Virtual Memory PDF

Virtual Memory Virtual Memory Definition: Virtual memory is a memory management technique that creates an illusion of a large, continuous block of memory for the user , exceeding the actual physical memory (RAM) available. It achieves th is by using a combination of hardware and software to...

Virtual Memory Virtual Memory Definition: Virtual memory is a memory management technique that creates an illusion of a large, continuous block of memory for the user , exceeding the actual physical memory (RAM) available. It achieves th is by using a combination of hardware and software to map virtual ad dresses to physical addresses.  Why We Need Virtual Memory: Efficient Memory Utilization: Definition: Virtual memory allows for more efficient use of physical memory by allocating space only when needed. Benefit: Programs can use more memory than physically available, avoiding the need to load entire programs into RAM at once. Isolation and Security: Definition: Each process operates in its own virtual address space. Benefit: Enhances security and stability by isolating processes, preventing one from accessing or corrupting the memory of another. Convenience and Flexibility: Definition: Simplifies memory management for programmers. Benefit: Provides a large, contiguous address space regardless of actual physical memory layout. Handling Large Programs: Definition: Allows execution of programs larger than physical memory. Benefit: Breaks programs into smaller, manageable pages, making it feasible to run large applications on limited RAM. Is Virtual Memory Part of RAM or Secondary Memory? Combination of Both: Definition: Virtual memory uses both RAM and secondary storage (e.g., hard drive, SSD). RAM: Stores actively used pages. Secondary Memory: Stores inactive pages. Mechanism: Paging: Moves pages between RAM and secondary storage as needed. Swapping: Transfers entire processes between RAM and disk when memory is limited. Advantages: Expanded Memory: Virtually increases available memory. Cost-Effective: Uses cheaper secondary storage to extend memory. From Big Program to Processes and Pages Process Conversion: Definition: Large programs are divided into smaller processes. Execution: Each process can be executed independently and schedule d by the operating system. Page Conversion: Definition: Processes are further divided into fixed-size pages. Size Match: Pages are typically the same size as cache lines or as defin ed by the system (e.g., 4KB). Terminologies Virtual Address: Definition: Address generated by the CPU, used by the program. Purpose: Provides a consistent and isolated address space for each proc ess. Physical Address: Definition: Actual address in physical memory (RAM). Purpose: Location where data is actually stored. Translation Mechanism: Page Table: Maps virtual addresses to physical addresses. Translation Lookaside Buffer (TLB): Caches recent translations for faster access. Pages: 1. The virtual memory is divided into fixed-size blocks called pages. 2. Each page typically ranges from 4KB to 4MB. Page Frames: 1. The physical memory is divided into fixed-size blocks called frames. 2. A page is mapped to a page frame when it is loaded into physical memory. Page Table: 1. A data structure used to store the mapping between virtual addresses and physical a ddresses. 2. Each process has its own page table. Translation Lookaside Buffer (TLB): 1. A cache that stores recent translations from virtual memory to physical memory. 2. Speeds up address translation. 1.Initial Setup: 1. Three processes, each divided into pages. 2. Physical memory (RAM) divided into frames. 3. Cache lines are assumed to be empty.  Loading a Memory Block into RAM: Let's say Process 1's Page 1 needs to be loaded into RAM. The operating system (OS) selects an empty frame (e.g., Frame 0) and loads Page 1 into it. The page table is updated to reflect this mapping.