	ACADEMIC COMPUTING and COMMUNICATIONS CENTER

IBM HELP Glossary of CMS and Computer Related Terms

ABSTRACT
GLOSSARY
-- Purpose
-- Notice

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ABSTRACT

This document is a glossary of computer terms with emphasis on VM and CMS related terms. It is part of the VM/ESA CMS 9 online HELP system (it is available through the HELP LIBRARY TASK menu), and is copyright by IBM.

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GLOSSARY

Purpose

This glossary contains technical terms that refer to the components, functions, operation, and use of VM/ESA Release 2 and related products. You can find additional information on IBM terminology in the Dictionary of Computing, SC20-1699.

Notice

Some entries in this glossary that are from the Dictionary of Computing include definitions from:

The American National Standard Dictionary for Information Systems, ANSI X3.172-1990, copyright 1990 by the American National Standards Institute (ANSI). Copies may be purchased from the American National Standards Institute, 11 West 42nd Street, New York, New York 10036. Definitions are identified by the symbol (A) after the definition.
The Information Technology Vocabulary, developed by Subcommittee 1, Joint Technical Committee 1, of the International Organization for Standardization and the International Electrotechnical Commission (ISO/IEC JTC1/SC1). Definitions of published parts of this vocabulary are identified by the symbol (I) after the definition; definitions taken from draft international standards, committee drafts, and working papers being developed by ISO/IEC JTC1/SC1 are identified by the symbol (T) after the definition, indicating that final agreement has not yet been reached among the participating National Bodies of SC1.

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A-type address constant: In assembler language, an address constant used for branching within a module or for getting data.
abend: (1) Abnormal end of task. (2) Synonym for abnormal termination.
abend dump: The contents of main storage, or part of main storage, written to an external medium for debugging an error condition that resulted in the termination of a task before its regular completion.
abnormal end of task (abend): Termination of a task before its completion because of an error condition that cannot be resolved by recovery facilities while the task is executing.
abnormal termination: The ending of processing before planned termination. Synonymous with abend.
accept: Allowing a connection to the user's virtual machine from another virtual machine or from the user's own virtual machine.
access list: A list of address spaces that are available to a program operating in access-register mode. For XC virtual machines, access lists are managed by CP and are formally called host access lists. See also host access list.
access method services (AMS): The utility that allocates and configures VSAM data sets. AMS is invoked through the AMSERV command in CMS.
access mode: A method VM/ESA uses to control user access to data files. Access modes let the user read and write data to a file, or only read data from a file. See file mode.
access register: A 32-bit register that contains an ALET identifying an address space to be used for data references when in access-register mode. ESA/370, ESA/390, and ESA/XC architectures provide 16 access registers, numbered 0-15. When in access-register mode, a "B" field (or sometimes an "R" field) in an instruction designates both the general register to be used as a base register and the access register to be used to specify the address space containing the data.
access security: Information that a target LU and target transaction program use to verify whether a source program is authorized to make a connection. This information consists of a user ID and, possibly, a password.
access-list entry (ALE): Synonym for host access-list entry.
access-list-entry token (ALET): A 32-bit value that is an indirect specification of an address space. When a program is operating in access-register mode, an ALET contained in an access register designates the address space containing an instruction operand.
access-register (AR) mode: An address translation mode in which the contents of the access registers are used to determine the address space containing a storage operand. The SET ADDRESS SPACE CONTROL instruction can be used to change the PSW to enter and exit access-register mode. See also primary-space mode.
ACF/SSP: Advanced Communications Function for Systems Support Programs.
ACF/VTAM: Advanced Communications Function for Virtual Telecommunications Access Method.
active record: A record added or appended to a file currently in use.
active work unit: A work unit that has uncommitted work associated with it. A request was made on the work unit (other than an atomic request) and no commit or rollback has occurred.
ACU: Automatic calling unit.
ADCON: An A-type address constant used in calculating storage addresses.
address space: A collection of bytes that are allocated, and in many ways managed, as a single entity by CP. Each byte within an address space is identified by a unique address. An address space represents an extent of storage available to a program. Address spaces allocated by VM range in size from 64KB to 2GB.
address stop: See breakpoint and instruction address stop.
address-space-identification token (ASIT): An 8-byte token that uniquely identifies a particular address space allocated by CP. The ASIT is a system-wide, unique identification token--once an ASIT value has been assigned to an address space, that ASIT value is not used again within the scope of a VM/ESA system IPL (CP IPL).
addressing-capability exception: A program exception that arises as a result of an attempt to access an address space for which access permission has been revoked by the owner of the address space.
Advanced Communications Function for Systems Support Programs (ACF/SSP): An IBM licensed program made up of a collection of utilities and small programs. SSP is required for operation of the NCP.
Advanced Communications Function for Virtual Telecommunications Access
Method (ACF/VTAM): An IBM licensed program that controls communications and flow of data in an SNA network. It provides single-domain, multiple-domain, and interconnected network capability.
advanced function printer (AFP): An all-points-addressable printer, such as the IBM 3800-3 and IBM 3820 printers, capable of printing images and text.
advanced function printing data stream (AFPDS): A method of representing image data in a file that is destined for printing on a PSF-controlled printer.
Advanced Interactive Executive for PS/2 (AIX PS/2): An IBM licensed program that contains the features of AIX PS/2 Version 1.2.
Advanced Program-to-Program Communications (APPC): The inter-program communication service within SNA LU 6.2 on which the APPC/VM interface is based.
Advanced Program-to-Program Communications/VM (APPC/VM): An API for communicating between two virtual machines that is mappable to the SNA LU 6.2 APPC interface and based on IUCV functions. Along with the TASK virtual machine, AVS virtual machine, and VTAM, APPC/VM provides this communication within a single system, throughout a collection of systems, and throughout an SNA network.
AFP: Advanced function printer.
AFPDS: Advanced function printing data stream.
agent: (1) In CRR sync point processing, the role of the CRR sync point manager (SPM) when sync point requests are received from an initiator that is a partner in a protected conversation. (2) A task for an SFS file pool server or CRR recovery server. The SFS file pool server and CRR recovery server provide support for multitasking several agents.
AIX domain controller: (1) An AIX application that lets programs on AIX workstations communicate with resources on VM systems or other programmable workstations in a CS collection. (2) See also AIX PS/2 domain controller, OS/2 domain controller, VM/ESA domain controller.
AIX PS/2: Advanced Interactive Executive for PS/2.
AIX PS/2 domain controller: An AIX PS/2 application that lets user programs on AIX workstations running VM PWSCS connect to and communicate with resources on VM/ESA and OS/2 Extended Edition systems using ISFC or VM PWSCS through a channel-attached OS/2 domain controller running VM PWSCS.
AL: ANSI-labeled tape without user-label processing.
ALE: Access-list entry.
ALET: Access-list-entry token.
alias: A pointer to an SFS base file. An alias can be in the same directory as the base file or in a different directory. There must always be a base file for the alias to point to. The alias references the same data as the base file. Data is not moved or duplicated.
allocation tree: A conceptual structure showing the nodes that are allocating protected conversations within a CRR coordinated transaction.
alphanumeric: A character set that contains letters, digits, and usually other characters, such as punctuation marks.
alternate console: A console assigned as a backup unit to the system console.
alternate path support: The selection of a path to a device from any of the available paths, even though the primary path is busy. The selection is made in response to an I/O request for a device, through use of the two-channel switch, the two-channel switch additional features, and the string switch hardware feature.
alternating operating systems: Multiple operating systems loaded consecutively into a virtual machine. Information is passed between the operating systems by the CP spooling facilities. Synonymous with flip-flop.
American National Standard Code for Information Interchange (ASCII): The standard code, using a coded character set consisting of 7-bit coded characters (8 bits including parity check), used for information interchange among data processing systems, data communication systems, and associated equipment. The ASCII set consists of control characters and graphic characters.
AMS: Access method services.
anchor block: The CMS anchor block is an area in user storage that contains space for 16 anchor slots. It also includes space for the register save area used by the Anchor facility. The anchor block storage persists after an abend occurs.
anchor identifier: The anchor identifier uniquely identifies the program when calling the CMS Anchor facility. The ANCHOR macro accepts an anchor identifier 3 bytes in length and is padded on the right with a blank. The anchor identifier must be assigned by IBM.
anchor slot: Each anchor slot is 8 bytes in length. The first 4 bytes contain the anchor identifier. The second 4 bytes contain the anchor word. They are both initially zero.
anchor word: The anchor word contains the user's data and is usually the address of the caller's data area.
annotated flow diagram: A diagram consisting of logic labels and commentary. It describes the general flow and use of CP logic modules and their relationships to other modules while doing a specific function or task.
ANSI: American National Standards Institute.
ANSI Version 3: Term that refers to the ANSI standards, ANSI X3.27-1978, level 4.
AP: Attached processor.
AP/MP mode: A mode of VM/ESA while running in an AP or MP system.
APAR: Authorized program analysis report.
APAR number: The number that IBM assigns to an APAR and to the change resulting from it.
API: Application program interface.
APPC: Advanced Program-to-Program Communications.
APPC link: A logical TSAF link that is physically controlled by VTAM, and is established using the TSAF VTAM line driver.
APPC/VM: Advanced Program-to-Program Communications/VM.
APPC/VM VTAM Support (AVS): A component of VM/ESA that lets application programs using APPC/VM or CPI Communications communicate with programs anywhere in a network defined by IBM's SNA. AVS transforms APPC/VM into APPC/VTAM protocol.
application program: A program written for or by a user that applies to the user's work, such as a program that does inventory control or payroll.
application program interface (API): The formally defined programming language interface between an IBM system component or licensed program and its user.
apply: When servicing a product or component, to generate an auxiliary control structure from a PTF. See apply disk, apply ID, apply message log, apply list, apply status table, apply string.
apply disk: In VMSES/E, a minidisk or SFS directory containing the files that describe the maintenance levels: the apply status table, AUX files, version vector tables, the select data file, and the build status table.
apply ID: A 1- to 8-character, alphanumeric identifier that is used to name the software inventory files created during apply processing. The user can change this value to define different maintenance levels.
apply list: A file listing PTFs applied to a product or component.
apply message log: A file in which the VMFAPPLY EXEC writes status and error messages during apply processing.
apply status table: The software inventory table that identifies what PTFs have been applied to the product. The system level of the table identifies what product or component has been applied to the system. The file type of the system level inventory table is SYSAPPS and the file type of the service level inventory table is SRVAPPS.
apply string: In VMSES/E, the set of apply disks.
AR: Access register.
architected segment: A 1MB portion of real storage defined by the 370-XA, ESA/370, ESA/390, and ESA/XC architectures.
AR mode: Access-register mode.
area: A term acceptable for DASD space when there is no need to differentiate between space on count-key-data devices and FB-512 devices. See DASD space.
ASCII: American National Standard Code for Information Interchange.
ASIT: Address-space-identification token.
ASP: Asymmetric multiprocessing system.
assembler language: A source language that includes symbolic machine language statements in which there is a one-to-one correspondence with instruction formats and data formats of the computer.
asymmetric multiprocessing system (ASP): An extension to the IBM System/360 Operating System that provides increased automation of computer operations for large-scale data processing installations.
asynchronous communication: A method of communication in a conversation that lets a program issue functions on other conversations while waiting for a particular function to complete.
atomic request: An SFS command or program function (CSL routine) that completes execution immediately and does not affect work in other resources (such as other SFS file pools) on the work unit. Any file pool updates caused by an atomic request are committed (or rolled back) when the file pool server finishes processing the request. Atomic requests cannot be issued to a file pool that has uncommitted work for the specified (or default) work unit.
attached processor (AP): A processor that has no I/O capability and is always linked to the processor initialized for I/O handling.
attached processor operation: A mode in which VM/ESA runs tasks on two processors, only one of which has attached channels; however, the tasks come from a common dispatch queue and share common storage.
attachment interface: The logical interconnection and interaction between or to software programs that enable the programs to function together.
attention interrupt: An I/O interrupt caused by a terminal user pressing the attention key (or equivalent). See attention key (ATTN key) and signaling attention.
attention key (ATTN key): A function key on terminals that, when pressed, causes an I/O interruption in the processing unit. See signaling attention.
attention symbol: Under CP, for printing terminals only, the character (always the exclamation point (!) character) that indicates each time the attention key is pressed.
ATTN key: Attention key.
AUL: ANSI labeled tape with user label processing.
authority: In SFS, the permission to access a file or directory. You can have read authority or write authority (which includes read authority). You can also have file pool administration authority, which is the highest level of authority in a file pool.
authorized application: A GCS application, started with the OSRUN command, that operates in supervisor state, and can use privileged functions. Contrast with ordinary application.
authorized program: Synonym for privileged program.
authorized program analysis report (APAR): An official request to the responsible IBM Change Team to look into a suspected problem with IBM code or documentation. APARs describe problems giving conditions of failure, error messages, abend codes, or other identifiers. They also contain a problem summary and resolution when applicable. See program temporary fix (PTF).
authorized user ID: In GCS, a user ID that provides access to the GCS supervisor, supervisor state, and (at times) certain restricted CP commands. This access is provided by including the user ID on a list of authorized user IDs compiled with the GCS GROUP EXEC. The virtual machine associated with an authorized user ID is an authorized machine, and programs running in that machine are authorized applications.
authorized virtual machine: A GCS virtual machine identified by user ID.
automatic calling unit (ACU): A device that receives dial digits from the communications adapter and is associated with the switched line modem, enabling the modem to call its counterpart at the remote system.
automatic logon: A process by which an operator's or user's virtual machine is initiated by someone other than the user of that virtual machine. For example, the primary VM/ESA system operator's virtual machine is automatically started during VM/ESA initialization. A privileged user can enter the AUTOLOG command to start some other (disconnected) virtual machine.
automatic software re-IPL: The process by which the control program attempts to restart the system after abnormal termination. This process does not involve the hardware IPL process. See virtual=real machine recovery.
AUX file: Auxiliary control file.
auxiliary control file (AUX file): A file that contains a list of file types of update files applied to a particular source file or to control the service level used during build. See control file and preferred auxiliary file. Synonymous with auxiliary file.
auxiliary directory: In CMS, an extension of the CMS file directory for a minidisk, which contains the names and locations of certain CMS modules not included in the minidisk's CMS minidisk file directory.
auxiliary file: Synonym for auxiliary control file.
auxiliary storage: Data storage other than main storage; in VM/ESA, auxiliary storage is usually a direct access device.
AVS: APPC/VM VTAM Support.
AVS virtual machine: The virtual machine that manages a gateway that allows APPC communication between VM systems and an SNA network.

