L-5.13: 2-Level Paging in Operating System | Multilevel Paging

Summary

In this lesson, the concept of multi-level paging is introduced, focusing on two-level paging. The video explains the need for dividing large page tables into smaller, manageable pages when they exceed the size of memory frames. Through a detailed example, the script demonstrates how logical and physical addresses are mapped, explaining the division of the page table and the creation of outer and inner page tables. The process ensures that large page tables fit into memory by using multi-level indexing, offering a clear understanding of the paging process in memory management.

Takeaways

• 😀 Multi-level paging is introduced to manage large page tables that cannot fit into a single memory frame.
• 😀 In multi-level paging, when the page table size exceeds the available memory frame size, it is divided into smaller pages.
• 😀 Physical memory size and frame size are key factors in determining the number of frames in memory.
• 😀 A logical address is divided into a page number and an offset, where the page number maps to a page table entry, and the offset specifies the exact byte within the page.
• 😀 The total number of frames in physical memory can be calculated by dividing the total physical memory size by the frame size.
• 😀 A page table entry (PTE) typically stores the frame number where a page is located in memory, and its size is determined by the number of frames.
• 😀 When a page table becomes too large, it must be divided further into multiple pages, which are stored in frames of main memory.
• 😀 In two-level paging, an outer page table is used to map to an inner page table, which then maps to the actual frames in memory.
• 😀 The number of entries in an outer page table corresponds to the number of inner page tables, and the number of entries in an inner page table corresponds to the number of pages.
• 😀 A logical address is mapped using a two-level page table structure: the outer page table provides the address of the inner page table, and the inner page table provides the frame number.
• 😀 The entire process of address translation is similar to looking up an index in a book: first, find the outer index, then the inner index, and finally, the content in the desired frame.

Q & A

What is the main reason for using multi-level paging in computer systems?

The main reason for using multi-level paging is to manage large page tables that cannot fit into a single memory frame. When a page table becomes too large, it is divided into smaller, manageable tables, allowing the system to efficiently map logical addresses to physical memory.

How is a logical address divided in a multi-level paging system?

In a multi-level paging system, a logical address is divided into three parts: the outer index (used to access the outer page table), the inner index (used to access the inner page table), and the frame offset (used to locate the exact byte within the physical frame).

Why can't the page table fit into a single memory frame in the example given?

In the example, the page table is too large because it needs to store 1 million entries (one for each page), and each entry takes up 2 bytes. This results in a page table size of 2 MB, which is larger than a single 4 KB memory frame, requiring further division.

What is the size of a memory frame and how does it relate to the paging system?

The memory frame size in the example is 4 KB. The frame size is important because it determines how much data can be stored in one memory frame. In the paging system, both the page size and the page table entry size are designed to fit within these frame constraints.

How many frames are there in total for a 256 MB physical address space with a 4 KB frame size?

For a 256 MB physical address space and a 4 KB frame size, the total number of frames is 2^16, which equals 65,536 frames.

How is the logical address space of 4 GB divided in the example, and what is the total number of pages?

The 4 GB logical address space is divided into 4 KB pages. Since each page is 4 KB, the total number of pages is 2^20, which equals 1 million pages.

How does the page table entry size affect the overall size of the page table?

Each page table entry is 2 bytes, which means that for 1 million pages, the total size of the page table is 2 MB (2^20 entries × 2 bytes per entry). This is too large to fit into a single memory frame, necessitating the use of multi-level paging.

What happens when the page table size exceeds the size of a memory frame in multi-level paging?

When the page table size exceeds the size of a memory frame, the page table is divided into smaller sub-tables. These sub-tables are then stored in memory frames, allowing the system to efficiently map addresses using multiple levels of page tables.

How is the outer page table structured, and how does it help with address translation?

The outer page table contains entries that point to the inner page tables. In the example, the outer page table has 2^9 entries (512 entries), each of which points to an inner page table. The outer page table helps locate which inner page table contains the specific frame for a given page.

How is the address mapping process handled in multi-level paging?

In multi-level paging, the logical address is split into parts. The outer index is used to find the outer page table entry, which points to the inner page table. The inner index is used to find the specific entry in the inner page table, which contains the frame number. Finally, the frame offset specifies the exact byte within the frame, completing the address translation process.