Design issues:īellow is a simple cache which holds 1024 words or 4KB, memory address is 32 bits. The disadvantage of the direct mapped cache is that it is easy to build, but suffer the most from thrashing due to the 'conflict misses' giving more miss penalty. CPU uses different replacement policies to decide which block is to replace. If a miss occur CPU bring the block from the main memory to the cache, if there is no free block in the corresponding set it replaces a block and put the new one. Select location from block using block offset.ĭiagram of a direct mapped cache (here main memory address is of 32 bits and it gives a data chunk of 32 bits at a time):.Select set using index, block from set using tag.The memory address is divided into 3 parts- tag(most MSB), index, block offset(most LSB) in order to do the cache mapping. Number of bits to identify the correct set: log 2(m).Each cache block has a tag saying whichīlock of memory is currently present in it, each cache block also contain a valid bit to ensure whether a memory block is in the cache block currently. There are three methods in block placement-Ī given memory block can be mapped into one andīlock identification: let the main memory contains n blocks(which require log 2(n)) and cache contains m blocks, so n/m different blocks of memory can be mapped (at different times) to a cache block. So a procedure is needed for mapping main memory blocks into cache lines.cache mapping scheme affects cost and performance. ![]() Miss: a cache access does not find data resident, so it forces to access the main memory.Ĭache treats main memory as a set of blocks.As the cache size is much smaller than main memory so the number of cache lines are very less than the number of main memory blocks.Hit: a cache access finds data resident in the cache memory.The notion of cache memory actually rely on the correlation properties observed in sequences of address references generated by CPU while executing a programm(principle of locality).When a memory request is generated, the request is first presented to the cache memory, and if the cache cannot respond, the request is then presented to main memory. Cache memory is a small (in size) and very fast (zero wait state) memory which sits between the CPU and main memory.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |