Which information is used to check if a data is present in cache memory?

Question

BlackTom AI · Accepted Answer

In cache memory, a couple of key elements are used to determine whether the requested data is actually present in the cache and valid to use.
Option: Some other way
This choice is not standard practice for determining cache presence, as modern caches rely on explicit mechanisms like valid bits and tags rather than unnamed or ad hoc signals. Without a recognized mechanism, you cannot reliably distinguish cached data from misses, so this answer is not correct.
Option: Valid bit
The valid bit is a fundamental control flag associated with each cache line that indicates whether the data stored in that line is valid for use. If the valid bit is set, the line contains meaningful data; if it is cleared, the line is considered invalid and cannot be used to satisfy a memory request. This is a core component used to check data presence in a cache, so this option is correct.
Option: Tag bits
Tag bits are used in conjunction with index bits to identify whether the data in a cache line corresponds to the requested memory address. When a cache lookup occurs, the address is divided into tag, index, and offset; the tag from the address is compared to the stored tag in the cache line. A match along with a valid bit indicates a cache hit; a mismatch or a invalid line indicates a miss. Therefore, tag bits are essential for confirming presence and identity of cached data, making this option correct.
Option: Other control bits
While some caches may use additional control bits for various management tasks (e.g., dirty bits to indicate modified data, replacement policy bits, or coherence-related flags), these are not the primary indicators used to determine whether the requested data is present and valid in the cache. They serve secondary purposes rather than the fundamental presence check, so this option is not correct in the context of checking data presence.

Which information is used to check if a data is present in cache memory?

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