Introduction to Paged Virtual Memory

Study Notes

Introduction to Paged Virtual Memory

Paged virtual memory is a memory management technique that allows a computer to use more memory than physically available.
It achieves this by dividing the program's address space and available physical memory into fixed-size blocks called pages.
Each program's address space is called a virtual address space; physical memory is called physical address space.

Key Concepts

Virtual Address Space: The logical address space a process sees, independent of physical memory. Each process has its own virtual address space.
Physical Address Space: The actual physical memory available to the computer.
Page Table: A data structure mapping virtual page numbers to physical page numbers. Crucial for translating virtual addresses to physical addresses.
Page Frame: A fixed-size block of physical memory, same size as a page.
Page Fault: An interrupt when a requested page isn't in physical memory, requiring loading from secondary storage (e.g., hard drive).
Page Replacement Algorithm: A method for choosing which page in physical memory to replace when a new page needs to be loaded. Different algorithms impact performance.

Addressing Mechanism in Paged Virtual Memory

Memory access involves two steps:
- Translation of the virtual address to a physical address.
- Accessing the data at the translated physical address using the page table.

Page Table Structure and Management

The page table holds mappings between virtual page numbers and physical page numbers.
Typically implemented in hardware for faster translation.
Page table entries usually include:
- The physical frame number if the page is in memory.
- Status indicators (present/absent, read/write permissions).
- Other control bits.
The page table itself resides in main memory due to its size, requiring address translation cycles that need optimization.

Page Replacement Algorithms

FIFO (First-In, First-Out): Replaces the page in memory longest resident. Simple but often underperforming.
LRU (Least Recently Used): Replaces the least recently used page. Attempts to predict future page needs, ideally performing better than FIFO.
Optimal Algorithm: Replaces the page that won't be used for the longest time in the future. Theoretically optimal but impractical since the future isn't predictable.

Benefits of Paged Virtual Memory

Memory protection: Prevents processes from interfering with each other's memory.
Memory sharing: Allows processes to share pages, saving memory.
Efficient memory use: Enables programs larger than physical memory to run by swapping pages in and out.
Portability: The virtual address space is independent of physical memory, enhancing portability.

Challenges and Issues

Overhead: Translation lookaside buffers (TLBs) and page tables introduce processing time.
Performance issues: Page faults significantly impact performance when using secondary storage.
Complexity: Implementing and managing paged virtual memory is complex.
Page table management: Efficient strategy required to deal with large page tables' impact.

Relation to Secondary Storage

Paged virtual memory heavily relies on secondary storage (like hard drives) to store pages not currently in main memory.
Facilitates running larger processes by moving pages between memory and disk.
Disk swapping affects performance, creating a balance between memory use and performance.

Description

Explore the essential concepts of paged virtual memory, a memory management technique that enables computers to use more memory than physically available. Learn about virtual address space, physical address space, page tables, page frames, and page faults. This quiz will test your understanding of these foundational topics in computer memory management.