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The verb form of "backout" is "back out."

base disk: In VMSES/E, minidisk or SFS directory containing the original product files as shipped on the product tape.
base file: The first occurrence of an SFS file. It remains the base for the life of the file, even if the file has been renamed. Aliases point to base files. See base file type.
base file type: In VMSES/E, the file type used for a serviceable part when there is no service. The PTF number in the file type is set to "00000." For example, EXC00000 is the base file type for an exec.
base string: IN VMSES/E, the set of base disks.
basic control (BC) mode: A mode in which additional System/370 features, such as new machine instructions, are not operational. Contrast with extended control (EC) mode.
basic conversation: A conversation where data is sent in APPC-defined logical record formats. Programs must be coded to consider the amount of data sent. Contrast with mapped conversation.
basic sequential access method (BSAM): An access method for storing or getting data blocks in a continuous sequence (using either a sequential access or direct access device).
BC mode: Basic control mode.
BDT: Bulk data transfer. See MVS/Bulk Data Transfer.
binary digit: Either of the digits 0 or 1 when used in the pure binary numeration system. Synonymous with bit.
binary synchronous communication (BSC): Communication using binary synchronous line discipline in which transmission of binary-coded data between stations is synchronized by timing signals generated at the sending and receiving stations.
bit: (1) Either of the binary digits 0 or 1. See byte. (2) Synonym for binary digit.
block: (1) A unit of DASD space on FB-512 devices. For example, FB-512 devices can be the IBM 9335, 9332, 9313, 3370, and 3310 DASD using fixed-block architecture. (2) In CMS Multitasking, to stop the execution of a thread until a function has been completed or a condition is satisfied.
block extent: A continuous space on a direct access storage volume that does not have to be stored contiguously with a block (4096) of pages.
blocked file: A file in which each data block may contain one or more records. If a file is unblocked, each data block contains only one record.
bootstrap program: A technique or device that brings itself into a desired state by its own action. For example, a machine routine whose first few instructions are sufficient to bring the rest of itself into the computer from an input device.
border: A boundary around a window. The user can enter one-letter BORDER commands from the corners of the border. For example, the letter P entered from a border corner pops the window. The border corners are indicated by a + (plus) sign.
box: In CP, to shut off all communication with an I/O device because CP has identified a serious problem with the device, for example, exceeding the hot I/O rate.
bpi: Bits per inch.
Bpi: Bytes per inch.
break tree processing: The process of breaking off other parts of a sync point tree to ensure that a unique LUWID is used by each sync point tree after a protected conversation abnormally ends.
breakpoint: A place in a program, specified by a command or a condition, where the system halts execution and gives control to the workstation user or to a specified user.
broadcast message: Information that can be sent by the system operator to all terminal users that are enabled to receive messages. The three major classes of messages are: (1) log (LOGMSG) messages automatically displayed at the user's terminal when the user logs on, (2) optional lower priority log messages, and (3) informational warning messages that alert users to some imminent event or action.
BSAM: Basic sequential access method.
BSC: Binary synchronous communication.
buffer: An area of storage, temporarily reserved for performing input or output, from which data is read, or into which data is written. See forms control buffer (FCB), spool file buffer linkage block, terminal input buffer.
build: (1) In the installation and service of a product, to do the necessary steps to produce executable code or systems. This is often called the build process. (2) See build disk, build message log, build ID, build list, build status table, build string.
build disk: In VMSES/E, a minidisk or SFS directory where executable versions of objects serviced are built.
build ID: A 1- to 8-character, alphanumeric identifier that is used to name the software inventory files created during build processing. The user can change this value to define different maintenance levels.
build list: An exec that names the serviceable parts included in an object being built, and parameters and options used to build the object.
build message log: A file in which the VMFBLD EXEC writes status and error messages during build processing.
build status table: The software inventory table that identifies what products have been built, in the system level, and what individual objects have been generated for the product, in the service level. The file type of the system level inventory table is SYSBLDS and the file type of the service level inventory table is SRVBLDS.
build string: The set of build disks.
built-in function: A specialized function, invoked by a keyword, that has been built into the system program code because it is commonly required by many users.
built-in stage command: A stage command supplied by CMS Pipelines. See also user-written stage command.
byte: A unit of storage, consisting of eight adjacent binary digits that are operated on as a unit and constitute the smallest addressable unit in the system.
B1: One of several security designations that can be assigned to a computing system by the U. S. Department of Defense. The B1 designation indicates that the system:

