11.2. Cache controller

The cache controller is a hardware block responsible for managing the cache memory, in a way that is largely invisible to the program. It automatically writes code or data from main memory into the cache. It takes read and write memory requests from the core and performs the necessary actions to the cache memory or the external memory.

When it receives a request from the core, it must check to see whether the requested address is to be found in the cache. This is known as a cache look-up. It does this by comparing a subset of the address bits of the request with tag values associated with lines in the cache. If there is a match, known as a hit, and the line is marked valid, then the read or write occurs using the cache memory.

When the core requests instructions or data from a particular address, but there is no match with the cache tags, or the tag is not valid, a cache miss results and the request must be passed to the next level of the memory hierarchy, an L2 cache, or external memory. It can also cause a cache linefill. A cache linefill causes the contents of a piece of main memory to be copied into the cache. At the same time, the requested data or instructions are streamed to the core. This process occurs transparently and is not directly visible to a software developer. The core need not wait for the linefill to complete before using the data. The cache controller typically accesses the critical word within the cache line first. For example, if you perform a load instruction that misses in the cache and triggers a cache linefill, the core first retrieves that part of the cache line that contains the requested data. This critical data is supplied to the core pipeline, while the cache hardware and external bus interface then read the rest of the cache line, in the background.

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