- 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
as "guests" on the same machine. This enabled the
separation of environments (test and production) and
workloads long before
The VM operating system is no longer
marketed by IBM.
- 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
systems in a
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.
- Virtual Telecommunications Access Method (VTAM)
- IBM's proprietary telecommunications software. See
for further information.
- 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.