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cache fast-write: A 3990 Model 3 function where data is written directly to cache without using nonvolatile storage and the data is available for later destaging. This function should be used for data of a temporary nature, or data that is readily repeated.
callable services library (CSL): A package of CMS assembler routines that can be stored as an entity and made available to a high-level language, REXX, or an assembler program.
cascaded agent: An agent whose initiator is an agent of another initiator. In this role, an agent is responsible for responding to sync point requests from its cascaded initiator.
cascaded initiator: An agent that has protected conversations with cascaded agents. The cascaded initiator takes the role of initiator in relation to its agents and is responsible for propagating sync point requests to its cascaded agents.
catalog storage group: The storage group in a file pool that contains information about the objects (such as files and directories) and authorizations that exist in the file pool. See file pool catalog.
CAW: Channel address word.
CC: Condition code.
CCH: Channel-check handler.
CCS: Console communication service.
CCT: Communications control table.
CCW: Channel command word.
CE: (1) Channel end. (2) Correctable error. (3) (IBM) Customer Engineer.
Central Processing Complex (CPC): The boundaries of a system exclusive of I/O control units and devices, that can be controlled by a single operating system. A CPC consists of main storage, one or more central processing units, time-of-day clocks, and channels, which are or can be placed in a single configuration. A CPC also includes channel subsystems, service processors, and expanded storage.
central site/system: The main installation with skilled system support personnel such as system programmers and operations staff.
CETI: Continuously Executing Transfer Interface.
CF: Crypto Facility.
chain link record: In CMS, a record that contains pointers to the physical blocks currently assigned to a CMS file.
changes: In installation and service, service supplied by IBM and original equipment manufacturers (OEMs) for their programs. In the IBM service process, there are many ways users can receive information they need to fix (change) a portion(s) of a product they are running on a VM system. These include PTFs, APARs, user modifications, and information received over the phone. All these types of information are called changes.
channel: A path in a system that connects a processor and main storage with an I/O device.
channel address word (CAW): An area in storage that specifies the location in main storage at which a channel program begins.
channel command word (CCW): A doubleword at the location in main storage specified by the channel address word. One or more CCWs make up the channel program that directs data channel operations.
channel status word (CSW): An area in storage that provides information about the termination of I/O.
channel-check handler (CCH): In System/370, a feature that records information about channel errors and issues appropriate messages to the operator.
channel-set switching: A facility used in some attached processor environments. It lets processing continue in uniprocessor mode on the attached processor after the main processor enters a disabled WAIT state following an irrecoverable error (a hard machine or channel check), or after the system operator varies the main processor offline. CP switches all active channels on the main processor to the attached processor, and processing continues in uniprocessor mode.
channel-to-channel (CTC) device: A hardware device that connects two channels on the same computing system or on different systems. CTC devices include both CTCAs and 3088 MCCUs.
channel-to-channel adapter (CTCA): A hardware device that connects two channels on the same computing system or on different systems.
character delete symbol: Synonym for logical character delete symbol.
checkpoint: An internal file pool server operation during which the changes recorded on the log minidisks are permanently made to the file pool.
checkpoint (CKPT) start: A VM/ESA system restart that attempts to recover information about closed spool files previously stored on the checkpoint cylinders. The spool file chains are reconstructed, but the original sequence of spool files is lost. Unlike warm start, CP accounting and system message information is also lost. Contrast with cold start, force start, and warm start.
circumventive service: Information that IBM supplies over the telephone or on a tape to circumvent a problem by disabling a failing function until a PTF is available as a corrective service fix. See patch and zap.
CKD: Count-key-data.
CKPT start: Checkpoint start.
class A user: See primary system operator privilege class.
class Any user: Any user, regardless of class, who is allowed to use a subset of CP commands to log onto VM/ESA, have the terminal logically connected to a multiple-access virtual machine, or send messages to the operator or to other users.
class authority: Privilege assigned to a virtual machine user in the user's directory entry; each class specified allows access to a subset of all the CP commands. See privilege class and user class restructure (UCR).
class B user: See system resource operator privilege class.
class C user: See system programmer privilege class.
class D user: See spooling operator privilege class.
class E user: See system analyst privilege class.
class F user: See service representative privilege class.
class G user: See general user privilege class.
class override file: A file containing control statements defining changes in the privilege classes of CP commands and diagnose codes. The override program uses this file to establish a new class structure of commands under UCR.
clock comparator: A hardware feature (required by VM/ESA) that causes an interruption when the TOD clock has equaled or exceeded the value specified by a program or virtual machine.
CLP: Current line pointer.
CMR: Conversation management routine.
CMS: Conversational Monitor System.
CMS batch facility: A facility that lets the user run time-consuming or noninteractive CMS jobs in another CMS virtual machine dedicated to that purpose, thus freeing the user's own terminal and virtual machine for other work.
CMS blip facility: An indication to the user at a terminal, by way of a tilt/rotate of the IBM Selectric type ball (if using a printing terminal), or the display of one or more specified characters, every 2 seconds of either real or virtual execution time that has elapsed for the CMS virtual machine. Whether the time shown is real or virtual depends on the current setting of the REALTIMER option.
CMS editor: A CMS facility that lets the user create, change, insert, delete, or rearrange lines of data in a CMS file. See edit mode and input mode.
CMS EXEC: An EXEC procedure or EDIT macro written in the CMS EXEC language and processed by the CMS EXEC processor. Synonymous with CMS program.
CMS EXEC language: A general-purpose, high-level programming language, particularly suitable for EXEC procedures and EDIT macros. The CMS EXEC processor executes procedures and macros (programs) written in this language. Contrast with EXEC 2 language and Restructured Extended Executor (REXX) language.
CMS EXEC processor: The component of the VM/ESA operating system that interprets and executes procedures and EDIT macros written in the CMS EXEC language.
CMS file system: A way to create files in the CMS system. CMS files are created by using an identifier consisting of three fields: file name, file type, and file mode or SFS directory. These files are unique to the CMS system, cannot be read or written using other operating systems, and are stored either on minidisks or SFS directories.
CMS files: Refers exclusively to files in the fixed-block format used by the CMS file system. VSAM and OS data sets and DOS files are not compatible with the CMS file format and cannot be manipulated using CMS file system commands.
CMS loader work file: The CMS file loaded into storage by the LOAD command.
CMS minidisk file directory: A directory on each CMS disk that contains the name, format, size, and location of each of the CMS files on that disk. When a disk is accessed by the ACCESS command, its directory is read into virtual storage and identified with any letter from A through Z. Synonymous with master file directory block and minidisk directory.
CMS nucleus: The portion of CMS that is resident in the user's virtual storage whenever CMS is executing. Each CMS user receives a copy of the CMS nucleus when the user IPLs CMS. See saved system and shared segment.
CMS Pipelines: A part of CMS that processes pipelines and includes stage commands that can be used in a pipeline.
CMS program: Synonym for CMS EXEC.
CMS system disk: The virtual disk (file mode S) that contains the CMS nucleus and the disk-resident CMS commands. The CMS system disk can have extensions; usually as file mode Y.
CMS system file: Any file residing on the CMS system disk instead of on a user's disk or in a user's file space.
CMS system minidisk: The minidisk located at virtual address 190. It contains the system commands for which the user has read-only access.
CMS user disk: One or more virtual disks that contain CMS or read-only DOS or OS files that can be accessed by the user. If the user has read/write access to a disk, the user can create programs and data files on the disk. Files are retained until the user deletes them. The user can also link to and access other users' disks, usually on a read-only basis.
CMS-formatted disk: A formatted disk used specifically with the CMS file system. It does not contain a VTOC and is incompatible with OS and DOS formatted disks. OS and DOS access methods cannot be directly used with CMS formatted disks.
CMS/DOS: The functions of CMS that become available when the user enters the command SET DOS ON. CMS/DOS is a part of the regular CMS system and is not a separate system. Users who do not use CMS/DOS are sometimes called OS users, because they use the OS simulation functions of CMS. Synonymous with DOS simulation under CMS. Contrast with OS simulation under CMS.
CMS/DOS phase library: Synonym for DOSLIB library.
CMSDOS: The standard name of the CMS/DOS saved segment. See saved segment.
CMSSERV: A command that starts a CMS router in the Enhanced Connectivity Facilities environment of VM/ESA.
CNTRL file: Control file with file type CNTRL.
cold start: A VM/ESA system restart that ignores previous data areas and accounting information in main storage, and the contents of paging and spool files on CP-owned disks. Contrast with checkpoint (CKPT) start, force start, and warm start.
collection: See TSAF collection or CS collection.
command: A request from a user at a terminal for the execution of a particular CP, CMS, GCS, TSAF, Dump Viewing Facility, or AVS function. A CMS command can also be the name of a CMS file with a file type of EXEC or MODULE. See subcommand and user-written CMS command.
command abbreviation: A short form of the command name, operand, or option that is not a truncation of the word. For example, MSG instead of MESSAGE, RDR instead of READER. Contrast with truncation.
command line: The line at the bottom of display panels that lets a user enter commands or panel selections. It is prefixed by an arrow (====>).
command privilege class: See privilege class.
command scan: In CMS, a routine that scans the command line entered and converts it to a standard CMS parameter list. See parameter list (PLIST).
commit: (1) In the context of SFS, to change a resource (such as a file) permanently. (2) In the context of CRR, to make permanent changes to protected resources (such as SFS file pools) during a transaction (CRR logical unit of work). CRR commits changes made by an application program or transaction program.
commit level: In CMS Pipelines, an indication of a stage's progress. The commit level determines when a stage is dispatched relative to other stages and is used to report to other stages that an error has been detected.
common dump receiver: One user ID in a virtual machine group appointed to receive other group members' storage dumps. Unless the user specifies otherwise, all dumped information automatically goes to this user ID (identified with the GCS GROUP EXEC). It should be an authorized user ID to receive fetch-protected data and storage with a key other than 14.
common lock: A doubleword in storage, controlled by the GCS LOCKWD macro. When a program is using common storage, it can turn the common lock ON. Other programs that examine the lock and find it ON cannot gain access to common storage.
Common Programming Interface (CPI): Provides languages, commands, and calls that allow the development of applications that are more easily integrated and moved across environments supported by SAA. VM/ESA supports the communications and the resource recovery elements of the CPI.
Common Programming Interface (CPI) Communications: A set of program-to-program communication routines that let applications written in REXX and high-level languages access APPC/VM functions. These routines are part of IBM's SAA and are also known as the SAA communications interface.
Common Programming Interface (CPI) Resource Recovery: See SAA Resource Recovery Interface.
common storage: A shared segment of reentrant code that contains free storage space, the GCS supervisor, control blocks, and data that all members of a virtual machine group share.
communication adapter: A program or a part of a program that handles communications with other programs. See SFS communication adapter.
communication link: Synonym for data link.
communication services (CS) collection: A group of one or more domains made up of VM systems or LAN-based workstations.
communications directory: A CMS facility that lets APPC/VM applications connect to a resource using symbolic destination names and special NAMES files.
communications partner: The virtual machine on the other end of the local APPC/VM path, not necessarily the target of the communications.
communications scanner: A communications controller hardware unit that provides the connection between lines and the central control unit. The communications scanner monitors telecommunication lines and data links for service requests.
communications server: A virtual machine that provides APPC/VM services between systems within a TSAF collection, and allows for communication between an APPC/VM environment and an SNA-defined network. TSAF and AVS are communication servers. See intermediate communications server.
compare states: The function of conveying the state of the CRR logical unit of work to a participating resource manager, and conveying the state of the protected resource's logical unit of work, including heuristic actions, back to the CRR recovery server.
compile: To translate a program written in a high-level programming language into a machine language program.
component: A collection of objects that together form a separate functional unit. A product may contain many components. For example, CP, CMS, and TSAF are components of VM/ESA.
component override: Synonym for component parameter override.
component override area: An area of the product parameter file or of a product parameter override file that contains one or more component parameter overrides. Synonymous with override area.
component parameter override: A component parameter, defined in a component override area, that updates or replaces a component parameter defined in a component area of the product parameter file. Synonymous with component override and override.
concurrently: Concerning a mode of operation that includes doing work on two or more activities within a given (short) interval of time.
condition code (CC): A code that reflects the result of a previous I/O, arithmetic, or logical operation.
condition variable: In an application using CMS multitasking services, a variable whose value indicates a state of a mutex-protected shared resource. This state, or condition, is defined by the application. It can be waited on and signaled as being true.
connect: Establishing a path to communicate with another virtual machine or with the user's own virtual machine.
connected: In CMS Pipelines, pertaining to a stream that is attached to a stream from another stage, so that data can flow from one stage to another. If streams are not connected, then data cannot flow from one stage to another.
connectivity program request block (CPRB): An interface control block that requesters and servers use to communicate information.
connector: In CMS Pipelines, a symbol that connects the input or output stream of a stage to the beginning or end of a pipeline specified on a CALLPIPE or ADDPIPE pipeline subcommand called by the stage. The minimal connector is an asterisk and a colon (*:).
console: A device used for communications between the operator or maintenance engineer and the computer.
console communication service (CCS): A group of CP modules that interfaces with the VTAM service machine, providing full VM/ESA console capabilities for SNA terminal users.
console function: The subset of CP commands that lets the user simulate almost all of the functions available to an operator at a real system console.
console spooling: Synonym for virtual console spooling.
console stack: Refers collectively to the program stack and the terminal input buffer.
contention: The situation where two LUs try to allocate a conversation over the same session at the same time.
context editing: A method of editing a file without using line numbers. To refer to or change a particular line or a group of lines; all or part of the contents of that line are specified.
Continuously Executing Transfer Interface (CETI): An interface that uses continuously executing channel programs to transfer messages between two systems, or between an application and a control unit.
control block: A storage area that a computer program uses to hold control information.
control data: In a file pool, the data that controls the DASD space and objects within a file pool. Control data consists of the POOLDEF file, the control minidisk, and all minidisks allocated to storage group 1.
control file: (1) In service, a file with file type CNTRL that contains records that identify the updates to be applied and the macro libraries, if any, needed to assemble that source program. (2) A CMS file that is interpreted and directs the flow of a certain process through specific steps. For example, the control file could contain installation steps, default addresses, and PTF prerequisite lists and many other necessary items.
control minidisk: In a file pool, the minidisk that tracks the physical DASD blocks allocated to the file pool.
control processing complex (CPC): One or more processors, main storage, or channels, where the processors can be configured to share main storage.
control program: A computer program that schedules and supervises the program execution in a computer system. See Control Program (CP).
Control Program (CP): A component of VM/ESA that manages the resources of a single computer so multiple computing systems appear to exist. Each of these apparent systems, or virtual machines, is the functional equivalent of an IBM System/370, 370-XA, or ESA computer. Also, XC virtual machines provide functions beyond the ESA architecture. See also virtual machine.
control register: A register used for operating system control of relocation, priority interruption, program event recording, error recovery, and masking operations.
control section (CSECT): The part of a program specified by the programmer to be a relocatable unit, all elements of which are loaded into adjoining main storage.
control statement: A statement that controls or affects program execution in a data processing system.
control terminal: Any terminal currently controlling system resources and used by the primary system operator (privilege class A), the resource operator (privilege class B) or the spooling operator (privilege class D).
control unit: A device that controls I/O operations at one or more devices.
control unit terminal (CUT): An operational mode that allows one logical terminal session. Contrast with distributed function terminal (DFT).
conversation: A connection between two transaction programs over an LU-LU session that lets them communicate with each other while processing some transaction. The programs establish a conversation, send and receive data in the conversation, and then terminate the conversation.
conversation characteristics: The attributes of a conversation that determine the functions and capabilities of programs within the conversation.
conversation correlator: A value that identifies an APPC conversation and is unique at the LU that generates it. The conversation correlator is established when the APPC conversation is established.
conversation management routine (CMR): A routine that resides in the GCS load library and controls the routing of inbound connections through AVS to available service pool virtual machines.
conversation partner: One of the two programs involved in a conversation.
conversation state: See program state.
Conversational Monitor System (CMS): A virtual machine operating system and component of VM/ESA that provides general interactive time sharing, problem solving, program development capabilities, and operates only under the control of the VM Control Program (CP).
Coordinated Resource Recovery (CRR): A CMS facility that implements the LU 6.2 sync point architecture, which ensures that transactions can update multiple protected resources with integrity. This means that all updates, within the transaction, are either completed (committed) or not completed (rolled back or backed out). CRR consists of the coordination function (see synchronization point processing), the resynchronization function (see resynchronization), and the logging function (see log minidisks). The coordination function resides in the application program's virtual machine. The resynchronization and logging functions reside in the CRR recovery server.
coordinated transaction: See logical unit of work (in terms of CRR) or see LUWID.
coordinated update: See logical unit of work (in terms of CRR) or see LUWID.
copy file: A file having file type COPY that contains nonexecutable real storage definitions that are referred to by macros and assemble files.
copy function: The function initiated by a PF key to copy the contents of a display screen onto an associated hardcopy printer. A remote display terminal copies the entire contents of the screen onto a printer attached to the same control unit. A local display terminal copies all information from the screen, except the screen status information, onto any printer attached to any local display control unit.
COR: Corrective service tape.
corequisite change: A change that must be applied to the user's product along with another change. For example, if the user needs to apply change1 to the system and change1 has a corequisite of change2, then the user must apply both change1 and change2 to the system, but not in a specific order. A corequisite change corrects a problem that requires changes to one or more elements of a product or component.
corrective service: Service that IBM supplies on tape to correct a specific problem.
corrective service tape (COR): A tape, supplied by IBM at the user's request, containing a fix for a specific problem and any requisites for the fix.
count-key-data (CKD) device: A DASD that stores data in the format: count field, usually followed by a key field, followed by the actual data of a record. The count field contains the cylinder number, head number, record number, and the length of the data. The key field contains the record's key (search argument).
CP: Control Program.
CP assist: A hardware function, available only on a processor with ECPS, that reduces CP overhead by doing the most frequently used tasks of CP routines.
CP command: A command available to all VM users. Class G CP commands let the general user reconfigure their virtual machine, control devices attached to their virtual machine, do input and output spooling functions, and simulate many other functions of a real computer console. Other CP commands let system operators, system programmers, system analysts, and service representatives manage the resources of the system.
CP directory: Synonym for VM directory.
CP privilege class: See privilege class.
CP read: The condition when CP is waiting for a response or request for work from the user. On a typewriter terminal, the keyboard is unlocked; on a display terminal, the screen status area indicates CP READ.
CP READ screen status: For a display terminal used as a virtual console under VM/ESA, an indicator located in the lower right of the screen, that indicates CP is waiting for a response or request for work from the user.
CP system disks: Any disk space CP uses, other than the CP paging, spooling, and temporary disk space; for example, virtual disks.
CP trace table: A table VM/ESA uses for debugging. Its size is a multiple of 4096 bytes and depends on the size of real storage or a user-specified value. This table contains the chronological occurrences of events that take place in the real machine, recorded in a wraparound fashion within the trace table. Synonymous with trace table.
CP-accessed disk: Any CMS-formatted minidisk that is available to CP after installation.
CP-owned disk: Any disk formatted by the CP Format/Allocate program and designated as system-owned during system generation; for example, the CP system residence volume, or any disk that contains CP paging, spooling, saved systems, or temporary disk space.
CPC: Control processing complex.
CPI Communications: Common Programming Interface Communications.
CPI Resource Recovery: Common Programming Interface Resource Recovery.
CPRB: Connectivity program request block.
CPU timer: A hardware feature that measures elapsed processor time and causes an interruption when a previously specified amount of time has elapsed. The CPU timer is decremented when the processor is executing instructions, is in a WAIT state, and is executing program loading instructions, but not when the processor is in a stopped state. A virtual machine that uses the CPU timer must have the EC mode and REALTIMER options active.
critical section: In CMS Multitasking, a block of code that manipulates a shared resource, such as a data structure or device.
cross system extensions (CSE): An environment in which end users attached to a single system can participate with additional systems as though all participating systems were one complex.
cross system extensions exclusion list: A list of user IDs excluded from the cross system extensions environment.
cross system extensions protected volume: A cross system extensions (CSE) volume not participating in the CSE environment.
CRR: Coordinated Resource Recovery.
CRR recovery server: A virtual machine, which is generated during VM/ESA installation, that provides the CRR logging and resynchronization functions. Every processor must have only one CRR recovery server.
CRT: Cathode ray tube.
Crypto Facility (CF): A hardware feature that provides cryptographic capability.
cryptography: A method of protecting data in computer and communication systems from unauthorized disclosure and modification. It involves the process of transforming plain-text into cipher-text (encipherment) and the reverse process of turning cipher-text into plain-text (decipherment).
CS collection: See Communication Services collection.
CSE: Cross system extensions.
CSECT: Control section.
CSL: Callable services library.
CSL routine: An assembler program that resides in a CSL and that can be invoked from a high-level language, REXX, or an assembler program to do a specific function.
CSR: (IBM) Customer Service Representative.
CSW: Channel status word.
CTC: Channel-to-channel.
CTCA: Channel-to-channel adapter.
current line pointer (CLP): A pointer that indicates the line of a CMS file on which the CMS Editor or the XEDIT editor is currently working.
CUT: Control unit terminal.
CVT: Communications vector table.
cylinder: In a disk pack, the set of all tracks with the same nominal distance from the axis about which the disk pack rotates.
C2: One of several security designations that can be assigned to a computing system by the U. S. Department of Defense. The C2 designation indicates that the system:

