Final answer:
A direct mapping structure with a 128KB cache and 128MB memory size can be designed. The address limits of main memory, line numbers of cache, block numbers, and number of tag bits can be calculated. The structure has advantages of simplicity and low cost, but also drawbacks of cache conflicts.
Step-by-step explanation:
In a direct mapping structure, each block of main memory is mapped to a specific line in the cache. Given a 128KB cache and 128MB memory, we need to determine the address limits of main memory, line numbers of cache, block numbers, and number of tag bits.
The cache size is 128KB, which is equivalent to 2^17 bytes. As the cache is direct mapped, there will be 2^17 cache lines. Each cache line contains a block from main memory.
Now, let's calculate the block size. The main memory size is given as 128MB, which is equivalent to 2^27 bytes. To determine the block size, we divide the memory size by the number of cache lines. In this case, the block size is 2^10 bytes.
Next, we can calculate the number of blocks (block numbers) in main memory. Dividing the main memory size by the block size gives us 2^17 blocks.
In a direct mapping structure, the tag bits are used to identify the location of a block in main memory. In this case, since there are 2^17 blocks, we need 17 bits for the tag.
The advantage of a direct mapping structure is its simplicity and low cost. Each block in main memory has a fixed location in the cache, making it easy to determine where to store and retrieve data. The drawback is that if multiple memory addresses map to the same cache line, there will be cache conflicts, leading to reduced cache performance.