Paging in Operating Systems: Efficient Virtual Memory Management

Understand paging, a virtual memory management technique that divides processes and memory into fixed-size blocks (pages and frames). This guide explains how paging works, its advantages (efficient memory utilization, non-contiguous allocation), and its role in supporting multitasking and virtual memory.

Paging in Operating Systems

What is Paging?

Paging is a memory management technique that allows an operating system to retrieve processes from secondary storage (like a hard drive) into main memory. It works by dividing both the processes and the main memory into fixed-size blocks called pages and frames, respectively.

Imagine dividing a large document into smaller, equally sized pages. Paging does the same for processes, breaking them into pages. The main memory is also divided into frames of the same size. Each page from a process is then loaded into an available frame in main memory.

Pages don't need to be stored contiguously in memory (next to each other). They can be scattered throughout, making better use of available space. Pages are loaded into memory only when needed, improving efficiency and allowing more processes to run concurrently.

Page and Frame Sizes

Operating systems define the size of each frame, and these sizes must be equal for all frames within a system. Since a page is mapped to a frame, the page size must be identical to the frame size.

Example

Let's say we have 16KB of main memory and a frame size of 1KB. This means the memory is divided into 16 frames. If we have four 4KB processes (P1, P2, P3, P4), each divided into four 1KB pages, these pages can be loaded into frames.

Initially, all frames might be empty, so pages are loaded contiguously. However, if some processes move to a waiting state (e.g., P2 and P4), their frames become free. A new process (e.g., P5) can then be loaded, even if there aren't contiguous free frames, because paging allows pages to be placed in non-contiguous locations.

Memory Management Unit (MMU)

The Memory Management Unit (MMU) is a hardware component that translates logical addresses (generated by the CPU) into physical addresses (actual memory locations). A logical address has two parts:

• Page Number: Indicates which page of the process is being accessed.
• Offset: Specifies the location within that page.

The MMU uses the page number to find the corresponding frame number and then combines it with the offset to obtain the physical address.

Example: Address Translation

If the CPU requests the 10th word of the 4th page of process P3, and that page is loaded into frame 9, then the MMU translates the logical address to physical address at the 10th word of frame 9.

C Code Example: Declaring a Character Variable

char ch = 'a';

Example Output

She said "Hello!" to me.

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