This makes it work much faster and also quite cheaper in comparison to the fully associative cache. Normally, a direct mapped cache is much simpler because it needs only one multiplexer and one comparator.
DIRECT MAPPED VS SET ASSOCIATIVE FREE
If the line is not free or available, a replacement algorithm is used to transfer data and the new data replaces any one of the lines. If it is free, the data is transferred immediately without using any rule or formula.
On the other hand, in a fully associative cache, the transfer of data to the cache lines from the main memory is done by checking the availability of the line. The word of the main memory represents the mapping between the main memory address and the cache addresses with reference to the information of the same unit. In a direct mapped cache, when data is transferred from the main memory to the cache memory, a line is replaced by using a formula KmodN where K signifies the number of the main memory block and N stands for the number of cache lines. On the other hand, one of the most significant disadvantages of a fully associative cache is that it is quite expensive as compared to a direct mapped cache which is mainly because these particular types of caches need to store the address as well along with the data. One of the greatest disadvantages of the direct mapped caches is that the level of performance offered by them is quite low as compared to the level of performance of a fully associative cache because it needs replacement for the data-tag value. On the other hand, in comparison, a fully associative cache is also quite fast but is also very easy to implement. It is also very fast because it needs matching only the tag field to search for a word and it is relatively less expensive. The direct mapped cache is considered to be the simplest of all due to the simple mapping technique. On the other hand, in the case of a fully associative cache, the search time is more because the cache control logic has to examine each individual tag of the block while looking for a match. Read Also: What is Associative Memory? Types, Pros, Cons & More This is due to the fact that there is only one possible location in the cache organization to look for in every block from main memory. In a direct mapped cache, the search time is less. On the other hand, there is no such effect in the cache hit ratio in the case of a fully associative cache even if the processor has to access the same locations in the memory from two distinct memory pages repeatedly. In a direct mapped cache, there is a significant decrease in the cache hit ratio experienced when it is needed by the processor to access the same locations in the memory from two distinct memory pages repeatedly.
On the other hand, in a fully associative cache, the main memory block is mapped with any of the available blocks of the cache. This is because a fixed formula is used in its mapping. In a direct mapped cache, there can be only one possible location for every block in the cache organization from the main memory. On the other hand, in a fully associative cache, the main memory address is divided into Tag and Word fields. The Block and the Word together make the Index. In a direct mapped cache, there are three particular fields in which the memory address is divided into, namely, Tag, Block, and Word. In other words, the cache control logic needs to look at every tag of the block in order to find a match and also find out whether or not a block is in the cache at the same time. On the other hand, in a fully associative cache, it needs comparison with every tag bit to find a match.
In a direct mapped cache, there is only one comparison required by using a direct formula to find the effective cache address for mapping. On the other hand, in a fully associative cache, each block of the main memory is loaded into any available line of cache. In simple terms, a direct mapped cache is that where each block of the main memory is mapped into only one particular cache line possible. Direct Mapped Cache vs a Fully Associative Cache – The 13 Differences A fully associative cache has high hardware requirements and is therefore not easy to build.The direct mapped cache is simplest of all mapping techniques and is less expensive.In a direct mapped cache the memory address is divided into three fields such as Tag, Word, and Block but in fully associative cache there are only Tag and Word fields.Which One is Better to Use – Direct Mapped Cache or Fully Associative Cache?.Direct Mapped Cache vs a Fully Associative Cache – The 13 Differences.
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