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I'm stuck on a point while reading about these different accessing methods. As per author.

Sequential access

Memory is organized into units of data, called records.Access must be made in a specific linear sequence.

I understand that it will take variable time to access due to sequential ordering.

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. Again, access time is variable.

Here I'm Stuck that if we have unique address why it is taking variable time and why we can't directly access data.

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.

What is the difference between this and direct access method.

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Sometimes, depending on the sources,direct access and random access are used to express the same memory access mode and therefore do not present any substantial difference in the speed of data access.

More in detail, the differences between the two modes can be sought in their implementation: Random Access can be realized directly via RAM memory wiring, while direct access can use secondary storage; in this case is obvious that random access is faster than direct access.

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