, group, and account of each permanent file on
the system. The directory contains the size of
each file, its location on the disk, who may access
it, and other information.
System generator The MPE/iX utility used to create or modify system
(SYSGEN) and I/O configurations; add, remove, and replace
program files and boot files; replace system
libraries; generate a full system backup, and
create a boot tape to bring up an MPE/iX system
with the new configuration. Equivalent to MPE V/E
SYSDUMP and INITIAL.
system halt The condition of MPE following a system shutdown or
system failure, indicated by no response at the
system console or to any user command.
system image See system configuration.
system loader A piece of software that brings a program into
memory and binds it to addresses before execution.
system load tape A system load tape, also referred to as a boot
tape, contains the system load utilities, base
system files, and user files. It is generated with
the system generator (SYSGEN) utility. Equivalent
to the MPE V/E coldload tape.
system logging The MPE/iX system logging facility records details
of system resource requests in a series of log
files on disk. The system manager or operator can
select which system events to record such as
job/session initiation/termination, program
termination, file closing, file spooling
completion, and system shutdown. I/O device
failures are recorded in the system log and are
used to detect problems before they interfere with
overall system operation.
system manager The person who manages the computer installation,
responsible for creating accounts and assigning
capabilities and resource-use limits to each
user/account.
system manager A capability that allows execution of all commands
capability (SM necessary to manage the system. This capability is
capability) usually only given to a system manager, since that
is the person responsible for the structure,
security, and overall operation of the system.
system master volume Any disk volume that has to be mounted for the
system to boot. This is different from nonsystem
volumes, which can be removed while the system is
up and the system remains running. A system volume
is always named MPEXL_SYSTEM_MASTER.
system measurement See Glance/XL.
tools (SMT)
system performance The efficiency of the system as measured by
response time and transaction throughput.
system processor The chip in which all the processor boards and
unit (SPU) cards reside.
system services A focal point within a system network architecture
control point (SSCP) (SNA) network for managing the configuration,
coordinating network operator problem requests, and
providing directory support and other session
services for end users of the network.
systems network An IBM architectural model. It defines the rules,
architecture (SNA) protocols, and procedures for communication between
devices within a network.
systems programming A programming language for the HP 3000 Series 37
language (SPL) through 70 that provides control of machine
instructions in a code segment. It is used for
many applications, including MPE V itself. Most
SPL programs execute in MPE/iX compatibility mode.
system startup To load the MPE operating system from either disk
or tape by bringing a subset of the MPE initiator
program into memory.
system supervisor A capability assigned by the system manager to the
capability (OP system supervisor's user name and account. The
capability) system supervisor is responsible for performing
backups, altering the system configuration, and in
general, tuning the computer so that it continues
to perform well and meet the needs of users.
system volume An MPE/iX system volume set. It contains a
bootable system image and system configuration on
its master volume. It is the only volume needed to
load and start the system. It is always mounted
and named MPEXL_SYSTEM_VOLUME_SET. This is
equivalent to an MPE V/E system domain.
tape mark The uniquely formatted area on a magnetic tape that
is used to separate files; it also may be used to
delimit the end of the tape (two file or tape
marks).
tape request A printed message at the console asking for a
backup device to be assigned to a user.
temporary file A file that exists only for the duration of a
session or job.
terminal A hardware device connected to a computer, used for
entering and receiving data. A terminal consists
of a keyboard and a display screen.
Text and Document An HP 3000 line editor (with a screen editor
Processor/V (TDP/V) option) used to create, manipulate, and format
ASCII text files.
thermal printer A printer that forms characters by heating paper.
The printer requires special heat-sensitive paper.
ThickLAN cables ThickLAN cables (IEEE802.3 10BASE5) are used to
connect Datacommunications and Terminal Controllers
(DTCs) to the system. Thicknet cables require a
media attachment unit (MAU) for each DTC. Use of
ThickLAN cabling is most appropriate when the DTCs
are distributed throughout the facility.
ThinLAN cables ThinLAN cables (IEEE802.3 10BASE2) are used to
connect Datacommunications and Terminal Controllers
(DTCs) to the system. Use of ThinLAN cabling is
most appropriate when the DTCs are in close
proximity to the system.
throughput A measurement of the amount of useful work
performed by a system in a given amount of time.
Toolset/XL Toolset/XL provides an integrated programming
environment for COBOL II/XL and Pascal/iX
programming.
topology The physical structure of a network (for example,
star or ring network).
track A data area on disk that forms a concentric circle,
divided into sectors. One full track passes under
the disk head during each rotation of the disk.
trailer The last page printed every time output is directed
to a line printer. It contains the session number,
session name, logon identification, day of the
week, date, and time. It corresponds to the header
printed as the first page.
