11.2.3. Physical Page Frame Number Mapping

A process generates virtual addresses to access memory. The CPU must convert these virtual addresses to physical memory addresses to retrieve the required data.

Virtual addresses are used by the CPU to access memory, but they need to be converted to physical addresses to actually access the data stored in memory. This conversion process is essential for efficient computing as the CPU works with virtual addresses while the memory storage is based on physical addresses.

Consider ordering a book from a library. You use the library's catalog (virtual address) to find the book's location on a shelf (physical address). Just like you need the actual shelf location to retrieve the book, the CPU needs to convert virtual addresses to physical addresses to access data.

The virtual address consists of two parts: the page offset and the virtual page number. The page offset is directly translated without modification since the page sizes in both virtual and physical memory are the same.

The virtual address is divided into two components: the page number, which identifies which page the data resides in, and the page offset, which specifies the exact location of the data within that page. Since the size of pages in both virtual memory and physical memory is the same, the offset can be used directly without any conversion.

Imagine a file cabinet where each drawer is a page. The drawer number corresponds to the virtual page number and the exact file within that drawer corresponds to the page offset. You need both to find your document effectively.

Each process has a page table indexed by the virtual page number. When the valid bit is 1 in the page table entry, it indicates a valid mapping to a physical page frame number.

The page table acts as a mapping device between virtual addresses and physical addresses. It contains entries for each virtual page number, and if the valid bit in an entry is set to 1, it indicates that the corresponding entry has a valid physical page frame number. If the valid bit is 0, it signifies that the page is not present in the physical memory.

Think of the page table as a directory for a hotel. The valid bit indicates whether a room (physical frame) is available. If the room is available (valid bit is 1), you can check in (access the data). If it’s not (valid bit is 0), you can’t check in because the room isn’t ready yet.

If the valid bit is 0, the page is not present in physical memory and must be fetched from secondary storage. The page table must also contain the disk address of the page.

When the valid bit is 0, it indicates that the requested virtual page is not currently in physical memory. In this case, the system must locate the page on secondary storage (e.g. a hard drive) and bring it into RAM. This process can slow down operations if it happens frequently, known as a page fault.

Imagine you are looking for a book but realize it's not in your library (physical memory). You’ll have to order it from an online store (secondary storage), which takes time, comparing it to fetching data from memory.

We maintain two distinct data structures: one to map virtual addresses to physical memory page frames and another to map virtual addresses to secondary storage page addresses.

Maintaining two page tables helps efficiently manage the mapping of virtual pages to their respective locations, whether in physical memory or on disk. This system distinguishes between pages currently in use and those that need to be retrieved from secondary storage.

Consider a restaurant menu that lists available items (physical memory) and also provides a special request section for items that need to be ordered from a supplier (secondary storage). This way, customers know what’s immediately available and what needs to be fetched.

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