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 MMUhasatranslation lookaside buffer (TLB). Assume that when a page is evicted from the main memory, the corresponding blocks in the cache are marked as in valid. For a given memory reference, which of the following sequences of events can NEVER happen?

The correct sequences of events that can NEVER happen are:

1. TLB hit, Page table miss, Cache hit

2. TLB miss, Page table miss, Cache hit

Let's analyze why these sequences are impossible step-by-step:

1. Understanding the Components:
- Translation Lookaside Buffer (TLB): A high-speed cache that stores recent virtual-to-physical address translations. Since it is a subset of the Page Table, a translation can only exist in the TLB if it already exists in the Page Table.
- Page Table: A data structure in main memory that maps virtual addresses to physical addresses for all pages currently resident in main memory. A page table hit means the page is present in main memory. A page table miss (page fault) means the page is not in main memory.
- Cache Memory: A small, fast memory unit that holds copies of data from frequently used main memory locations. In this system, the cache is indexed using physical addresses.

2. Analyzing "TLB hit, Page table miss, Cache hit":
- A TLB hit means the translation from virtual address to physical address was successfully found in the TLB.
- Since the TLB only caches valid translations that exist in the page table, a TLB hit implies that the translation must also exist in the Page Table (a Page Table hit). Therefore, a Page table miss cannot occur simultaneously with a TLB hit.
- Furthermore, if there were a Page table miss (meaning the page is not in main memory), the page would have to be loaded from disk. Since the cache is physically addressed, and the corresponding blocks are marked invalid upon page eviction, we cannot have a Cache hit for a page that is not present in memory. Thus, this sequence is impossible.

3. Analyzing "TLB miss, Page table miss, Cache hit":
- A TLB miss means the address translation is not in the TLB, so the system looks it up in the Page Table.
- A Page table miss indicates that the page containing the requested address is not present in main memory (a page fault occurs).
- When a page is not in main memory, any block belonging to that page cannot be in the cache. In fact, the problem states that when a page is evicted from main memory, its corresponding blocks in the cache are marked invalid. Therefore, it is impossible to have a Cache hit if there is a Page table miss. Thus, this sequence can never happen.