This mapping technique is intermediate to the previous two techniques. Blocks of the cache are grouped into sets, and the mapping allows a block of main memory to reside in any block of a specific set. Therefore, the flexibity of associative mapping is reduced from full freedom to a set of specific blocks. This also reduces the searching overhead, because the search is restricted to number of sets, instead of number of blocks. Also the contention problem of the direct mapping is eased by having a few choices for block replacement.

Consider the same cache memory and main memory organization of the previous example. Organize the cache with 4 blocks in each set. The TAG field of associative mapping technique is divided into two groups, one is termed as SET bit and the second one is termed as TAG bit. Each set contains 4 blocks, total number of set is 32. The main memory address is grouped into three parts: low-order 5 bits are used to identifies a word within a block. Since there are total 32 sets present, next 5 bits are used to identify the set. High-order 6 bits are used as TAG bits.

The 5-bit set field of the address determines which set of the cache might contain the desired block. This is similar to direct mapping technique, in case of direct mapping, it looks for block, but in case of block-set-associative mapping, it looks for set. The TAG field of the address must then be compared with the TAGs of the four blocks of that set. If a match occurs, then the block is present in the cache; otherwise the block containing the addressed word must be brought to the cache. This block will potentially come to the cooresponding set only. Since, there are four blocks in the set, we have to choose appropriately which block to be replaced if all the blocks are occupied. Since the search is restricted to four block only, so the searching complexity is reduced. The whole arrangement of block-set-associative mapping technique is shown in the figure 3.15.