Associative Mapping

Summary

In this session on computer organization and architecture, the lecturer explores the concept of **associative memory mapping** to resolve **conflict misses** in cache systems. The lecture covers various types of cache misses—compulsory, conflict, and capacity—explains their causes, and discusses the advantages and challenges of fully associative mapping. The session also includes a hardware implementation example, where a physical address is used to calculate **hit latency** in a fully associative memory system. This is followed by a detailed breakdown of how to approach numerical problems related to associative mapping in exams.

Takeaways

• 😀 Conflict misses occur in direct memory mapping when multiple memory blocks are assigned to the same cache line, leading to data eviction and replacement.
• 😀 Cache misses can be categorized into compulsory misses, conflict misses, and capacity misses, with compulsory misses being unavoidable on the first access to a block.
• 😀 Conflict misses, also known as collision or interference misses, happen when blocks previously loaded into the cache are evicted and need to be reloaded.
• 😀 Capacity misses happen when the cache is too small to hold the working set, leading to frequent evictions even though space is available in the cache.
• 😀 The associative memory mapping technique, also known as fully associative mapping, resolves conflict misses by allowing any block to be assigned to any cache line.
• 😀 In fully associative mapping, the physical address is split into tag bits and block/line offset, with the tag bits used to identify the memory block in the cache.
• 😀 The major drawback of associative mapping is the increased hit latency, as all cache lines must be compared to check for a cache hit.
• 😀 A potential solution to reduce the hit latency in associative mapping involves using multiple comparators in parallel, but this increases hardware cost.
• 😀 The hardware implementation of associative mapping involves using comparators and multi-input OR gates to detect cache hits, although XOR gates would be more efficient but costlier.
• 😀 To calculate the hit latency in fully associative memory mapping, we consider the delay of the comparators and the OR gate, with the final hit latency being the sum of these delays.
• 😀 Numerical problems involving associative memory mapping often require calculations based on the physical address size, block size, and comparator delays to determine the hit latency.

Q & A

What is the primary objective of the associative memory mapping technique?

The primary objective of associative memory mapping is to resolve the conflict miss problem that arises in direct memory mapping by allowing any block to be mapped to any cache line, providing greater flexibility in memory access.

What are the main types of cache misses discussed in the session?

The main types of cache misses discussed are compulsory misses (cold misses), conflict misses (collision or interference misses), and capacity misses. Additionally, coherence and coverage misses are briefly mentioned.

How do compulsory misses occur in cache memory systems?

Compulsory misses occur when a memory block is referenced for the first time. These misses cannot be avoided unless the block has been prefetched into the cache.

What are conflict misses, and how do they impact direct memory mapping?

Conflict misses occur when a cache line is evicted and replaced with a new block, causing previously stored blocks to be evicted. In direct memory mapping, multiple memory blocks may be mapped to the same cache line, leading to conflict misses.

What is the solution to the conflict miss problem in direct mapping?

The solution to conflict misses in direct mapping is associative mapping, where any memory block can be mapped to any cache line, thus eliminating mapping conflicts.

What is fully associative mapping, and how does it differ from direct mapping?

Fully associative mapping allows any block to be mapped to any cache line, using the entire block number bits as tag bits. Unlike direct mapping, which assigns specific blocks to fixed cache lines, fully associative mapping offers more flexibility.

What is the main disadvantage of fully associative mapping?

The main disadvantage of fully associative mapping is that it increases hit latency and hardware cost due to the need for multiple comparators and complex logic circuits to identify which cache line contains the required block.

How does the hardware implementation of associative mapping work?

In the hardware implementation of associative mapping, the physical address is divided into tag bits and block offset bits. The tag bits are compared to those in the cache using multiple comparators. If a match is found, a cache hit occurs.

What is the significance of the OR gate in associative mapping hardware?

The OR gate in associative mapping hardware is used to combine the results from the multiple comparators to determine if a cache hit occurs. An OR gate is chosen over an XOR gate due to cost considerations.

How is the hit latency calculated in fully associative memory mapping?

The hit latency in fully associative memory mapping is calculated by adding the delay caused by the comparators and the delay from the OR gate. For example, with a comparator delay of 15 ns and 20 tag bits, the total hit latency would be 307 ns, considering the OR gate delay.