11.5. Virtual Address Structure

In this lecture, we continue our discussion with virtual memories. A process generates virtual addresses in order to access memory. Therefore, the CPU generates virtual addresses, and to get the required data, this virtual address must be converted to a physical memory address.

Virtual addresses are used by a CPU when a process needs to access memory locations. The virtual address generated must be translated into a physical memory address to retrieve the actual data stored in RAM. This process is crucial for allowing multiple processes to use memory efficiently without interfering with each other.

Think of virtual addresses like a home address that directs a postal service to a mailbox in a large building. The virtual address is the home address, while the physical address is the actual location of the mailbox where the letters are delivered.

The virtual address is divided into two parts: the page offset and the virtual page number. The page offset is translated directly to the physical page offset without any modification. The virtual page number is then floated to the page table of the process.

A virtual address consists of a virtual page number and a page offset. The virtual page number helps locate the page in the page table, while the page offset specifies the exact location within that page. This division simplifies memory management and enables efficient data retrieval.

Imagine a library where the books are organized by section (the page number) and each page within a book is like a specific part (offset) of that section. To find a specific sentence in a book, you first need to know which section the book is in and then locate the page where that sentence can be found.

Each process has a page table that maps each virtual memory page to either a page in main memory or a page on disk. If the valid bit is 1, the page table supplies the corresponding physical page frame number. If it is 0, the page must be brought from secondary storage.

The page table is a crucial structure that holds the mapping of virtual pages to their physical counterparts. The valid bit indicates whether the mapping is legal; if it is set to 0, it means the desired page is not currently in RAM and must be fetched from disk. This system helps maintain the efficiency of memory usage.

Consider the valid bit as a library card. If the card is active (valid), you can check out a book (access physical memory). If the card is inactive (invalid), it means that the book is not available, and you need to request it from the library's reserve (disk storage).

Page table sizes can be huge. For a 32-bit computer with a page size of 4 KB, the page table can contain 1 million entries, consuming 4 MB of physical memory.

In modern computing, managing memory efficiently is critical. A 32-bit system can theoretically address a large amount of memory, leading to significant page table sizes. For example, with a 4 KB page size, each page table entry can demand considerable memory space, necessitating techniques to manage this effectively.

Picture a large school with many classrooms. If every classroom needs its own directory (like a page table), a large school could use up a lot of space just for those directories. Managing directory space effectively is as important as managing classroom resources.

The virtual memory of a process can grow over time as more data is required. A page table length register (PTLR) tracks the current size of the allocated virtual memory for a process.

As applications run, they may require more memory dynamically. The PTLR helps manage this growth effectively by indicating how much memory is currently allocated. This flexibility prevents wasting memory on pages that are not in use while still allowing applications to access more memory as needed.

Think of the PTLR as a suitcase that expands or shrinks based on the traveler’s needs. If the traveler (the application) buys more souvenirs (data), the suitcase can expand to fit them; if fewer items are bought, the suitcase can compress, saving space.

To handle scattered virtual memory, segmentation with paging allows for different modules to be organized, with separate page tables for each segment and length registers to manage their sizes.

This approach combines segmentation with paging to improve memory management and organization, allowing for logical grouping of related modules (like code and data). Each segment can now grow independently while keeping track of size and access, which optimizes memory usage.

Imagine a toolbox where each compartment is dedicated to a specific tool type (segments) and the tools within each compartment can be different sizes (paged). This organization makes it easier to find tools and allows each compartment to grow or shrink based on the number of tools needed.

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