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DAC: discretionary access control.
DASD: Direct access storage device.
DASD Dump Restore (DDR) program: A service program that copies all or part of a minidisk onto tape, loads the contents of a tape onto a minidisk, or sends data from a DASD or from tape to the virtual printer.
DASD space: (1) Area allocated to DASD units on CKD devices. (2) Area allocated to DASD units on FB-512 devices. Note that DASD space is synonymous with cylinder when there is no need to differentiate between CKD devices and FB-512 devices.
DAT: Dynamic address translation.
data control block (DCB): A control block access method routines use to store and retrieve data.
data link: The equipment and rules (protocols) used for sending and receiving data. Synonymous with communication link.
data space: Informal term for an additional address space created by CP at the request of a program running in an XC virtual machine. This term is usually used to refer to these additional address spaces because they can contain data (operands of instructions) but instructions cannot be executed from within them.
data stream: A set of logical records sent one after the other.
DBCS: Double-byte character set.
DCB: Data control block.
DCF: Document Composition Facility.
DCSS: Discontiguous saved segment.
DDP: Distributed data processing.
DDP logical operator: (1) The person who handles nonroutine (nonprogrammed) messages sent from the local systems' programmable operator. (2) The person who is logically responsible for DDP system operations. See distributed data processing (DDP).
DDP operator: (1) The person who handles nonroutine (nonprogrammed) messages sent from distributed systems' programmable operators. (2) The person who is logically responsible for the DDP network. See distributed data processing (DDP).
DDR program: DASD Dump Restore program.
deadline priority: A value that sorts a virtual machine into the eligible lists and the run lists. This value determines when users (virtual machines) get their next time slice. This value is calculated each time a user is dropped from a queue and is based on paging activity, processor usage, and the load on the system as well as on user priority.
dedicated channel: A channel attached to a virtual machine, for its sole use, so that CP can bypass translating the addresses of virtual devices.
dedicated device: An I/O device or line not being shared among users. The facility can be permanently assigned to a particular virtual machine by a VM/ESA directory entry, or temporarily attached by the resource operator to the user's virtual machine.
dedicated maintenance mode: In a file pool server machine, a mode of file pool server processing during which the file pool server machine has exclusive use of the file pool. The file pool is unavailable to other users. Contrast with multiple user mode.
default file attributes: In CMS, certain reserved file type names that cause the CMS editors (EDIT and XEDIT) to assume certain values for record length, tab setting, uppercase or lowercase setting, record sequence numbering, verification mode, fixed- or variable-length record type, truncation column, and other related record characteristics.
default operand: An operand that has a preset value if a value is not specified on the CP or CMS command line.
delimited string: In CMS Pipelines, a character string beginning and ending with a delimiter. See also null string.
delimiter: (1) A flag that separates and organizes items of data. Synonymous with separator. (2) A character used to indicate the beginning and end of a character string. (T) (3) A character that groups or separates words or values in a line of input. (4) In CMS Pipelines, a character used before and after a string of characters to define a delimited string. The delimiter cannot be blank and it must not occur within the string. Two adjacent delimiters represent a null string.
delta disk: In VMSES/E, a minidisk or SFS directory containing control files used for servicing. The control files can include: apply lists, exclude lists, PTF parts lists, auxiliary control files, update files, and text files.
delta string: In VMSES/E, the set of delta disks.
dependent PTF: A PTF that has another PTF as a prerequisite or corequisite.
derived file: A CMS file created from selected versions of elements. The build process creates the derived file. Some derived files can be provided by IBM on the PUT tapes. The customer installation can also create them.
description table: The software inventory table that contains the descriptive name for a product, in the system level, and APARs in the service level. The file type of the system level inventory table is SYSDESCT and the file type of the service level inventory table is SRVDESCT.
destage: The asynchronous writing of new or updated data from cache or nonvolatile storage to DASD. This method of writing data is used only for the fast-write function of the 3990 Model 3 Storage Control.
device: (1) A mechanical, electrical, or electronic contrivance with a specific purpose. (2) See channel-to-channel (CTC) device, count-key-data (CKD) device, dedicated device, device block, device driver, device support facilities, direct access storage device (DASD), display device, draining a unit record device, fixed-block architecture (FBA) device, primary paging device, spooling devices, unit record device, virtual spooling device.
device block: A storage area used by a computer program to hold device information.
device driver: In CMS Pipelines, a stage command that reads data from or writes data to I/O and storage devices, host environments (including CP, CMS, and XEDIT), and REXX and EXEC 2 variables. Contrast with filter.
device support facilities: A program for doing operations on disk volumes so that they can be accessed by IBM and user programs. Examples of these operations are initializing a disk volume and assigning an alternate track.
DFT: Distributed function terminal.
DIAGNOSE interface: A programming mechanism that lets any virtual machine, including CMS, directly communicate with CP by way of the DIAGNOSE instruction. Specific interface codes let a virtual machine more efficiently request specific CP services.
DIRCONTROL directory: Synonym for directory control directory.
direct access storage device (DASD): A storage device in which the access time is effectively independent of the location of the data.
directory: See auxiliary directory, CMS minidisk file directory, directory control directory, file control directory, SFS directory, or VM directory.
directory control directory: A type of SFS directory with functional characteristics like a minidisk's. A single access authority applies to the directory and all the files in the directory. When you access a directory control directory in read-only mode, you cannot see changes made until you release and reaccess the directory. When you access the directory in read/write mode, changes become available as they are made. Synonymous with DIRCONTROL directory. Contrast with file control directory.
directory identifier (dirid): A fully qualified directory name (in which the file pool ID and user ID can be allowed to default), a file mode letter, or plus (+) or minus (-) file mode syntax (used in commands).
directory name (dirname): A fully qualified directory name that can incorporate a period (.) to indicate the user's own top directory (used in commands).
dirid: Directory identifier.
dirname: Directory name.
disable: Synonym for lock, as applied to file spaces and storage groups in the CMS Shared File System.
disconnect mode: The mode of operation in which a virtual machine is executing without a physical line or terminal connected as an operator console. Any attempt to issue a read to the console causes the virtual machine to be logged off after 15 minutes have elapsed, unless the user logs on again within the 15-minute interval. Note that with the SCIF, a user can be disconnected from a primary virtual console but still have console communications through the console of the secondary user.
discontiguous saved segment (DCSS): A saved segment that begins and ends on a megabyte boundary and is not a segment space or a member of a segment space. A DCSS may contain logical saved segments. Contrast with segment space and member saved segment.
discretionary access control (DAC): A method of restricting access to data objects based upon authorizations granted to users or groups of users. Discretionary access control protects all system objects from unauthorized access. Normally, authorization to access an object is granted by its owner. Occasionally, it can be granted by someone else, such as a privileged administrator.
disk: A magnetic disk unit in the user's CMS virtual machine configuration. See virtual disk.
disk operating system (DOS): An operating system for computer systems that use disks and diskettes for auxiliary storage of programs and data.
Disk Operating System/Virtual Storage Extended (DOS/VSE): An operating system that is an extension of DOS/VS. A VSE system consists of: (a) licensed VSE/Advanced Functions support, and (b) any IBM-supplied and user-written programs required to meet the data processing needs of a user. VSE and the hardware it controls form a complete computing system.
dispatch: To allocate time on a processor to jobs or tasks that are ready for execution. (I) (A) See dispatch list, dispatch request queue, dispatch/scheduler favoring scheme.
dispatch list: See run list.
dispatch request queue: A queue of executable CP tasks, I/O tasks, and timer requests ready to be dispatched.
dispatch/scheduler favoring scheme: A set of criteria that the dispatcher and scheduler use to create a bias in favor of queue 1 (Q1) users. Q1 users are usually highly interactive users.
dispatcher: The program in CP that places virtual machines or CP tasks into execution. The dispatcher selects the next virtual machine to run and prepares the virtual machine for problem state execution.
dispatching: (1) In VM/ESA, the starting of virtual machine execution. (2) In CMS Multi-tasking, giving each thread a turn at running on a processor. See dispatching class.
dispatching class: A set of threads with two properties:
display device: An I/O device that gives a visual representation of data.
display mode: A type of editing at a display terminal in which an entire screen of data is displayed at once and in which the user can access data through commands or by using a cursor. Contrast with line mode.
display paging: A technique used for scanning through a CMS file on a display terminal in display mode. When using the CMS Editor, scanning can include up to 20 lines at a time. When using the XEDIT editor, use the BACKWARD or FORWARD subcommands; if a number is not given with either subcommand, only one screen is shown.
display terminal: A terminal with a component that can display information on a viewing surface such as a screen or gas panel.
distributed data processing (DDP): Data processing in which processing, storage, and control functions, in addition to I/O operations, are distributed among remote locations and connected by transmission facilities.
distributed function terminal (DFT): An operational mode that allows multiple concurrent logical terminal sessions. Contrast with control unit terminal (CUT).
distributed logical operator: A virtual machine at the distributed system to which machine intervention requests are sent. A machine attendant mounts forms and tapes and turns the power ON or OFF for that site.
distributed processing: A transaction that involves more than one node to complete its processing. A node might be a VM/ESA virtual machine in the same processor or a different processor or it may be an LU partner in a non-VM system (for example, MVS). We can think of the use of SFS as distributed processing (a user machine and a server machine cooperating) and the file pool as a distributed resource.
distributed resources: See distributed processing.
distribution code: In the VM/ESA directory, a one- to eight-character identification word printed or punched with the user ID in the separator page (or punched card) to further identify the location or department of the user.
DMSPARMS file: A CMS file with a file type of DMSPARMS that contains the start-up parameters that SFS file pool server and CRR recovery server processing uses.
Document Composition Facility (DCF): A text processing program; its main component is the text formatter, called SCRIPT/VS. See SCRIPT/VS.
domain: A set of transaction programs and servers that allocate shared resources within a single logical system.
domain controller: A communications server that manages communications within a domain and between domains. See AIX domain controller, AIX PS/2 domain controller, OS/2 domain controler, VM/ESA domain controller.
domination rule: The rule defining when one security label dominates another:
dormant state: A state in which the active pages of a virtual machine have been paged out.
DOS: Disk operating system.
DOS simulation under CMS: Synonym for CMS/DOS.
DOS/VSE: Disk Operating System/Virtual Storage Extended.
DOSLIB library: A CMS library that contains the executable phases produced by the DOS Linkage Editor under CMS. These phases are equivalent to, but not usable in the DOS/VS core image library. These phases can be fetched and executed only under CMS/DOS. Synonymous with CMS/DOS phase library.
double key protection: This program support associates each 2KB block of real storage with two storage protect keys. Two keys are needed to protect a 4KB frame.
double-byte character set (DBCS): A character set that requires 2 bytes to uniquely define each character. This contrasts with EBCDIC, in which each printed character is represented by 1 byte.
double-density DASD volume: A DASD volume with twice the standard number of cylinders for its device type, for example, a 3380-E4 with 1770 cylinders, or a 3390-2 with 2226 cylinders. See single-density DASD volume and triple-density DASD volume.
DPA: Dynamic paging area.
drain: (1) For spooling devices: to bring spooling operations on a real device to a halt after the current spool file is processed. (2) For DASD: to bring input/output operations on a DASD to a halt after the present input/output operations are completed.
dual address space assist: Synonym for cross memory service.
dump: To write the contents of part or all of main storage, or part or all of a minidisk, to auxiliary storage or a printer. See abend dump.
Dump Viewing Facility: A component of VM/ESA that lets users display, format, and print data interactively from CP hard and soft abend, stand-alone, and virtual machine dumps; process CP trace table data stored on tape or in a system trace file; and display symptom records.
duplex pair: A logical volume comprised of two physical devices with all data recorded twice, once on each device. A 3990 Model 3 Storage Control automatically ensures that both devices are updated with each write operation to the dual copy volume.
dyadic processor: A processor complex containing two processors in one unit. Both processors share central storage, are controlled by a single operating system, directly communicate with each other, execute I/O operations through a common element, and can run with one central processor if the other is removed from the configuration because of an error. A dyadic processor cannot be configured into two independent uniprocessor units. Note that each processor has access to its own assigned channel set of up to 16 channels.
dynamic address translation (DAT): In System/370 and ESA virtual storage systems, the change of a virtual storage address to a real storage address during execution of an instruction.
dynamic configuration: Configuring a collection or reconfiguring a collection when a system enters or leaves the collection after a link goes up or down within the collection. See CS collection and TSAF collection.
dynamic paging area (DPA): An area of real storage that CP uses for virtual machine pages and pageable CP modules.
dynamically modified channel program: A channel program changed by the program or by data being read in from a channel during the interval between the execution of the START I/O (SIO) instruction and the channel end interruption.

