Page 151 - DCAP403_Operating System

Operating System

Notes unformatted, you must use unformatted I/O statements only. The last record of a sequential fi le
is the end-of-fi le record.

Direct Access

Direct access is based on a disk model of a file. For direct access, the file is viewed as a numbered

sequence of block or records. A direct-access file allows arbitrary blocks to be read or written.
Thus, after block 18 has been read, block 57 could be next, and then block 3. There are no
restrictions on the order of reading and writing for a direct access fi le. Direct access fi les are of
great use for intermediate access to large amounts of information.

The file operations must be modified to include the block number as a parameter. Thus, you have
“read n”, where n is the block number, rather than “read next”, and “write n”, rather that “write
next”. An alternative approach is to retain “read next” and “write next” and to add an operation;

“position file to n” where n is the block number. Then, to effect a “read n”, you would issue the
commands “position to n” and then “read next”.
Not all OS support both sequential and direct access for files. Some systems allow only sequential

file access; others allow only direct access. Some systems require that a file be defined as sequential

or direct when it is created; such a file can be accessed only in a manner consistent with its
declaration.
Direct-access files support both formatted and unformatted record types. Both formatted and

unformatted I/O work exactly as they do for sequential fi les.
Other Access Methods
Other access methods can be built on top of a direct-access method. These additional methods
generally involve the construction of an index for a file. The index contains pointers to the various

blocks. To find an entry in the file, the index is searched first and the pointer is then used to access

the file directly to find the desired entry. With a large file, the index itself may become too large

to be kept in memory. One solution is to create an index for the index file. The primary index fi le

would contain pointers to secondary index files, which would point to the actual data items. For
example, IBM’s indexed sequential access method (ISAM) uses a small master index that points
to disk blocks of a secondary index. The secondary index blocks point to the actual fi le blocks.

The file is kept sorted on a defined key. To find a particular item, I first make a binary search of

the master index, which provides the block number of the secondary index. This block is read

in, and again a binary search is used to find the block containing the desired record. Finally, this
block is searched sequentially. In this way, any record can be located from its key by at most
direct access reads.
8.4 Directory Structure
The directories themselves are simply files indexing other files, which may in turn be directories

if a hyerarchical indexing scheme is used. In order to protect the integrity of the file system in
spite of user of program error, all modifications to these particular directory files are commonly

restricted to the file management system. The typical contents of a directory are:

1. file name (string uniquely identifying the file), type (e.g. text, binary data, executable,

library), organization (for systems that support different organizations);
2. device (where the file is physically stored), size (in blocks), starting address on device (to

be used by the device I/O subsystem to physically locate the fi le);

3. creator, owner, access information (who is allowed to access the file, and what they may do
with it);
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