Consider a system that has a cache memory unit and a memory management unit (MMU). The address input to the cache memory is a physical address. The MMU has a translation lookaside buffer (TLB). Assume that when a page is evicted from the main memory, the corresponding blocks in the cache are marked as invalid. For a given memory reference, which of the following sequences of events can NEVER happen?

Let us analyze the validity of each sequence of events:

1. Option A (TLB miss, Page table hit, Cache hit):

   • A TLB miss occurs, so the MMU accesses the page table in main memory.

   • The page is present in main memory (Page table hit), yielding the physical address.

   • The physically addressed cache is searched with the physical address and finds the block (Cache hit).

   • This is a valid and common scenario.

2. Option B (TLB hit, Page table miss, Cache hit):

   • A TLB hit means the virtual-to-physical address mapping is present and valid in the TLB, which implies the corresponding page must be in main memory.

   • A page table miss (page fault) cannot occur if the mapping is present and valid in the TLB (and the page table is not even consulted on a TLB hit).

   • Hence, this sequence can NEVER happen.

3. Option C (TLB miss, Page table miss, Cache hit):

   • A page table miss means the page is not present in main memory (a page fault).

   • Since the problem states that when a page is evicted from main memory, its corresponding blocks in cache are invalidated, a block belonging to a page not present in main memory cannot produce a Cache hit.

   • Hence, this sequence can NEVER happen.

4. Option D (TLB miss, Page table miss, Cache miss):

   • The page is not in the TLB, not in main memory (page fault), and not in the cache.

   • This is the standard sequence for a page fault.

   • This is a valid scenario.

Therefore, the sequences that can never happen are B and C.