Specifications

OpenVMS Operating System for Alpha and VAX (Versions 7.1, 7.1–1H1, 7.1–1H2, and 7.1–2) SPD 41.87.06
Power Failures (VAX Only)
If the power fails, the system shuts down automatically.
When power is restored, the system restarts automati-
cally and resumes processing at the point of interruption
under these circumstances:
• If the system has a time-of-day clock and a memory
battery backup unit
• If the contents of memory are still valid
• If the system is set to permit automatic rebooting
The system restarts devices and communications lines
and all I/O operations in progress, including magnetic
tape I/O operations. On request, programs can be noti-
fied of power restoration. An optional battery-operated
hardware clock resets the date and time of day when
the system restarts. If the system does not have a bat-
tery backup unit, or if the memory contents are not valid
on power restoration, the system reboots automatically
if it is set to permit automatic rebooting.
If, for any reason, after a power failure the system disk
does not come back on line within a specific time after
the CPU regains power, the system shuts down.
Input/Output
The QIO system service and other related IO services
provide a direct interface to the operating system’s I/O
routines. These services are available from within most
OpenVMS programming languages and can be used to
perform low-level I/O operations efficiently with a mini-
mal amount of system overhead for time-critical appli-
cations.
Device drivers execute I/O instructions to transfer data
to and from a device and to communicate directly with
an I/O device. Each type of I/O device requires its own
driver. Compaq supplies drivers for all devices sup-
ported by the OpenVMS operating system and provides
QIO system service routines to access the special fea-
tures available in many of these devices.
OpenVMS supports a variety of disk and tape periph-
eral devices, as well as terminals, networks, and mail-
boxes (virtual devices for interprocess communication),
and more general I/O devices.
With OpenVMS Alpha, users can write drivers for I/O
devices that do not have inherent OpenVMS support.
OpenVMS Alpha allows device drivers to be written
in DEC C and MACRO-32. Documentation is avail-
able that describes how to write OpenVMS Alpha de-
vice drivers and how to convert existing OpenVMS VAX
drivers to run on OpenVMS Alpha systems. The DEC
C compiler for OpenVMS Alpha is an optional layered
product.
With OpenVMS VAX, users can write their own drivers
in MACRO-32 for I/O devices that do not have inher-
ent OpenVMS support, as described in the appropriate
documentation.
I/O Performance Features
Fast I/O provides a suite of additional system services
that applications can use to improve I/O throughput. The
fast I/O services minimize the CPU resources required
to perform I/O.
Fast Path provides a streamlined mainline code path
through the I/O subsystem to improve both uniprocessor
and multiprocessor I/O performance. On multiprocessor
systems, Fast Path allows all CPU processing for spe-
cific I/O adapters to be handled by a specific CPU. This
can significantly lower the demands on the primary CPU
and increase the I/O throughput on multiprocessor sys-
tems with multiple I/O ports. The CI port and DSA disk
drivers have been enhanced to take advantage of the
Fast Path capability. No user application changes are
needed to take advantage of Fast Path. Fast Path can
be utilized by the $QIO system service or the Fast I/O
services.
Virtual I/O Cache
OpenVMS provides a standalone or clusterwide, file-
oriented disk cache. Applications benefit from the ad-
vantages of the virtual I/O cache without any special
coding. The virtual I/O file-caching algorithm is chosen
based on the type of clusterwide access currently in
progress. Virtual I/O caching reduces current and po-
tential I/O bottlenecks within OpenVMS systems. It re-
duces the number of I/Os to the disk subsystem, thereby
reducing systemwide bottlenecks.
Record Management Services (RMS)
RMS is a set of I/O services that helps application pro-
grams to process and manage files and records. Al-
though it is intended to provide a comprehensive soft-
ware interface to mass storage devices, RMS also sup-
ports device-independent access to unit-record devices.
RMS supports sequential, relative, and indexed file or-
ganizations in fixed-length and variable-length record
formats. RMS also supports byte stream formats for
sequential file organization.
RMS record access modes provide access to records
in four ways:
• Sequentially
• Directly by key value
• Directly by relative record number
• Directly by record file address
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