Introduction to Paged Virtual Memory

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.