Virtual Memory is like a Library Catalog
- Physical Memory (RAM) is the actual library with a limited number of shelves and books. All the programs on your computer have to share these same shelves.
- Virtual Memory is like a personal, magical catalog given to every person (or program) who enters the library. This catalog lists every single book the library system owns, making it seem like you have the entire library all to yourself.
Note
Your program interacts with this magical catalog (virtual memory), not the actual, crowded library shelves (physical memory).
Important
The computer’s Virtual Memory Manager (VMM) is the librarian who takes your request from the catalog and finds the actual book on the physical shelf for you.
This system:
- Prevents programs from messing with each other’s books
- Makes memory management much easier
Reserved vs. Committed Memory: Booking a Table 🍽️
Let’s think about going to a restaurant.
-
Reserved Memory: You call the restaurant and say, “I’d like to reserve a table for four at 7 PM.”
The restaurant now holds that table for you. No one else can take it. But right now, it’s just an empty table. You haven’t used any of the restaurant’s resources (food, waiter’s time, etc.) yet. You’ve simply claimed the space. -
Committed Memory: You arrive at your table and place your order.
Now, the kitchen has to actually get the ingredients (physical storage) and cook your meal. You are now committing to using the restaurant’s resources.
Note
To store your data in memory, you first reserve the space, and then you commit it so the system backs it up with real storage.
Paging: Your Desk vs. a Filing Cabinet 🗄️
Imagine your desk is your computer’s fast RAM, and a big filing cabinet in the corner is your slower hard drive.
- You keep the documents you are actively working on right on your desk (this is your working set). It’s super fast to grab them.
- When your desk gets too cluttered with documents you aren’t using, you move them to the filing cabinet to free up space.
- If you suddenly need one of those documents from the cabinet, you have to get up, walk over, find it, and bring it back to your desk. This is much slower.
Note
This is exactly what paging is.
When your RAM gets full, the computer moves the least-used “pages” of memory to a “page file” on the hard drive. Getting that data back is a slow operation that can make your computer feel sluggish.