File Access Methods
When a file is used, information is read and accessed into computer memory and there are several ways to access this information of the file. Some systems provide only one access method for files. Other systems, such as those of IBM, support many access methods, and choosing the right one for a particular application is a major design problem.
It is the simplest access method. Information in the file is processed in order starting at the beginning of the file, one record after the other. This mode of access is by far the most common; for example, editor and compiler usually access the file in this fashion.
Read and write make up the bulk of the operation on a file. A read operation reads from the current file pointer position. Usually, the software reads some pre-determined amount of bytes. The read operation also moves the file pointer to the new position where the system has stopped reading the file. Similarly, for the write command writes at the point of the current pointer.
Another method is direct access method also known as relative access method. A fixed-length logical record that allows the program to read and write records rapidly, in no particular order. Direct access is based on the disk model of a file since disk allows random access to any file block. For direct access, the file is viewed as a numbered sequence of block or record. Thus, we may read block 14 then block 59 and then we can write block 17. There is no restriction on the order of reading and writing for a direct access file.
A block number provided by the user to the operating system is normally a relative block number, the first relative block of the file is 0 and then 1 and so on.
Index Sequential Access
It is a method of accessing a file which is built on top of the sequential access method. This method constructs an index for the file. The index, like an index in the back of a book, contains the pointer to the various blocks. To find a record in the file, we first search the index and then by the help of pointer we access the file directly.