Memory Management & Virtual Memory

Memory Management

Main Memory (RAM) is a critical resource. The CPU can only execute instructions that are currently loaded into RAM. The Operating System's Memory Manager is responsible for allocating space to processes when they start, and freeing it when they finish.

1. Fragmentation

1.1 External Fragmentation

• The Problem: There is enough total free memory in RAM to satisfy a request, but the memory is not contiguous (it is scattered in small chunks). Because programs usually need to be loaded in one continuous block, the OS cannot load the program.
• Solution: Compaction (shuffling memory contents to place all free memory together in one large block) or using Paging.

1.2 Internal Fragmentation

• The Problem: Memory is divided into fixed-sized blocks (e.g., 4KB). If a process only needs 3KB, it is given the whole 4KB block. The remaining 1KB inside the block is wasted.

2. Paging

• How it works:
  1. Physical memory (RAM) is broken into fixed-sized blocks called Frames.
  2. Logical memory (the process/program) is broken into blocks of the exact same size called Pages.
  3. When a program is executed, its pages can be loaded into ANY available frames in RAM, even if they are scattered all over the place.
• Page Table: The OS uses a Page Table to keep track of which Page is loaded into which Frame. The CPU uses this table to translate logical addresses into physical addresses on the fly.

3. Virtual Memory

• The Concept: Virtual memory creates an illusion for the user (and the program) that there is a massive amount of RAM available. It does this by using a portion of the Hard Drive/SSD as an extension of RAM.
• Demand Paging: Instead of loading the entire 10GB program into RAM at once, the OS only loads the Pages that are currently needed. The rest of the program stays on the Hard Drive.
• Page Fault: If the CPU tries to access a page that is NOT currently in RAM, a 'Page Fault' occurs. The OS pauses the process, fetches the required page from the Hard Drive, puts it into RAM, and then resumes the process.
• Swapping: If the RAM is full and a new page needs to come in, the OS kicks out an old, unused page back to the Hard Drive to make room. This is called swapping (or page replacement). Thrashing: If a system is spending more time swapping pages in and out of the hard drive than actually executing instructions, the computer will freeze or become incredibly slow. This state is called Thrashing (usually happens when you open too many heavy apps with very little RAM).