L-3.6: Direct Mapping with Example in Hindi | Cache Mapping | Computer Organisation and Architecture

Summary

In this video, the concept of Direct Mapping in cache memory is thoroughly explained. The speaker begins by reviewing the architecture of main memory and cache, highlighting how words and blocks are organized. The focus is on understanding how CPU addresses are generated, divided into block number and block offset, and how the K mod N formula determines which cache line a block will be mapped to. Additionally, the video covers cache hits and misses, as well as how cache replacement works. The session concludes with an invitation to explore related numericals in the next video.

Takeaways

• 😀 Direct mapping is a cache mapping technique where blocks of memory are mapped to specific lines in the cache, using the formula K mod N, where K is the block number and N is the number of cache lines.
• 😀 In direct mapping, the main memory is divided into blocks, and each block can only be mapped to one specific cache line based on its block number.
• 😀 Main memory in the example consists of 128 words, divided into blocks of 4 words each, resulting in a total of 32 blocks.
• 😀 Cache memory in this example is 16 words, which is divided into 4 lines, each holding 4 words, and the line number is determined using the mod formula.
• 😀 Direct mapping uses a fixed mapping between memory blocks and cache lines, meaning each block can only fit into one specific line in the cache.
• 😀 When the CPU generates an address, the address is divided into block number and block offset to identify the specific memory location and word within the block.
• 😀 Physical addresses in memory are represented by 7 bits, with 5 bits for the block number and 2 bits for the block offset, as each block contains 4 words.
• 😀 Cache addresses are divided into tag, line number, and block offset. The tag indicates which block in memory the line corresponds to, the line number specifies the cache line, and the block offset points to the specific word within the block.
• 😀 When a CPU accesses a word, it uses the physical address to find the block number and offset, and if the block is found in the cache, it’s a cache hit; otherwise, it’s a cache miss.
• 😀 Cache misses trigger a cache replacement, where the current block in the cache is replaced with the block needed by the CPU. The process ensures that the most relevant blocks are always in the cache for faster access.

Q & A

What is direct mapping in cache memory?

Direct mapping is a technique used in cache memory where each block of main memory is mapped to exactly one line in the cache. The mapping is determined using a formula K mod N, where K is the block number, and N is the number of cache lines. This ensures that each block from memory is placed in a specific cache line.

How is the block number determined in direct mapping?

The block number in direct mapping is determined by dividing the total number of words in main memory into blocks. Each block consists of a fixed number of words. The block number is calculated using the formula K mod N, where K is the block number and N is the total number of cache lines.

How are words and blocks organized in main memory?

In the example, main memory is divided into blocks, with each block containing 4 words. There are a total of 128 words in the main memory, so the memory is divided into 32 blocks, labeled 0 to 31, with each block containing consecutive words.

What is the role of the 'mod' operation in direct mapping?

The 'mod' operation in direct mapping is used to determine which cache line a particular block from main memory will be mapped to. The block number (K) is divided by the total number of cache lines (N), and the remainder of the division gives the corresponding cache line.

How is the size of cache and its organization in lines explained?

The cache size is given as 16 words, which is divided into 4 lines, with each line containing 4 words. This organization ensures that the cache can hold a total of 16 words, and the lines are mapped to corresponding blocks of main memory.

What is the significance of the 'block offset' in memory addressing?

The block offset specifies the position of a particular word within a block. For a block containing 4 words, 2 bits are used to represent the block offset, where the values 00, 01, 10, and 11 are used to address the words within the block.

How does the CPU use the physical address to access data in memory?

The CPU generates a physical address, which is divided into two parts: the block number and the block offset. The block number helps locate the block in main memory, and the block offset determines the specific word within the block. The physical address is 7 bits long in this case, with 5 bits for the block number and 2 bits for the block offset.

What happens when a block is referenced in the cache?

When a block is referenced in the cache, the cache first checks if the block is already present. If the block is found, it is a cache hit, and the data is retrieved. If the block is not found, it is a cache miss, and the block is fetched from the main memory, potentially replacing an existing block in the cache.

How is the tag used in cache memory addressing?

The tag in cache memory helps identify which block from main memory is currently stored in a cache line. It is part of the address format and helps differentiate between blocks that are mapped to the same cache line. The tag, line number, and block offset together make up the full address in the cache.

What is the difference between a cache hit and a cache miss?

A cache hit occurs when the CPU requests data that is already present in the cache, allowing for faster access. A cache miss occurs when the requested data is not in the cache, causing the data to be fetched from main memory. In the case of a miss, the cache may also replace an existing block with the new block from memory.