What are the differences among sequential access, direct access, random access and associative access?

Question is:

What are the differences among sequential access, direct access, random access and associative access?

Solution is:

There are four types of method to access units of data. They are Sequential access, Direct access and random access, associative access. The difference among them are given below:

See first about these access in details then come to the difference.

Sequential access: 
    Memory is organized into units of data, called records. Access must be made in a specific linear sequence. Stored addressing information is used to separate records and assist in the retrieval process. A shared read– write mechanism is used, and this must be moved from its current location to the desired location, passing and rejecting each intermediate record.Thus, the time to access an arbitrary record is highly variable. Tape units are sequential access.

Direct access: 
    As with sequential access, direct access involves a shared read–write mechanism. However, individual blocks or records have a unique address based on physical location. Access is accomplished by direct access to reach a general vicinity plus sequential searching, counting, or waiting to reach the final location. Again, access time is variable. Disk units are direct access.

Random access: 
    Each addressable location in memory has a unique, physically wired-in addressing mechanism. The time to access a given location is independent of the sequence of prior accesses and is constant. Thus, any location can be selected at random and directly addressed and accessed. Main memory and some cache systems are random access.

Associative access: 
    This is a random access type of memory that enables one to make a comparison of desired bit locations within a word for a specified match, and to do this for all words simultaneously. Thus, a word is retrieved based on a portion of its contents rather than its address. As with ordinary random-access memory, each location has its own addressing mechanism, and retrieval time is constant independent of location or prior access patterns. Cache memories may employ associative access.


So,the main difference or key terms of different among sequential access, direct access and random access are:

Sequential access: Memory is organized into units of data, called records. Access must be made in a specific linear sequence.
Direct access: Individual blocks or records have a unique address based on physical location. Access is accomplished by direct access to reach a general vicinity plus sequential searching, counting, or waiting to reach the final location.
Random access: Each addressable location in memory has a unique, physically wired-in addressing
mechanism. The time to access a given location is independent of the sequence of prior accesses and is constant.


Tags: Computer Organization and architecture, differences among sequential access, direct access, random access and associative access, access of data of memory units, data accessing in computer memory