The simplest way of associating main memory blocks with cache block is the direct mapping technique. In this technique, block k of main memory maps into block k modulo m of the cache, where m is the total number of blocks in cache. In this example, the value of m is 128. In direct mapping technique, one particular block of main memory can be transfered to a particular block of cache which is derived by the modulo function.

Since more than one main memory block is mapped onto a given cache block position, contention may arise for that position. This situation may occurs even when the cache is not full. Contention is resolved by allowing the new block to overwrite the currently resident block. So the replacement algorithm is trivial.

The detail operation of direct mapping technique is as follows:

The main memory address is divided into three fields. The field size depends on the memory capacity and the block size of cache. In this example, the lower 5 bits of address is used to identify a word within a block. Next 7 bits are used to select a block out of 128 blocks (which is the capacity of the cache). The remaining 4 bits are used as a TAG to identify the proper block of main memory that is mapped to cache.

When a new block is first brought into the cache, the high order 4 bits of the main memory address are stored in four TAG bits associated with its location in the cache. When the CPU generates a memory request, the 7-bit block address determines the corresponding cache block. The TAG field of that block is compared to the TAG field of the address. If they match, the desired word specified by the low-order 5 bits of the address is in that block of the cache.