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early warning system (EWS): An information system for getting information about programming problems to the field. The information includes:
EBCDIC: Extended binary-coded decimal interchange code.
EBT: Element build table.
EC level: Engineering change level.
EC mode: Extended control mode.
ECB: Event control block.
ECC: Error checking and correction.
ECF: Enhanced connectivity facilities.
ECKD: Extended count-key data.
ECPS:VM/370: Extended Control Program Support:VM/370.
ECSW: Extended channel status word.
EDF: Enhanced disk format.
edit: A function that makes changes, additions, or deletions to a file on a disk. These changes are interactively made. The edit function also generates information in a file that did not previously exist.
EDIT macro: (1) A procedure defined by a frequently used command sequence to do a commonly required editing function. A user creates the macro to save repetitious reentering of the sequence, and invokes the entire procedure by entering a command (that is, the macro file's file name). The procedure can consist of a long sequence of edit, CMS, and CP commands, along with EXEC 2 or CMS EXEC control statements to control processing within the procedure. (2) A CMS file whose file name starts with a dollar sign ($) character and whose file type is EXEC.
edit mode: The environment in which CMS EDIT subcommands and XEDIT subcommands can be entered by the user to insert, change, delete, or rearrange the contents of a CMS file. Contrast with input mode.
eligible list: The list of virtual machines waiting to get into the run list. They are runnable but cannot fit into the run list because of the current system load.
ELN: Exchange log names.
emulation: The use of programming techniques and special machine features to permit a computing system to execute programs written for another system.
emulation program (EP): A control program that lets an IBM 3704 or 3705 Communications Controller emulate the functions of an IBM 2701 Data Adapter Unit, an IBM 2702 Transmission Control Unit, or an IBM 2703 Transmission Control Unit.
enable: Deprecated term for removing a lock from an object in the CMS Shared File System, in particular from a file space or storage group.
end of file: In CMS Pipelines, a condition in which no data can flow through a stream because the stream has been severed.
enhanced connectivity facilities (ECF): A set of programs for interconnecting IBM Personal Computers and IBM System/370 host computers running the MVS/XA, VM/SP, VM/SP HPO, VM/XA SP, and VM/ESA operating systems.
enhanced disk format (EDF): A CMS file storage format that supports files consisting of 512-, 1K-, 2K-, or 4K-byte CMS blocks.
entry point: An address or label of an instruction performed on entering a computer program, a routine, or a subroutine. A program can have several different entry points, each corresponding to a different function or purpose.
environmental record editing and printing program (EREP): A program that makes the data contained in the system recorder file available for further analysis.
EOF: End of file.
EOV: End of volume. The logical end of a physical tape volume.
EP: Emulation program.
erased alias: An alias that no longer points to a base file because the base file was erased.
ERDS: Error recording data set.
EREP: Environmental record editing and printing program.
EREP file: A collection of error records produced by the malfunctioning of hardware components on a computer and stored for processing by the EREP program.
error checking and correction (ECC): In a processing unit, the detection and correction of all single-bit errors, plus the detection of double-bit and some multiple-bit errors.
error recording area: The DASD space that the system programmer defines during system generation on the system residence volume that CP uses to record formatted outboard error recordings, machine check records, and channel check records. For CKD devices, this area is between 2 and 9 contiguous cylinders in size; for FB-512 devices, the size of this area can be any number of contiguous pages.
error recording data sets (ERDS): A central processor repository for hardware error records.
ESA virtual machine: A virtual machine that simulates ESA/370 or ESA/390 functions. Contrast with 370 virtual machine, XA virtual machine, and XC virtual machine
ESA/XC: Enterprise Systems Architecture/Extended Configuration.
ESA/XC architecture: A virtual machine architecture in which DAT-off programs, like CMS and CMS applications, can create and access additional address spaces called data spaces. These additional address spaces can also be shared with programs running in other virtual machines. The ESA/XC virtual machine architecture is available only when VM/ESA is running on an ESA/390 processor.
escape symbol: Synonym for logical escape symbol.
escape to CP: Under VM/ESA, a transfer of control to CP when either the terminal user or the machine stops virtual machine operation. This can occur by entering a CP command (such as #CP), by invoking a DIAGNOSE function, or by signaling attention. See DIAGNOSE interface, signaling attention, and attention interrupt.
ESCM: ESCON Manager.
ESCON: Enterprise systems connection.
ESCON architecture: An architecture that uses fiber optic channels to interconnect computer systems.
ESCON directors: Dynamic switches that connect multiple control units and channels to implement the ESCON architecture. These switches are integral for the attachment of devices to both single and multiple systems.
ESCON Manager (ESCM): A licensed program that provides host control to help manage connections that use ESCON Directors.
Ethernet: See IEEE 802.3.
event: (1) An occurrence of significance to a task; for example, the completion of an asynchronous operation, such as an input/output operation. (2) In CMS Multitasking, an occurrence defined to be an event by CMS or an application. Such defined events can be monitored and signaled by CMS and by processes using CMS multitasking services. See event control block (ECB), event name, event signal, system event.
event control block (ECB): A control block that represents the status of an event.
event name: In applications using CMS multitasking services, a character string of arbitrary length and composition that is the primary identifier of an event.
event signal: In applications using CMS multitasking services, a signal that originates with CMS or an application to indicate that an event has occurred. The signal consists of the event name and additional data to be matched against keys specified by processes that want to monitor the event.
EWS: Early warning system.
exchange log names: An exchange of log data (log names, LU names, and TPN) for validating levels of log data between the CRR recovery server and participating resource managers, or between the CRR recovery server on one processor and another CRR recovery server on another processor.
exclude list: A file listing PTFs to be omitted from a product or component.
exclusive access: In addition to the data on the disk remaining unchanged by others, no one else has or can access the specified minidisk until the person holding the stable read or write releases access.
exclusive segment: A range of pages within a saved system, member saved segment, or discontiguous saved segment that can be accessed by only one virtual machine at a time.
EXEC procedure: (1) A procedure defined by a frequently used sequence of CMS and CP commands to do a commonly required function. A user creates the procedure to save repetitious reentering of the sequence, and invokes the entire procedure by entering a command (that is, the exec file's file name). The procedure could consist of a long sequence of CMS and CP commands, along with REXX, EXEC 2, or CMS EXEC control statements to control processing within the procedure. (2) A CMS file with a file type of EXEC.
EXEC 2 EXEC: Synonym for EXEC 2 program.
EXEC 2 language: A general-purpose, high-level programming language, particularly suitable for EXEC procedures and XEDIT macros. The EXEC 2 processor runs procedures and XEDIT macros (programs) written in this language. Contrast with CMS EXEC language and Restructured Extended Executor (REXX) language.
EXEC 2 processor: A program in VM/ESA that interprets and executes procedures, EDIT macros, and XEDIT macros written in the EXEC 2 language.
EXEC 2 program: An EXEC procedure, EDIT macro, or XEDIT macro written in the EXEC 2 language and processed by the EXEC 2 processor. Synonymous with EXEC 2 EXEC.
executable phase library: See DOSLIB library.
executable statement: In an EXEC procedure, EDIT macro, or XEDIT macro, any statement that the REXX Interpreter, the EXEC 2 processor, or the CMS EXEC processor processes. This can be a control statement or assignment, a CMS or CP command, or a null line.
exit: See user exit and installation-wide exit.
expanded storage: Optional integrated high-speed storage. Expanded Storage can be shared by CP and one or more virtual machines. It can also be dedicated to CP or to a particular virtual machine.
expanded virtual machine assist: A hardware assist function, available only on a processor that has ECPS, that handles many privileged instructions not handled by VMA, and extends the level of support of certain privileged instructions beyond that provided by VMA.
explicit lock: A lock on a file or directory that a user explicitly created by entering a CREATE LOCK command or executing a DMSCRLOC CSL routine.
extended binary-coded decimal interchange code (EBCDIC): A set of 256 characters, with each character represented by 8 bits.
extended communications: A set of program-to-program communication routines that are an extension to CPI-Communications routines. These routines are not part of IBM's Systems Application Architecture.
extended control (EC) mode: A mode in which all features of a System/370 computing system, including dynamic address translation, are operational. Contrast with basic control (BC) mode.
Extended Control Program Support (ECPS:VM/370): A hardware assist feature that improves the performance of CP by reducing CP overhead. ECPS:VM/370 consists of CP assist, expanded virtual machine assist, and virtual interval timer assist.
extended count-key data (ECKD) architecture: A storage architecture specifically designed to support nonsynchronous DASD control units.
extended PLIST (untokenized parameter list): Four addresses that indicate the extended form of a command as it was entered at a terminal.
external object: In SFS, an object within a file pool that an SFS server can refer to, but does not control. An external object contains a remote name of an entity residing outside the file pool. The external entity may be a file, but it can be something else.
external page storage: The portion of auxiliary storage that contains 4K-byte pages.
external page storage management: A set of routines in the paging supervisor that control the transfer of data between real storage and external page storage.
external security manager: A program that either augments or completely replaces the authorization checking done by file pool server processing.

