Inlägg "Hi, Me and my excellent laboration partner faced ..." | Datorteknik, grundkurs (IS1200)

Hi,

Me and my excellent laboration partner faced a question as we were discussing how cache memories worked. Providing we have an empty cache memory (i.e. the valid bits of all sets respectively are equivalent to 0), with a block size of 2 words and an associativity of 1, to which we write 1 word to a specific address – what will happen if we try to read from the other word, in the same block?

We have considered the following realistic possibilities, but we have not yet been able to figure out which one (if any) that's the correct:

The cache memory reads from the RAM and copies the content of the second word upon writing to the first, setting V = 1.
The cache memory still has V = 0 upon reading from the second word (after we have written to the first), yielding a cache miss and hence a full block read from the RAM.

Thanks in advance!

Kind regards,
Martin Pola

Maybe I missunderstand what you mean, but if I understand correctly I think your first alternative is correct.

When you first write to the memory address it will result in a cache miss and a full block of 2 words will be fetched from the main memory(RAM) and the valid bit will be set to 1 for this set(row). Next when you try to read from the other word which resides on the same row it will result in a hit because it was fetched together with the first word.

Thanks! You seem to have comprehended the full scope of our question. Just to make sure; would that imply a write operation could, in cases when V = 0, execute a read operation in addition to the requested write operation?

I'm not sure what you mean by "execute a read operation in addition to the requested write operation". Do you mean that the write and read operations are operated in parallel or sequentially? I think V = 0 only when the cache is empty and as soon as a block has been fetched V is set to 1. So when V=0 it will always fetch data from Main memory.