TRANSACT A high-level programming language for MPE V and
MPE/iX.
transaction A logical unit of work. It may consist of one or
more operations, but either all or none of them
will be performed.
transaction logging The process of keeping track of all database
operations occurring within defined transactions.
The resulting transaction log file can be used to
recover data and ensure database integrity when a
program aborts or the system crashes.
transaction Ensures data consistency and integrity by providing
management facility transaction locking. Transaction locking ensures
that only one transaction at a time is allowed to
update given data, and all changes that are part of
a transaction must be completed before the changes
are committed to a permanent record.When a
transaction abnormally ends (before it is actually
complete), the changes made from the beginning of
the transaction to the abort point are undone. The
database is restored to the state it was in just
prior to the beginning of the transaction.
transient space Disk space used for temporary processes such as
stacks and operating system data structures.
translation The translation lookaside buffer (TLB) is a
lookaside buffer high-speed random access memory (RAM) buffer. It
(TLB) optimizes the task of translating virtual addresses
to physical addresses. The
MPE/iX processor generates 48-bit virtual addresses
and memory access is provided by 28-bit physical
(real) addresses.
transparent Unseen by the user. A process or action with which
the user need not be concerend.
transport backup Transport refers to the movement of files between
MPE V/E and MPE/iX systems using backup media.
MPE/iX backup and recovery provides a compatibility
option with which an MPE V/E-compatible tape may be
created or read.
transport layer Layer four of the Open Systems Interconnection
(OSI) network model. The transport layer provides
for transport of messages between end-users.
tuple A row, record, or data entry in a DBCore relation.
unconfiguring a The process of logically removing a device from the
device system.
unblocked record A physical record that contains just one logical
record. The opposite of blocked record.
undefined-length See variable-length records.
records
unit number A part of an address used for devices. A number
whose meaning is software-dependent and
device-dependent, but which is often used to
specify a particular disk drive in a device with a
multidrive controller.
UNIX An operating system developed by AT&T Bell
Laboratories. Hewlett-Packard's implementation of
UNIX is called HP-UX.
UNKNOWN A disk pack without a volume label recognized by
MPE/iX.
unscratch a volume To make data available on a previously "scratched"
volume. See scratch a volume.
UPDATE 1) The MPE/XL ISL UPDATE utility performs a system
load from tape. It replaces the current base
system files on disk, and optionally replaces
configuration files. The ISL UPDATE CONFIG option
is equivalent to the MPE V/E COLDSTART procedure,
and the UPDATE NOCONFIG option is equivalent to the
MPE V/E UPDATE operation.
2) An MPE V/E coldload option that loads all files
in the PUB group of the SYS account from the backup
media. I/O configuration data, the directory, and
user files are loaded from the system disk. UPDATE
is typically used to install a new version of
system software or to load MPE from another
computer. Equivalent to the MPE/XL ISL UPDATE
utility UPDATE NOCONFIG option.
use communications A capability assigned to accounts and users
subsystems allowing access to the MPE communications
capability (CS subsystems.
capability)
user Anyone logged onto a session, using a local or
remote terminal to interact with the computer.
Each user identified by a user and account name can
access files in the logon group.
user asynchronous A method of performing I/O whereby a process
I/O launches an I/O request and continues to execute
while the I/O is performed. The process's buffers
should not be changed or used by the process until
the system informs the process that they are free.
user command A set of MPE/iX commands that a user has grouped
together to perform a specific task. A user
command is stored in a command file or UDC. UDCs
are first in MPE's search path for commands.
user-defined command A command that executes a set of one or more
(UDC) commands that the user has grouped together into a
single, named procedure file to perform a specific
task. See also command file.
user-level security The file access modes permitted the user.
User-level security must duplicate, or be a subset
of, the file access permitted the user's account
and group.
user logging A facility that enables users and subsystems to
record additions and modifications to files. If
necessary, user logging also provides the means
whereby recorded entries can be used to recover the
files themselves.
use volumes A capability assigned at the account and user level
capability (UV allowing users to access nonsystem disk volumes.
capability)
utility program A program that performs specific functions such as
file copying, sorting and merging, memory dump
analysis, or monitoring available disk space.
value added network A common carrier service that has been upgraded by
(VAN) a communications service. Also referred to as a
packet switching network.
variable A value that can be changed, as opposed to a
constant, usually represented by a letter or a
group of alphanumeric characters.
variable-length A record whose length (in bytes or words) can vary
record and is defined only by a maximum allowable file
size. A file with undefined length records must
have a blocking factor of one, and is written
without buffering.