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favored execution performance option: A virtual machine performance option that lets an installation force the system to allocate more of its resource to a given virtual machine than would usually be the case.
FB-512: An FBA device that stores data in 512-byte blocks (refers to DASD devices such as the IBM 9335, 9332, 9313, 3370, and 3310).
FBA: Fixed-block architecture.
FCB: (1) Forms control buffer. (2) Function control block. (3) File control block.
feature: A part of an IBM product that can be ordered separately by the customer.
fence: A logical boundary that separates one or more paths or elements from the remainder of the logical DASD subsystem. This separation allows isolation of failing components so that they do not affect normal operations.
fetch protection: A storage protection feature that determines right-of-access to main storage by matching the protection key associated with a main storage fetch reference with the storage keys associated with those frames of main storage.
field-developed program: An IBM licensed program that does a function for the user. It can interact with IBM licensed programs, or it can be a stand-alone program.
FIFO (first-in-first-out): A queuing technique in which the next item to be retrieved is the item that has been on the queue for the longest time. Contrast with LIFO (last-in-first-out).
file access mode: A file mode number that designates whether the file can be used as a read-only or read/write file by a user. See also file mode.
file control block: A control block used by OS Simulation to contain information about an I/O device or file, such as: name, device type, format, and I/O control data.
file control directory: A type of SFS directory for which separate access authorities are granted to the directory and to the individual files in the directory. When you access a file control directory, changes to the directory become available as they are made. Synonymous with FILECONTROL directory. Contrast with directory control directory.
file definition: (1) Equating a CMS file identifier (file name, file type, file mode) with an OS data set name by the FILEDEF command; or equating a DOS file ID with a CMS file identifier by the DLBL command. (2) Identifying--by way of either the FILEDEF or DLBL commands--the input or output files used during execution of an OS or DOS simulation program.
file ID: A CMS file identifier that consists of a file name, file type, file mode, or directory ID. The file ID is associated with a particular file when the file is created, defined, or renamed under CMS. See file name, file type, and file mode.
file mode: (1) A two-character CMS file identifier containing the file mode letter (A through Z) followed by the file mode number (0 through 6). The file mode letter indicates the minidisk or SFS directory on which the file resides. The file mode number indicates the access mode of the file.
file name: A one-to-eight character alphanumeric field, containing A through Z, 0 through 9, and special characters $ # @ + - (hyphen) : (colon) _ (underscore), that is part of the CMS file identifier and serves to identify the file for the user.
file pool: A collection of minidisks managed by SFS. It contains user files and directories and associated control information. Many users' files and directories can be contained in a single file pool.
file pool administration machine: A virtual machine with file pool administration authority.
file pool catalog: The part of a file pool that contains information about the objects stored in the file pool and the authorizations granted on those objects. See catalog storage group.
file pool ID: The name of a file pool. It is part of a fully-qualified directory name, identifying where the directory and all files in it are located. It has up to eight characters, followed by a colon (:).
file pool server machine: A virtual machine that is properly configured to manage a file pool. (Its VM/ESA system directory entries must, for example, contain the MDISK statements for a file pool.)
file space: A user's allocation of space within a file pool.
file status table (FST): A table that describes the attributes of a file on a CMS disk, including file name, file type, file mode, date last written, and other status information.
file system command: A CMS command that creates or manipulates CMS files.
file system control block (FSCB): A collection of information about a CMS file, which CMS OS simulation commands and user programs use. A file system control block is established for a file by the FILEDEF command or the FSCB macro instruction.
file tag: A character string associated with a VM/ESA spool file that contains information about that spool file.
file type: A one-to-eight character alphanumeric field, containing A through Z, 0 through 9, and special characters $ # @ + - (hyphen) : (colon) _ (underscore), that is used as a descriptor or as a qualifier of the file name field in the CMS file identifier. See reserved file types.
file type abbreviation table: The software inventory table that identifies the mapping between PTF-numbered file types and the real CMS file type. The service level inventory does not contain this table.
FILECONTROL directory: Synonym for file control directory.
filter: In CMS Pipelines, a stage command that transforms data but does not move data into or out of the pipeline. Contrast with device driver. See filter package.
filter package: In CMS Pipelines, a MODULE file that contains stage commands.
first-level storage: Refers to real main storage. Contrast with second-level storage and third-level storage.
fixed-block architecture (FBA) device: A disk storage device that stores data in blocks of fixed size or records; these blocks are addressed by block number relative to the beginning of the particular file.
flat file: A file that consists of a set of records ordered by record number or as sequentially entered in the file; a two dimensional file.
flip-flop: Synonym for alternating operating system.
floating-point register: A register that can manipulate data in a floating-point representation system.
flush list: A set of pages available to replenish the free list.
FMH5: Function Management Header 5.
force start: A VM/ESA system restart that attempts to recover information about closed spool files previously stored on the checkpoint cylinders. All unreadable or incorrect spool file information is ignored. Contrast with checkpoint (CKPT) start, cold start, and warm start.
format program: (1) In CMS, the service program that creates 1024-, 2048-, or 4096- byte blocks on a minidisk, counts or redefines the number of cylinders on a virtual disk, or creates a DASD label for the virtual disk. (2) In CP, the service program that formats (in 4096-byte blocks), allocates, and creates DASD labels for CP-owned disks for paging, spooling, and CP system residence. Contrast with virtual disk initialization program.
forms control buffer (FCB): In the 3800 Printing Subsystem, a buffer for controlling the vertical format of printed output. The FCB is analogous to the punched-paper, carriage-control tape that IBM 1403 Printers use.
forward recovery: The capability of a resource to recover all the data in the event of a media failure, including updates to the data that have been made since the last backup of the data.
free list: A list maintained by CP that points to a set of pages that can be allocated to satisfy both virtual machine and system page requests.
free storage: Storage not allocated. The blocks of central storage available for temporary use by programs or by the system.
FSCB: File system control block.
FST: File status table.
full recording mode: The mode of operation where transient processor and main storage errors that are corrected or circumvented by hardware retry or error correction code logic, are then recorded on the VM/ESA error recording cylinders.
full-pack minidisk: A virtual disk that contains all of the addressable cylinders of a real DASD volume.
full-screen CMS: When a user enters the command SET FULLSCREEN ON, CMS is in a window and can take advantage of 3270-type architecture and windowing support, and various classes of output are routed to a set of default windows. Also, users can type commands anywhere on the physical screen and scroll through commands and responses previously displayed. See windowing.
full-screen editor: An editor used at a display terminal where an entire screen of data is displayed at once and where the user can access the data through commands or by using a cursor. See full-screen CMS.
full-screen mode: In VM, the environment in which an entire 3270 display screen is under the control of a program running in a virtual machine.
fully-qualified LU name: A name that identifies each LU in an SNA network. It consists of a network ID followed by a network LU name. Contrast with locally-known LU name.
function control block (FCB): In Subsystem Support Services (SSS), a control block that contains information such as a function's status, event control block, task I/O queue, and I/O queue.
Function Management Header 5 (FMH5): A field at the beginning of an application request that carries control information for the target LU in an SNA network.

