Tree-Structured Directories Guide in Operating Systems

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Tree-Structured Directories Guide in Operating Systems

PC/DOS allows the user to create a tree-structured directory to assist in handling files and to make the search process more efficient. In contradistinction ot mainframe and minicomputers, all third generation microcomputers with disk operating sy stems provide a directory facility.

The Directory Concept

A directory is simple a table of files on a diskette or hard disk system. The end user assigns a name to a file and it is thereafter referenced by that name. Clearly, files can be deleted or renamed and the directory is always updated accordingly. Sometimes, a directory is called a catalog.

On each diskette or hard disk, there is a fixed area for the directory so the microcomputer operating system knows where to go to locate a file name specified for access. In this sense, the directory is analogous to the volume table of contents (VTOC). There is no concept that is analogous to the system master catalog, such as in MVT or TSS or MVS in microcomputer systems.

The space on disk for the directory is allocated when the disk is formatted and the directory space is fixed in size. For diskette, it may contain 64 or 112 file names-depending on whether single- or dual-sided diskettes are used. For hard disk, the directory size is essentially a formatting parameter.

To find out what is on a disk, the user normally enters a simple command-such as DIR or CATALOG. The list of file names with their attributes are then displayed or printed. 

The Tree Concept

The idea of a simple directory is satisfactory if the number of file names is small. With a hard disk, which is that state of the art in the third generation, the number of files in a 10-megabyte system can run in thousands, making the contents cumbersome to view and inefficient to access.

In PC/DOS, the directory may contain subdirectories, which may in turn contain more subdirectories, and so fortj. The lowest subdirectory level specifies file names. As before, the master or "root" directory may contain a fixed number of entries- 64 or 112 on diskette, as before. However, subdirectories are normal files so the number of file names that can be stored with a tree structured system is only limited by the size of the volume.

The tree-structured directory concept allows the file structure to model the application structure. If, for example,  there are sales and payroll departments, then there could be a subdirectory for SALES and one for PAYROLL. Similarly, if two users named Jones and Smith were using the system, then each person would have separate directory.

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Posted on Jan 12, 2012