Page 216 - DCAP103_Principle of operating system

Unit 6: File Management

processes are reading or writing from the same file, then the system may define one read/write Notes
mark or several.
In the former case, the read/write mark is kept at a central place, while in
the latter case it is kept with the process table entry. In Unix, a combination
of the two schemes is provided, as we shall see later.

6.4.2 Direct Access

This access allows a user to position the read/write mark before reading or writing. This feature
is useful for applications such as editors that need to randomly access the contents of the file.

6.4.3 Mapped Access

The Multics operating systems provide a novel form of access which we shall call mapped access.
When a process opens a file, it is mapped to a segment. The open call returns the number of
this segment. The process can thus access the file as part of its virtual store. The Close Segment
call may be used to close the file.

6.4.4 Structured Files
So far, we have treated files as byte streams. Database applications often wish to treat them as
records that may be accessed by some key. To accommodate these applications, some systems
support typed or structured files that are considered streams of records. If a file is structured,
the owner of the file describes the records of the file and the fields to be used as keys. OS/360
for IBM computers and DEC VMS provide such files.
6.4.5 Binding of Access Methods
An access method may be specified at various times:

When the operating system is designed. In this case, all files use the same method. When the
file is created. Thus, every time the file is opened, the same access method will be used. When
the file is opened. Several processes can have the same file open and access it differently.

6.5 Directory Structures

A disk is typically partitioned, also known as slices and minidisks. The device directory or volume
table of contents records and maintains the file properties such as name, size, type and location.
Overall Campus Directory Structure

Metadirectory
Border
Directory

Enterprise
DIrectory
Application-Specific
Directories

OS Directories
Directory (MS, Novell, etc.)
Database Departmental
Directories

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