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GAM/SP: Graphics Access Method/System Product.
gateway: The LU name of a VM system, CS, or TSAF collection that is a source for communications to an SNA-defined network or the target of communications from an SNA-defined network.
gateway manager: A virtual machine in which one or more gateways are active. AVS is a gateway manager.
GB: gigabyte.
GCS: Group Control System for ESA/370 or ESA/390 architecture.
GDDM: Graphical data display manager.
general register: In CMS, a register that does operations such as binary addition, subtraction, multiplication, and division. General registers primarily compute and modify addresses in a program.
general user privilege class: The subset of CP commands that lets the Class G user manipulate and control a virtual machine.
generic file ID: A file identifier that allows the use of asterisks. The asterisk replaces any string of characters when the system is searching for a match on the rest of the file identifier.
gigabyte (GB): 1,073,741,824 bytes.
global gateway: (1) A gateway that programs outside a CS or TSAF collection can use to access global resources inside the collection. (2) A gateway that global resource manager programs use to access resources outside a CS or TSAF collection. Contrast with private gateway and system gateway.
global resource: A resource accessible from anywhere within a CS or TSAF collection and whose identity is known throughout the collection. A shared file system file pool is an example of a global resource. Contrast with local resource, private resource, and system resource.
global resource manager: (1) An application that runs in a server virtual machine and identifies itself to the CS or TSAF collection as a global resource owner using *IDENT. Contrast with local resource manager, private resource manager, and system resource manager. (2) An application that runs in a server workstation and identifies itself to its domain controller as a global resource owner.
global system lock: A defer lock that provides system integrity for AP and MP support of command processing and code executed by IOBLOCK, TRQBLOK, or CPEXBLOK.
GOS: Guest operating system.
GPR: General-purpose register.
graphical data display manager (GDDM): (1) A group of routines that let pictures be defined and procedurally displayed through function routines that correspond to graphic primitives. Contrast with presentation graphics routines (PGR). (2) An IBM licensed program that creates page segments.
group: Synonym for virtual machine group.
group configuration file: A file that the GROUP EXEC creates. It contains the blueprint for building the user's virtual machine group. The name of the file is systemname GROUP, where systemname is the name of the user's GCS saved system.
Group Control System (GCS): A component of VM/ESA, consisting of a shared segment that the user can IPL and run in a virtual machine. It provides simulated MVS services and unique supervisor services to help support a native SNA network.
GROUP EXEC: A GCS installation tool that prompts you for the specifications needed to build a GCS configuration file.
guest: An operating system running in a virtual machine managed by a VM control program. Contrast with host.
guest operating system (GOS): A second operating system that runs on the user's primary operating system. An example of a GOS is VSE running on VM/ESA to support VM/VCNA.
guest real storage: The storage that appears real to the operating system running in a virtual machine. Contrast with guest virtual storage, host real storage, and host virtual storage.
guest virtual machine (GVM): A virtual machine in which an operating machine is running.
guest virtual storage: The storage that appears virtual to the operating system running in a virtual machine. Contrast with guest real storage, host real storage, and host virtual storage.
GVM: Guest virtual machine.

