问题
To start off, the first cache has 16 one-word blocks. As an example I will use 0x03 memory reference. The index has 4 bits (0011). It is clear that the bits equal 3mod16 (0011 = 0x03 = 3). However I am getting confused using this mod equation to determine block location in a cache with offset bits.
The second cache has a total size of eight two-word blocks. This means that there is 1 offset bit. Since there are now 8 blocks, there are only 3 index bits. As an example, I will take the same memory reference of 0x03. However now I am having trouble mapping to the block using the mod equation I used before. I try 3mod8 which is 3, however in this case, since there is an offset bit, the index bits are 001. 001 is not equal to 3 so what did I do wrong? Does mod not work when there are offset bits? I was under the impression that the mod equation would always equal the index bits.
回答1:
Its all in the address. You get the address, then mask off number of bits from the end, for following reasons.
- Number of words in the cacheline. If you've got 2 word cacheline (take a bit out, 4 word - 2 bts etc)
- Then how many cacheline entries you have. (If is a 1024 cacheline, you takeout 10 bits. This 10 bits is your index, remaining bits are for your Tag)
Now, you also need to consider 'WAY' as well. If its a direct mapped cache, above applies. If its a 2 way set associative cache, you dont have 1024 lines, what you have a 512 blocks with each having 2 lines in them. Which means you only need 9 bits to determine the index of the block. If its 4 way, you've got 256 blocks with 4 lines in them, meaning you only need 8 bits for your index.
In a set associative cache, index are there to choose a block, once a block is chosen, use can use a policy like LRU to fill an entry in case of a cache miss. Hits are determined by comparing the tag in the selected block.
Bottom line, block location is not determined by the address, only a block is selected by the address and thereafter its Tag comparison to find the data.
来源：https://stackoverflow.com/questions/40893097/how-do-i-map-a-memory-address-to-a-block-when-there-is-an-offset-in-a-direct-map