I know that the clock speed is determined by the slowest stage within the processor (usually fetch) because one clock cycle will take as much time as the slowest pipeline stage to ensure everything is synchronized.

So as fetching from DRAM is usually the slowest stage (for example during LDR instruction to memory which isn't in the cache) the time it takes for this should be the time for one clock cycle. So whats the point of caching?

I mean the processor has to wait anyway for the next clock cycle so even though the processor has fetched memory from cache very quickly wouldn't it have to wait anyway for the end of the clock cycle?


You are right that the clock speed is determined by the slowest stage. But on most architectures it is not true that fetching(or any kind of memory access in fact) takes one cycle. This is an illusion provided by the CPU to makes things manageable. In practice if your CPU tries to fetch and the operation cannot be completed in one cycle then it will stall(on pipelined processor it will introduce a "bubble") until it gets the memory content. Thus if your memory takes 100 cycles to read, then the CPU will stall 100 times in the fetching stage. The same applies to read(and to some extent write) stages.

This is why caching is so important: by caching the processor can reduce the fetch/read/write delay and maintain this illusion that it takes one cycle when really it does not. Example latencies for memory read are:

  • L1 cache: 1 cycle
  • L2 cache: 10 cycles
  • DRAM: 100 cycles

Do not take those figures literally, it is very architecture dependent. But you can see that without caching, your CPU would take 100 cycles to fetch each instruction, this is obviously unacceptable.

  • $\begingroup$ ahh ok so what your saying is that the time for fetching from memory does not effect the clock speed (only components within the processor are taken into account right?). So now my question is how does the processor know when the data is received from memory? $\endgroup$
    – Fady
    Nov 20 '17 at 9:16
  • $\begingroup$ Basically yes. How the processor knows is very technology dependent but it is always a variant of: the processor sends a a read requests to the piece of hardware responsible for memory accesses, and it gets a signal together with the data when it is ready. $\endgroup$ Nov 20 '17 at 9:56

When some stage is quite slow, it is divided up into multiple cycles. RAM access can take > 100 cycles. So the processor may do 100 accesses to cache in the time it takes to read one item from RAM.


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