![]() ![]() Tl dr performance depends on the access pattern. 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. i have 3 caches (one for each technique) made by 8 blocks of 4 bytes, and im trying to insert those values : 0, 16, 0, 24, 32 and understand which one will hit or miss. However, set associative caches are usually slower and somewhat more expensive to build because of the output multiplexer and additional comparators. i have troubles trying to understand how direct mapped, set associative and fully associative caching techniques works. Direct Mapping: Each block from main memory has only one possible place in the cache organization in this technique. 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. Set associative caches generally have lower miss rates than direct mapped caches of the same capacity because they have fewer conflicts. The mapping techniques can be classified as : Direct Mapping Associative Set-Associative 1. 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. Because fully associative, set associative, and direct mapped caches have different block placement constraints, the block re-placement policy for one cache. when evicting a line: if D0 (memory data is NOT stale), just set V0. Larger sets and higher associativity lead to fewer cache conflicts and lower miss rates, but they also increase the hardware cost. Fully associative: block can be anywhere in the cache Direct mapped: block can. Set sizes range from 1 (direct-mapped) to 2k (fully associative). Here, the cache is divided into many sets, so the "set number" part consists of the number of bits required to identify each set uniquely.ĭepending on the number of lines in each set ( a K-way set associative mapping will contain K lines in each set), the number of sets can be found out.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. the cache, but not to any specific block within that set. I don't know how that would make a difference in computing byte offset, index and cache Set-associative mapped cache allows any given main memory block to be mapped into two or more cache locations.įully-associative mapped caching allows any given main memory block to be mapped into any cache location. What is the size in bits of the cacheline offset, cachline index and tag?ĭirect mapped caching allows any given main memory block to be mapped into exactly one unique cache location. This cache uses write-back scheme, and the address is 32 bits wide.Īnswer the next questions for Direct Mapped cache, Fully Associative and Way Set Associative Direct Mapped Fully Associative Cache Unifying Theory Cache Design and Other Details Line Size Types of Misses Writing to Memory Sub-Blocks Cache Aware Programming The purpose of this document is to help people have a more complete understanding of what memory cache is and how it works. Consider a 512-KByte cache with 64-word cachelines (a cacheline is also known as a cache block, each word is 4-Bytes). ![]()
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