14.3.3. Possible and Impossible Cases

During a page fault when I do not have the required data in memory, I incur a page fault. The page table entry corresponding to the page I want to access shows that it is invalid. This means the valid bit is 0, indicating that the corresponding physical page number is not mapped to the page table entry.

A page fault occurs when the operating system accesses a part of memory that is not currently in RAM. The page table, which tracks where data is stored, indicates that the desired data isn't available by showing a valid bit of 0. This indicates that the data must be loaded from disk or secondary storage into RAM.

Think of it like searching for a book in a library. If the book is on the shelf, you find it easily. But if it's checked out or misplaced, the library system tells you it's not available, and you’ll need to wait for it to be returned before accessing it.

I need to decide whether this translation that I want to do is for an invalid reference or for a valid reference. If it is an invalid reference, that indicates the virtual address is not part of the virtual address space for the process itself. If the valid bit is 0 and the reference is valid, then the page is just not in memory and it has to be brought from disk.

When a page fault occurs, the operating system must determine if the request is valid. If it isn't a valid request (the data isn't part of the process's address space), the system will abort the operation. If it is valid but still not in memory, it means the system will have to retrieve the data from storage.

Imagine you're ordering a pizza. If you order a pizza that doesn't exist on the menu (invalid reference), the restaurant will let you know they can't fulfill that order. However, if you order something on the menu that they just don't have in stock currently (valid reference), they will go and fetch it from the storage.

If a valid reference is made and it is a page fault, the operating system must find a physical page frame in memory to load the page into. It may involve replacing an existing page that is currently in memory and swapping the needed page from the disk to RAM.

When a valid reference results in a page fault, the operating system goes through a series of steps. It finds a spot in RAM to put the new data. If RAM is full, it may need to remove some existing data (page replacement) to make room for the new information being fetched from disk.

This is similar to when your backpack is full, and you need to take something out to put in a new book you just bought. You can't carry everything at once, so you choose to remove something that you don’t need as much.

Once the desired page is loaded into memory, the OS must update the page table to indicate the new page's physical address and set the valid bit to 1, showing that the page is now in memory.

After the data is successfully loaded into RAM, the system finishes by updating the page table. This update gives the address of the newly loaded page in RAM and changes its status from invalid to valid, showing it is now ready to be used in processing.

Consider this like updating your calendar. Once an important meeting is confirmed and added to your schedule, you mark it down as confirmed. Before it was just a tentative note that might not happen, but now it's official.

After updating the page table, the instruction that caused the page fault can be restarted, and the data can be accessed from physical memory.

Once everything is in place and updated, the system goes back to the instruction that originally triggered the page fault. With the needed data now in memory, the instruction can be processed without any further interruptions.

This resembles a situation where you stop to refill your gas tank while on a road trip. Once you're done filling up, you can get back on the road and continue your journey without any other delays.

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