11.5.1. Three-Part Virtual Address

In this lecture, we continue our discussion with virtual memories. In the last lecture we said that a process generates virtual addresses in order to access memory; a process generates virtual addresses.

This introductory chunk sets the stage for understanding how processes use virtual addresses to access memory. It emphasizes that virtual addresses are essential for memory management in computer systems, allowing processes to request data without concerning themselves with the actual physical locations in memory.

Think of virtual addresses like the postal addresses for letters. Instead of sending a letter to a friend's specific house address (real memory), you write their name and postal code (virtual address) on the envelope. The postal service handles the delivery to the correct place.

This virtual address is divided into two parts: one is the page offset; the other is the virtual page number.

A virtual address comprises two critical components: the page offset and the virtual page number. The page offset specifies the exact location within a page, while the virtual page number identifies which page to reference in memory. This division helps in efficient memory management and data retrieval.

Imagine a library where each book is a page in a large volume. The virtual page number tells you which book to pull from the shelf (the library), and the page offset tells you which page within that book to read.

The page offset is directly translated... I generate the complete physical address.

To locate the physical address in memory, the system first retrieves the physical page frame number using the virtual page number. Then, it adds the page offset to this frame number to create a complete physical address. This process is crucial for data access and retrieval.

Continuing the library analogy, once you know which book (physical page frame) to take from the shelf, the offset is like knowing the specific page number to turn to in that book. Together, they allow you to find the information you need quickly.

The valid bit what does it tell us? It tells us whether the mapping is legal... physical memory and must be brought from the secondary storage.

The valid bit indicates whether a virtual page is currently mapped to a physical page in memory. If the valid bit is 1, the mapping is legal, and the data can be accessed. If it is 0, this means that the page is not in physical memory and must be loaded from secondary storage, like a hard drive.

Consider this like a library where some books are checked out (not in physical memory) and others are available. The valid bit is like a marker indicating whether a book is on the shelf or has been borrowed. If the book is borrowed, you'd need to wait until it's returned to access it.

We come to a discussion; we will now do a discussion on the structure of a page table... consuming 4 MB of space just for storing the page table.

This chunk explains the structure and size of page tables, which are used to map virtual pages to physical memory addresses. In modern computers, especially with a 32-bit addressing scheme, page tables can become large and require significant memory space, sometimes several Megabytes just for one process.

Imagine maintaining a detailed inventory of a large library. As more books (pages) are added, your inventory (page table) grows. If each book page requires separate entries and you have thousands of them, the inventory can take up significant space!

However, by using this page table length register... that is possibly addressable by the process.

The page table length register helps manage the size of the page table efficiently. It allows the operating system to adjust according to the actual size of the virtual address space used by a process, preventing unnecessary memory allocation for unutilized space.

Think of it as a folder that can expand or shrink based on the documents you are actively using. If you only need a few pages of a large capacity folder, it doesn't make sense to keep it full all the time.

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