Write Back Cache Quiz - Georgia Tech HPCA Part 3

Summary

This video script focuses on a quiz involving a direct-mapped write-back cache. The cache starts with a valid bit of 0, a dirty bit of 1, and an initial tag set to 'A'. The processor performs a sequence of memory accesses: reading and writing to memory locations A, B, C, and D. The quiz challenges viewers to determine the new state of the cache after these operations, as well as the number of cache misses and write-backs that occur during the process. This scenario tests the understanding of cache behavior and memory management in a write-back cache system.

Takeaways

• 😀 The quiz focuses on write-back caches and their behavior during memory accesses.
• 😀 The cache in the scenario is a direct-mapped cache, where multiple accesses map to a single cache entry.
• 😀 Initially, the cache's valid bit is set to 0, the dirty bit to 1, and the tag is set to A.
• 😀 The processor performs the following sequence of accesses: reads A, reads B, writes B, reads C, reads D, writes D.
• 😀 All the memory locations A, B, C, and D map to the same cache entry in this direct-mapped cache.
• 😀 The quiz asks for the new state of the cache after all these memory accesses.
• 😀 The number of cache misses must be determined after completing the sequence of accesses.
• 😀 The number of write-backs to memory must also be calculated based on the sequence of writes.
• 😀 The valid bit indicates whether the data in the cache is valid, while the dirty bit shows if the cache data has been modified.
• 😀 Write-back caches involve writing modified data back to memory only when it's evicted, which is a key aspect of this scenario.

Q & A

What is the initial state of the cache when the sequence begins?

The initial state of the cache is as follows: the valid bit is 0 (invalid), the dirty bit is 1 (indicating the cache is dirty), and the tag is set to A.

What happens when the processor reads A for the first time?

Since the valid bit is 0, this causes a cache miss. The cache is then updated with A, the valid bit becomes 1, and the tag is set to A.

What occurs when the processor reads B after A?

Since B maps to the same cache entry as A, a cache miss occurs. The cache is updated with B, the valid bit remains 1, and the tag is updated to B.

What happens when the processor writes to B?

Since B is already in the cache, the write operation marks the cache entry for B as dirty (the dirty bit is set to 1). No cache miss occurs during this write.

How does the cache behave when the processor reads C?

Since C also maps to the same cache entry as A and B, a cache miss occurs. The cache is updated with C, the valid bit remains 1, and the tag is updated to C.

What happens when the processor reads D?

When the processor reads D, a cache miss occurs because D maps to the same cache entry. The cache is updated with D, the valid bit remains 1, and the tag is updated to D.

What occurs when the processor writes to D?

Since D is already in the cache, the cache entry for D is marked as dirty (the dirty bit is set to 1). No cache miss occurs during this write.

What is the final state of the cache after all operations?

After all the operations, the final cache state has the valid bit set to 1 (the cache is valid), the dirty bit set to 1 (indicating it needs to be written back), and the tag set to D (the cache holds D).

How many cache misses occurred during the sequence of operations?

A total of 4 cache misses occurred during the sequence: one for A (initial load), one for B (first read), one for C, and one for D.

How many write-backs to memory will occur during this sequence?

Two write-backs will occur during this sequence, for A and B, as they were marked as dirty before being evicted from the cache.