Q. Why are high-speed memories required? Is the main memory not a high-speed memory?
The answer to second question is absolutely 'No' though why so? Then if memories are slower then how slow are they? On an average it has been found that operating speed of main memories lack by a factor of 5 to 10 than the speed of processors (like CPU or Input / Output Processors).
Additionally every instruction needs numerous memory accesses (it may range from 2 to 7 or even more sometimes). If an instruction needs even 2 memory accesses even then nearly 80% of time of executing an expression, processors waits for memory access.
The main question is what can we do to increase this processor-memory interface bandwidth? There are four potential answers to the question which are:
a) Reduce memory access time; use a faster though costly technology for main memory.
b) Access more words in a single memory access cycle which is in place of accessing one word from memory in a memory access cycle access more words.
c) Insert a high-speed memory called as Cache between processor and main memory.
d) Use associative addressing in place of random access.