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half-duplex protocol: A communications protocol where only one communication partner can send data at a given time.
handshaking feature: See VM/VS handshaking feature.
hard requisite: (1) In VMSES/E, a prerequisite to a PTF that supplies a change required by the PTF. (2) In VMSES/E, a prerequisite to a PTF that affects the same lines of code as the PTF, so that the PTF cannot be applied without the prerequisite. (3) See also corequisite, if-requisite.
HDLC: High-level data link control.
help function: One or more display images that describe how to use application software or how to do a system operation.
heuristic damage: Damage caused to protected resources that corrupted data integrity. In a distributed system this occurs when parts of a transaction are disconnected from each other before sync point processing is complete and commit or rollback is forced to occur at one or more locations and at least one location made a decision different from the others. Heuristic damage can only occur by intervention from the outside (see heuristic decision) generally to force completion of a process that is holding locks on needed data.
heuristic decision: When distributed protected resources become disconnected during a coordinated update, and resynchronization is unable to effect a timely resolution because one or more of the resources is not active, it may be desirable to force the logical unit of work to commit or rollback. Forcing the logical unit of work to commit or rollback can cause locked resources to be made available for further processing. The decision to force the logical unit of work to commit or rollback unilaterally is called a heuristic decision. It is typically made by the system operator but it can also be a programmed decision.
high common storage: GCS common storage that resides above the 16MB line. High common storage can be used only for free storage. See common storage.
high private storage: GCS private storage that resides above the 16MB line. See private storage.
High-level data link control (HDLC): In data communication, the use of a specified series of bits to control data links in accordance with the International Standards for HDLC.
high-water mark: The highest contiguous address, starting from location zero, where the virtual system's real addresses equal the virtual system's virtual addresses.
history files: One or more CMS files that describe the changes (with a date and time stamp) made to the VM/ESA system and its installed software products.
HOLDING screen status: For a display terminal used as a virtual console under VM/ESA, an indicator located in the lower right of the screen that displays that the current contents of the screen remain on the screen until the user requests that the screen be erased. This status occurs either by pressing Enter, or it is triggered by a message or warning displayed on the screen.
host: A VM control program in its capacity as manager of a virtual machine in which another operating system is running. Contrast with guest.
host access list: A CP-maintained list which defines the address spaces that are available to an XC virtual machine when it is in access-register mode. Host access lists are not directly addressable by virtual machines; CP adds and removes entries from the host access list for the virtual machine when requested through VM/ESA services.
host access-list entry: The information in a host access list that identifies a particular address space and represents a virtual machine's capability to access it. An access-list-entry token (ALET) selects the host access-list entry to be used to access a particular storage operand.
host real storage: The storage that appears real to the control program. If VM is running native, this is real storage; if VM is running in a virtual machine, this is virtual storage. Contrast with guest real storage, guest virtual storage, and host virtual storage.
host system: A data processing system that prepares programs and the operating environments for use by another computer or controller.
host virtual storage: The storage that appears virtual to the control program. Contrast with guest real storage, guest virtual storage, and host real storage.
host-primary address space: The initial address space for a virtual machine, created by CP at virtual machine logon. This is the address space from which instructions are fetched, and from which all operands are fetched and stored when in primary-space mode. For 370, XA and ESA virtual machines, the host-primary address space represents virtual machine real storage. For XC virtual machines, the host-primary address space is one of a set of address spaces that the virtual machine may own.
hot I/O: A serious error condition caused by an I/O interruption that disrupts system operation. See hot I/O rate.
hot I/O rate: In VM/ESA, the maximum rate of consecutive unsolicited interrupts allowed by CP.

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I/O: Input/output.
I/O adapter (IOA): A hardware and microcode element which, when combined with an integrated I/O processor, enables I/O devices to be attached to a processor without requiring a physical channel.
I/O Configuration Data Set (IOCDS): The data set that contains an I/O configuration definition built by the I/O configuration program (IOCP).
I/O Configuration Program (IOCP): A program that defines to a system all available I/O devices and channel paths.
IA: Integrated adapter.
IBM System/370 to IBM Personal Computer Enhanced Connectivity Facilities
for VM/ESA: (1) A program that provides the SRPI and a communications manager on an IBM System/370 using VM/CMS. (2) The implementation of enhanced connectivity on a VM/ESA system with CMS installed.
ID card: Under VM/ESA, the identification card that indicates the destination user ID of a deck of real cards. These cards are read into the system card reader or into the card reader of an RSCS remote station.
IDRC: Improved data recording capability.
IEEE 802: A standard that describes the formats and protocols at the medium access level for an LAN. In this standard, hardware protocol requires carrier sense multiple access with collision detection. In addition, a transmitting data station that detects another signal while transmitting stops sending, sends a jam signal, and then waits for a variable time before trying again (Ethernet).
if-requisite: (1) At the system-level, a requisite of an optional product. (2) At the service-level, an out-of-component hard requisite
image library: A set of modules that define the spacing, characters, and copy modification data that a 3800 printer uses to print a spool file or that define the spacing and character set that an impact printer uses to print a spool file. See system data file.
IML: Initial microprogram load.
immediate command: A type of CMS command that, when entered after an attention interruption, causes program execution, tracing, or terminal display to stop. Another immediate command can be entered to resume tracing or terminal display. The immediate commands are HB (halt batch execution), HI (halt all REXX or EXEC 2 programs or macros), HO (halt tracing), HT (halt typing), HX (halt execution), RO (resume tracing), RT (resume typing), SO (suspend tracing), TE (trace end), and TS (trace start). They are called immediate commands because they are executed when they are entered; they are not stacked in the console stack. Within an exec, immediate commands can be established or canceled by the CMS command IMMCMD.
implicit lock: A lock automatically acquired and freed when you run CMS commands and program functions against files or directories that reside in an SFS file pool. Many readers and one writer can access a file or directory.
implied CP command: In CMS, a CP command invoked without preceding the command line with CP.
implied EXEC: An EXEC procedure invoked without identifying it as such; that is, the word exec is not used for the invocation. Only the file name is used, as if entering a CMS command.
improved data recording capability (IDRC): A feature used on 3480 and 3490 tape devices that provides an improved data recording format.
improved spool file recovery: Recovering from the disk the checkpoint data that was previously recorded for closed spool files. Thus, if warm start is not possible, the VM/ESA system operator may attempt a checkpoint start or force start. See checkpoint (CKPT) start and force start.
in-doubt: A protected resource is called in-doubt when it has successfully completed the first phase of the two-phase commit and it is waiting for a decision from the initiator to either commit or roll back the changes and therefore start the second phase of the two-phase commit.
in-memory file: An EXEC 2 term for a file resident in storage and similar in concept to a file on disk.
in-queue virtual machine: A virtual machine on the run list waiting to be dispatched.
inactive page: A page in real storage that has not been referenced during a predetermined period.
inactive work unit: A work unit on which no requests have yet been made, or an atomic request was made, or requests were made and have been committed or rolled back; that is, an inactive work unit has no uncommitted work associated with it.
indicator: A 1-byte area of storage that contains either the character "1" to denote a true condition or the character "0" to denote a false condition.
initial installation system: In VMSES/E, a functional subset of the VM/ESA system shipped on the VM/ESA system DDR tapes and used during installation of VM/ESA.
initial microprogram load (IML): The action of loading a microprogram, which is a set of elementary instructions maintained in special storage that corresponds to a specific computer operation.
initial program load (IPL): The initialization procedure that causes an operating system to begin operation. A VM user must IPL the specific operating system into the virtual machine that will control the user's work. Each v