A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

V

Virtual Machine (VM)

An IBM Mainframe operating system (or control program). The most important feature of the VM operating system was the ability to run other operating systems such as MVS as "guests" on the same machine. This enabled the separation of environments (test and production) and workloads long before PR/SM became available.

The VM operating system is no longer marketed by IBM.

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Virtual Storage Access Method (VSAM)

An access method for direct or sequential processing of fixed and variable length records on Direct Access Storage Devices (DASD). The records in a VSAM dataset or file can be organised in logical sequence by a key (key sequence), in the sequence in which they were written to the dataset or file (entry sequence) or by relative record number. VSAM files can be accessed using indexing and also alternate indexing.

VSAM Record Level Sharing (RLS) enables a VSAM file to be shared (with full integrity) by MVS systems in a parallel sysplex environment. The structures for locking and data caching are contained in the Coupling Facility (CF). An additional advantage of RLS is that the records are locked at the record level and not the control interval (CI) level, which reduces contention and the risk of deadlocks.

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Virtual Telecommunications Access Method (VTAM)

IBM's proprietary telecommunications software. See ACF/VTAM for further information.

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Volume Table of Contents (VTOC)

The name given to the area of a Direct Access Storage Device (DASD) which contains the table of contents or index for that device, effectively enabling the direct access.

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