One of a set of records that varies in size with
respect to each other.
VINIT An MPE V/E volume initialization subsystem
(accessed with the VINIT command). It contains
several commands for online initialization and
formatting of private volumes, serial disks, and
foreign disks. Equivalent to the MPE/XL VOLUTIL
utility.
virtual memory MPE/iX virtual memory refers to providing
programmers with the appearance that the available
memory space is many times larger than the actual
amount of main or even disk memory. MPE/iX
provides this capability by taking advantage of the
system's extremely large addressing potential.
virtual terminal An NS3000/XL service that provides interactive
(VT) access to other systems on a network.
VLSI Very large scale integrated circuit technology.
VLSI technology puts over 100,000 gates on a single
chip and is used for CPUs and memories.
volume A volume is one disk pack. Each volume is a member
of a volume set and contains a volume label, a
label table, and a free space map.
volume class Volume classes are used for the allocation and
restriction of disk space. A volume class is a
logical subset or partition within a volume set and
can include any number of physical member volumes
of a volume set.
volume name The name given to a volume set or volume class with
the NEWSET command. The name may be assigned only
by a user with CV (create volume) capability,
usually the system manager or account manager. The
volume name is an ASCII character string of up to
eight alphanumeric characters, beginning with an
alphabetic character.
volume failure See disk failure.
volume management A facility of MPE used to manage disk storage using
volumes, volume sets, and volume classes.
volume number A number used to specify a particular volume on a
multivolume disk drive.
volume set A volume set is a group of from 1 to 255 related
disk packs. One volume of the volume set must be
designated as the master volume for the set. Each
volume set is assigned a name by which it is
identified and referenced. MPE/iX recognizes both
system volume sets and nonsystem or mountable,
volume sets.
volume set A part of a master volume of a volume set
information table containing description and address information
(VSIT) about volume sets.
volume states The states in which a volume can exist on a system.
Accessible: MASTER and MEMBER. Inaccessible:
LONER, UNKNOWN, and SCRATCH.
volume testing The verification that the system will continue to
operate when loaded to each individual limit, the
determination of what occurs when those limits are
exceeded, and the determination of what occurs when
the system is emptied after being in a full state.
VOLUTIL The MPE/iX volume utility that provides volume
initialization and maintenance, volume label and
membership inquiries, and volume space/sector
status. Equivalent to the MPE V/E VINIT subsystem.
VPLUS/V An application designed to simplify forms design
and screen handling for interactive applications.
VPLUS/V is included in the fundamental operating
software (FOS) for the 900 Series HP 3000.
WARMSTART The process of restarting the HP 3000 by loading
MPE V/E from the system disk. A WARMSTART is used
if incompletely processed spooled jobs and spooled
files must be recovered, since other startup
options do not permit the recovery of spoolfiles.
MPE/iX equivalent is the START RECOVERY option of
the ISL START utility.
WELCOME message The message that appears when a user logs on to the
system.
wildcard A symbol that represents a variety of characters.
In MPE, the at sign (@), the pound sign (#), and
the question mark (?) are used as wildcard
characters. Other subsystems may use different
symbols.
window A portion of a file or image displayed on selected
portions of a CRT screen.
word A word consists of 32 bits (4 bytes) of information
in the 900 Series HP 3000. A word consists of 16
bits (2 bytes) of information in other HP 3000
systems.
word processor A program that creates, stores, recalls, and edits
text (used for letters, memos, reports, and other
documents).
work around A "scratch file" used for intermediate data storage
by a subsystem or program. It is usually purged
when the subsystem or program terminates.
work file A temporary file created when a text editor is
invoked. A work file is a copy of a permanent disk
file. Any changes to the work file are lost unless
the user saves the updated version of the file.
workstation Terminals, personal computers, or serial printers
that communicate with a host computer but have
inherent processing capabilities.
write What a computer does when it writes information to
a device.
write-enable To remove write-protection, allowing data to be
written upon a floppy disk or a tape.
write-protect To protect stored data so that it can not be
overwritten.
write ring A plastic ring that fits onto the inner groove of a
reel-to-reel tape, enabling you to write
information onto the tape.
XON/XOFF An industry standard protocol used by a peripheral
device to regulate data transmission. XON starts
data transmission and XOFF stops it.
X.25 A type of communication link that provides
connection of packet switching networks (PSNs),
also known as value added networks (VANs). X.25
links are useful for long-distance communication
and can be more economical than leased lines in
some applications.