An example of a virtual memory system is a computer operating system that allows a program to use more memory than is physically available (RAM).
Let's break down how this works:
How Virtual Memory Works
- Virtual Address Space: The operating system creates a "virtual" address space that is larger than the actual physical memory (RAM). This is the address space programs use.
- Paging: The virtual address space is divided into fixed-size blocks called "pages." Physical memory is also divided into corresponding blocks called "frames."
- Mapping: The operating system maintains a mapping between virtual pages and physical frames. Not all virtual pages need to be resident in physical memory at any given time.
- Secondary Storage (Disk): When a program tries to access a virtual page that is not currently in RAM (a "page fault"), the operating system retrieves the page from secondary storage (like a hard drive or SSD) and loads it into a physical frame. If all frames are occupied, a less recently used page is swapped out to the disk to make room.
Example Scenario
Suppose a program requires 5 GB of memory to run, but the computer only has 4 GB of RAM. Here's how a virtual memory system enables the program to execute:
- The operating system creates a virtual address space of 5 GB for the program.
- The program starts executing. Initially, only the necessary pages are loaded into the 4 GB of RAM.
- If the program tries to access data located in a virtual page that is not currently in RAM (because it's stored on the hard drive), a page fault occurs.
- The operating system handles the page fault by:
- Finding a free frame in RAM. If no frame is free, it selects a page to swap out (based on a replacement algorithm like Least Recently Used or LRU).
- Writing the contents of the selected frame to the hard drive (if it has been modified).
- Reading the required virtual page from the hard drive into the freed frame.
- Updating the page table to reflect the new mapping.
- The program can then resume execution as if all 5 GB of memory were physically present.
Benefits of Virtual Memory
- Allows running larger programs: Programs can run even if their memory requirements exceed the available physical RAM.
- Increased Multitasking: Enables multiple programs to run concurrently, even if their combined memory needs exceed RAM.
- Memory Protection: Prevents programs from accessing memory belonging to other programs, improving system stability and security.
Drawbacks of Virtual Memory
- Performance Overhead: Page faults and swapping can significantly slow down program execution, especially if the program frequently accesses pages not in RAM (thrashing).
- Increased Complexity: Virtual memory adds complexity to the operating system.
In summary, virtual memory creates the illusion of more RAM than is physically present by utilizing secondary storage and sophisticated memory management techniques.
