© Copyright IBM Corp. 2003. All rights reserved. 167
Glossary
Access method control block (ACB). A control
block that links an application to a VSAM data set
Active control data set (ACDS). A VSAM linear
data set that controls the storage management
policy for an installation. It is shared by each
system that is using the same SMS configuration
to manage storage.
Active lock. A lock obtained by an executing
subsystem. Requests for locks incompatible with
a held active lock are queued. See also retained
lock.
Alternate index. A key-sequenced data set
containing index entries organized by the
alternate keys of its associated base data
records. It provides an alternative way of locating
records in the data set on which the alternate
index is based.
Application owning region (AOR). A CICS
region that does not own terminals or files but
which purely runs CICS transactions, relying
upon terminal-owning and file-owning regions for
terminal and file access.
Automatic class selection (ACS). A facility that
allows a DFSMS system to decide which
constructs (data class, storage class,
management class or storage group) should be
associated with a data set based on definition
attributes.
Automatic restart manager (ARM). An OS/390
component that provides recovery and restart
capabilities.
Back-out. Undo all changes made to a protected
resource since the previous sync point.
Basic sequential access method (BSAM). An
access method for sequentially organized
non-VSAM data that relies on an application
program to block and unblock data records.
CEMT. A CICS-supplied transaction used to
invoke all the master terminal functions. These
functions include inquiring and changing the
value of parameters used by CICS, altering the
status of system resources, terminating tasks and
shutting down CICS.
Cluster. A named VSAM structure consisting of a
group of one or more related components.
Consistent read (CR). A read operation where a
share lock is obtained and released as part of the
read process. This ensures that the reader sees
the latest committed copy of the record as at the
time of the read.
Consistent read explicit (CRE). A share lock is
obtained as part of the read process and held
until explicitly released. This ensures that a
subsequent read while the lock is still held will
always return the same version of the data.
Control area (CA). A fixed-size area in which
VSAM stores control intervals. It is the unit of
allocation for VSAM data.
Control interval (CI). A fixed-length area in
which VSAM stores records. It is the unit of
transfer between VSAM and disk storage.
Coupling Facility (CF). The hardware that
provides services to support data shared
between OS/390 systems.
Coupling Facility control code (CFCC). The
software running in a Coupling Facility that
provides Coupling Facility services.
Cross-system coupling facility (XCF). An MVS
component that provides services for the
exchange of messages between members of a
sysplex.
DFHLSCU. A CICS-supplied utility to calculate
space needed and average buffer size for log
streams.
DFR. An ACB parameter affecting buffer writing
when a data set is opened for LSR or GSR. It
defers the writing of a buffer until explicitly

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