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Wednesday, May 29, 2013

Explain the various File Access methods?

One of the most important functions of the mainframe operating system is the access methods that make it possible for you to access the data from external devices such as the tape or disk. 

What are access methods?

- Access methods are very useful in providing an API for transferring the data from one device to another.
- Another best thing about this API was that it worked as the device driver for the operating systems on non-mainframe computers. 
- There have been a lot of reasons behind the introduction of the access methods. 
- A special program had to be written for the I/O channel and there has to be a processor entirely dedicated to controlling the access to the peripheral storage device as well as data transfer from and to the physical memory. 
- Special instructions constitute these channel programs and are known as the CCWs or the channel command words.
- To write such programs, very detailed knowledge is required regarding the characteristics of the hardware. 

Benefits of File Access Methods

There are 3 major benefits of the file access methods:
Ø  Ease of programming: The programmer does not have to deal with the procedures of the specific devices, recovery tactics and error detection. A program designed to process a particular thing will do it no matter where the data has been stored.
Ø  Ease of hardware replacement: A program cannot be altered by the programmer during the migration of data from older to newer model of the storage device provided the same access methods are supported by the new model.
Ø  Ease in sharing the data set access: The access methods can be trusted for managing the multiple accesses to the same file. At the same it ensures the security of the system and data integrity.

Some File/Storage Access Methods

Ø  Basic direct access method (BDAM)
Ø  Basic sequential access method (BSAM)
Ø  Queued sequential access method (QSAM)
Ø  Basic partitioned access method (BPAM)
Ø  Indexed sequential access method (ISAM)
Ø  Virtual storage access method (VSAM)
Ø  OAM (object access method)

- For dealing with the records of a data set both the types of access i.e., the queued and the basic are suitable. 
- The queued access methods are an improvement of the basic file access methods. 
- Read ahead scheme and internal blocking of data is well supported by these methods. 
- This allowed combining the multiple records in to one unit, thus increasing the performance. 
- In sequential methods, it is assumed that there’s only a sequential way for processing the records which is just the opposite of the direct access methods. 
There are devices like the magnetic tape that only enforce the sequential access 
- Sequential access can be used for writing a data set and then later the direct manner can be used for processing it.

Today we have access methods that are network-oriented such as the following:
Ø  Basic telecommunications access method or BTAM
Ø  Queued tele – processing access method or QTAM
Ø  Telecommunications access method or TCAM
Ø  Virtual telecommunications access method or VTAM

The term access method was used by the IMS or the IBM information management system for referring to the methods for manipulation of the database records. 
- The access methods used by them are:
Ø  GSAM or generalized sequential access method
Ø  HDAM or hierarchical direct access method
Ø  HIDAM or hierarchical indexed direct access method
Ø  HISAM or hierarchical indexed sequential access method
Ø  HSAM or hierarchical sequential access method
Ø  PHDAM or partitioned hierarchical direct access method
Ø  PHIDAM or partitioned hierarchical indexed direct access

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