![]() ![]() Tag: some set of bits attached to a block that define it's characteristics (i.e., the address it is currently mapped to, whether it is "dirty" or not, etc.) Source block: the block in main memory to be copied into the cache or the block in the cache being written back to main memoryĭiscard / discarded : a discard is a block that has been flushed or removed from the cache and replaced with a block newly read from memory. For small moves (accessing one byte in a block, or writing one byte in a block) copying the whole block wastes some memory bandwidth but this is necessary concession to design practicality.ĭestination block: the block in the cache or in memory to be written to (e.g., when loading, the destination block is in the cache, when writing a changed cache block to memory, the destination block is main memory). A block is the smallest unit that may be copied to or from memory. ![]() (5) Caches are loaded in terms of blocks. Blocks contain some number of words, depending on their size. Caches usually contain a power of 2 # of blocks but this isn't a requirement. Except as noted, the size of all caches is an integer multiple of the block size. (4) All caches are broken into blocks of fixed size. (This is relevant to comparisons in H&P of the 200MHz 21064 Alpha AXP to the IBM 70MHz POWER2. It isn't just a matter of cranking the clockspeed up. (3) Measures such as "work per cycle" or "instructions per clock" are meaningless metrics for comparing different architectures. There's a pretty damn good reason, since modern CPUs almost exclusively use set-associative and direct mapped caches. For instance, one might ask why direct and set-associative caches (see below) would even need to exist when fully associative caches are so much more flexible. If you could, somebody would already have done it and you wouldn't be debating it. (2) You usually can't improve anything in computers without giving up a little of something else. This is accomplished by copying the contents into the faster cache. Caches work by mapping blocks of slow main RAM into fast cache RAM. Square bitmaps are stored one scanline after another, rectangular arrays of numbers are stored in memory as a series of linearly appended rows, and so on. All memory is addressed, stored, accessed, written to, copied, moved, and so on in a linear way. What's different about a fully associative cache? What are some key points I need to understand this section?ĥ. Tl dr performance depends on the access pattern.1. There are other performance issues at play as well, set associative lookup latency is typically a little longer, even on hits, due to the extra logic required to do the lookup. Set associative will have 0 misses because there are two sets to hold the most recent two blocks that map to the same line. Count the misses that will occur in both designs.Īssume simple mapping that uses (x modulo 8) to calculate the cache block to assign memory block x to.ĭirect mapped will have 3 misses due to conflicts, since 0 and 8 occupy the same block in the cache. Memory block access pattern is 0, 8, 0, 8. In set associative caches, you can choose which memory block to evict, which can help avoid misses & evictions when the access pattern causes conflict evictions in the direct mapped case. In a direct mapped cache, if two memory blocks map to the same cache block, you have to evict the block and store another one. You are right in saying that their capacity will be the same, but you should think about the access pattern and how many evictions and misses will occur in the different designs with respect to the access pattern. But in the case of a direct mapped cache, once you fill a cache block with a single memory block, cache trashing becomes possible). In such a scenario, wouldn't both cases result in 2 memory blocks being mapped to a single cache block? How are they different in this sense? (The only thing I can think of is in a 2-way set associative cache, for each set, you can fit in 2 memory blocks before cache trashing becomes possible. In this case, there would be 4 memory blocks mapped to each cache set. This would mean that for each cache block, there will be 2 blocks of memory that can be mapped to it.įor a 2-way set associative cache (of the same size), there would be 4 sets, each set containing 2 cache blocks. Here's my question: If a Direct Mapped Cache has the same number of cache blocks (lines) as an N-way Set Associative Cache, wouldn't their performance be the same?įor example, say there are 16 blocks of memory and 8 cache blocks in a direct mapped cache. ![]()
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