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Just a basic question to ask

Does the write through cache copies the whole block or just the byte which is updated?

I went through the following question

Array A contains 256 elements of 4 bytes each. Its first element is stored at physical address 4096. Array B contains 512 elements of 4 bytes each. Its first element is stored at physical address 8192. Assume that only arrays A and B can be cached in an initially empty, physically addressed, physically tagged, direct mapped, 2K-byte cache with an 8 byte block size. The following loop is the executed

for(i=0; i<256; i++)
A[i] = A[i] + B[2*i];

How many bytes will be written to memory if the cache has a write-through policy?

I calculated it as follows:

The cache can store the whole array with 2 elements per block (block size = 8bytes, element size = 4bytes). For every write, the whole block will be copied. For $0^{th}$ element, the block containing $0^{th}$ and $1^{st}$ element would be written. The same would be done for $1^{st}$ element as well.

So, for every iteration the 2 elements would be written. This makes the number of bytes as $256*2* (4bytes / element) = 2048bytes$.

But in the solution, they have just calculated the number of loop iterations ($256$) multiplied by the element size ($4byte$) which makes the answer $1024bytes$. If this is true, then the cache would update only the updated byte (not the whole block).

Which is correct?

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This likely depends on the cache implementation, less so on a scientific principle. Therefore, I think the question is offtopic here. –  Raphael Jan 28 '13 at 10:19
I think data transfer always occur in blocks. Isn't it? –  Shashwat Jan 28 '13 at 10:25
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3 Answers

up vote 1 down vote accepted

The caching strategies could be very "domain specific" like efficient data caching for numerical data structure, graph data structures and algorithms, bio-informatics data structure, etc.. This is a nice Ph.D thesis about it: "Cache-efficient Algorithms and Data Structures: Theory and Experimental Evaluation"

For your specific program I think it will cache whole block of memory not only the specific data in array. It uses a general cache strategy. See Cache algorithms on Wikipedia.

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I would say that write-through cache always knows which addressable units need to be written, so there is absolutely no reason why it should write the rest of the block. Therefore it does not.

I am intentionally saying "addressable units", because (though I am not sure) it would generally be aligned units of the same width as the bus, i.e. usually 4-byte. But that has no effect here when the elements are 4-byte.

In contrast write-back cache is likely to only keep per-block modified status. However a write-back cache would be delaying the writes and since the consecutive passes through the loop end up overwriting whole blocks, it would still write just 1024 bytes.

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A cache works by transferring a block at a time from/to RAM, so it copies a full block.

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