Let’s break this down clearly—your question gets to a core detail of how hierarchical caches interact, so let’s walk through the two key scenarios that play out when an L1 cache miss occurs.

First, let’s recap the specs we’re working with to align on basics:

• L1 cache block size: 4 words
• L2 cache block size: 16 words (meaning one L2 block holds 4 full L1 blocks)
• Data bus between L1 and L2: 4 words wide (can move an entire L1 block in one bus cycle)

情况1：L2缓存命中时的传输流程 #

When an L1 miss happens, the CPU first checks if the needed data exists in L2. If it does (an L2 hit):

• Since the bus width exactly matches the L1 block size, we don’t need multiple transfers. The L2 cache sends the full 4-word L1 block over the bus in a single cycle directly to L1.
• No memory access is required here at all—all data movement stays within the cache hierarchy.

情况2：L2缓存缺失时的传输流程 #

If the data isn’t in L2 (an L2 miss), we have to reach out to main memory:

1. The cache controller requests the entire 16-word L2 block from main memory in one logical memory access. Memory is optimized to serve larger block sizes efficiently, so it sends all 16 words to L2 in one burst (counted as a single memory access).
2. Once the 16-word block is stored in L2, the controller sends the specific 4-word subset that L1 needs over the bus to L1—again, this is a single bus transfer because the bus width matches the L1 block size.
3. Total memory accesses here: 1, not 4. The 4-word transfer to L1 is just moving data within the cache hierarchy, not hitting memory again.

关键误区澄清 #

You asked if we’d need 4 memory accesses to get 4 words to L1—this isn’t the case for two reasons:

• Memory is accessed in chunks matching the lower-level cache’s block size (L2’s 16 words here), not the upper-level’s. This leverages spatial locality: if you need one word from a block, you’re likely to need nearby words soon, so grabbing the whole L2 block at once is more efficient.
• The L1-L2 bus is sized to move an entire L1 block at once, so once the data is in L2, transferring to L1 is a single, quick step.

内容的提问来源于stack exchange，提问作者NoSuchUserException