Lesson on Cache memory and memory access with conceptual diagram,Lessons2all

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Lesson on Cache Memory and Memory Access modes

Cache memory is a small, fast acting buffer memory between the processor and the main memory. Cost of this memory is very high.

Why do we need this kind of arrangement?

The performance of a computer system will be severely affected if the speed disparity between the processor and the main memory is significant. So to improve the performance of the processor this memory is used

Why the performance of the Computer System increases because of Cache memory

Cache memories are static memories so the contents need not be refreshed so processor time is saved and the contents are readily available whenever needed by the processor
Since Cache memory is closer to processor than RAM and it is faster than RAM so the processing becomes faster

Whether all the contents of RAM are also stored in Cache?

No but only frequently used contents are stored in Cache memory
Processor first looks at Cache memory for the required instruction and if it finds it there then it takes that instruction from there otherwise it gets it from RAM
For example since the instructions within the body of a loop are executed frequently these statements may be stored in Cache memory

Memory can be accessed in two modes Random Access and Serial Access

Memory locations can be accessed randomly
Access time is independent of the location to be accessed.
Semiconductor memories are of this type
Since there is a separate access mechanism (read-write head) for each location, these different storage locations can be accessed independent of others, hence these memories are costlier

Storage locations can be accessed only in a certain pre-determined manner. Such memories are called as serial access memory
In this kind of memories, the access mechanism (read-write head) is shared by the storage locations. They must be assigned to different locations at different times either by moving the storage locations or by moving the read write head or both.
These operate by continuously moving the stored information around a closed path also known as track.
A particular location can be accessed only when it passes the fixed read-write head.
Hence the time required to access a particular location depends upon its position relative to read-write head.
Semi random: If each track has it own read-write head, but within the track the access will be serial. CD-ROM, HD are examples .