User`s guide

Understanding Device Structures
RT–11 devices are categorized according to two characteristics:
• How devices process information
• How devices are physically structured
How Devices Process Information
All RT–11 storage devices are either randomly accessed or sequentially accessed.
• Random-access devices process blocks of data in random order; that is,
independent of the data’s physical location on the device or its location relative
to any other information. All disks, diskettes, and VM devices fall into this
category.
Random-access devices are sometimes called block-replaceable devices, because
you can manipulate (rewrite) individual data blocks without affecting other data
blocks on the device.
• Sequential-access devices require sequential processing of data; the order in
which the system processes the data must be the same as the physical order
of the data. RT–11 sequential devices are magtape and printer.
How Devices Are Physically Structured
Devices are structurally categorized according to whether or not they can store files:
• File-Structured Devices
File-structured devices store data under assigned file names. RT–11 devices that
are file structured include all disk and diskette devices.
File-structured devices that have a standard RT–11 directory at the beginning
are called RT–11 directory-structured devices. A device directory consists of a
series of directory segments that contain the names, lengths, and dates of the
files on that device. The system updates the directory each time a program
moves, changes, adds, or deletes a file on the device. (The RT–11 Volume and
File Formats Manual contains a more detailed explanation of a device directory.)
RT–11 directory-structured devices include all disks.
• Nonfile-Structured Devices
Nonfile-structured devices, however, do not store files; they contain a single
logical collection of data. These devices, which include the printer and terminal,
are generally used for reading and listing information.
Special Directory Devices
Some devices that do not have the standard RT–11 directory structure, such as
magtape, store directory information at the beginning of each file, but RT–11 must
read the device sequentially to obtain all information about all files.
Using Files, Volumes, and Devices 3–5