Operating System Reference Manual Introduction The basic operaUons provided by the Flle System are as follows: mount and unmount - make a volume accessible/Inaccessible open and close - make an object accessible/lnaccesslble read and write - transfer Information to and from an object device control functions - control device-specific functions Some operations apply only to storage devices:
allocate and deallocate - specify slze of an object manipulate catalog - control naming of objects and creation and destruction of objects manipulate attributes - look at or change the characteristics of the object In addition to the data in an object, the object ltself has certain characteristics called attributes, such as the length and creation date of a file. Calls are available to access the attributes of any File System object. In addition to its system-defined attributes, an object on a storage device can have a label The label is available for programs to store information that they can Interpret Non-storage devices such as printers are accessed with a limited set of operations.
They must be mounted and opened before they can be accessed. Sequential read and/or write operations are available as appropriate for the device. Device-control functions are available to perform any device- specific functions needed. The file-name portion of the complete pathname for a nor^storage device is not used by the File System, although you do have to provide one when you open the devica For storage devices, the same sequential read and write operations are valid as for non-storage devices, storage devices also must be mounted, and particular files opened, before the files can be used. They have appropriate device-control functions available.
When writing to a disk file, space for the file is allocated as needed. Space for a file does not need to be contiguous, and in some cases this automatic allocation can result in a fragmented file, which may slow file access. To insure rapid access, you can pre-allocate space for the file. Pre-allocating the flie also ensures that the process wlll not run out of space on the disk. Four types of objects can be stored on storage devices. These are files, pipes, data segments, and event channels.
Files, already discussed, are simply arrays of stored data Pipes are objects that provide interprocess communicatioa Data segments are special cases of files that are loaded into memory along with program code. Event channels are pipes with a specialized structure Imposed by the system. 1.4 Process Management A process is an executing program and the data associated with it. Several processes can exist at one time, and they appear to run simultaneously because the CPU ls multiplexed among them. The Scheduler decides what 1-3