Inverted Page Table in Operating System (OS)

The Inverted Page Table (IPT) is an advanced memory management technique used in Operating Systems (OS) to reduce the memory required for page tables. Unlike a traditional page table, which maintains a separate page table for each process, an Inverted Page Table maintains only one page table for the entire system. This approach is especially useful in systems with large virtual address spaces.

In this article, you will learn the Inverted Page Table in Operating System, its working, structure, advantages, disadvantages, and solved example in simple words.

What is an Inverted Page Table?

An Inverted Page Table (IPT) is a page table that contains one entry for each physical memory frame, rather than one entry for each virtual page of every process.

Each entry stores information about the virtual page currently loaded into that physical frame, along with the process that owns the page.

The primary goal of an Inverted Page Table is to reduce memory consumption by maintaining a single system-wide page table.

Why is an Inverted Page Table Needed?

A traditional page table requires one entry for every virtual page of every process. As the number of processes and the size of virtual memory increase, the page tables consume a significant amount of memory.

An Inverted Page Table solves this problem by:

• Maintaining only one page table for the entire system.

• Creating one entry for each physical frame instead of each virtual page.

• Reducing memory overhead.

• Supporting large virtual address spaces efficiently.

Structure of an Inverted Page Table

Each entry in an Inverted Page Table typically contains the following information.

How Does an Inverted Page Table Work?

The operating system follows these steps to translate a virtual address into a physical address.

Step 1: CPU Generates a Virtual Address

The CPU generates a virtual address containing:

• Process ID (PID)

• Virtual Page Number (VPN)

• Offset

Step 2: Search the Inverted Page Table

The operating system searches the Inverted Page Table for an entry where:

• The Process ID matches.

• The Virtual Page Number matches.

Step 3: Find the Physical Frame

If a matching entry is found, the corresponding physical frame number is obtained.

Step 4: Generate the Physical Address

The physical address is created by combining:

• Physical Frame Number

• Offset

Step 5: Handle a Page Fault

If no matching entry exists, a page fault occurs. The required page is loaded into a free frame, and the Inverted Page Table is updated.

Example of an Inverted Page Table

Suppose physical memory contains 4 frames.

Inverted Page Table

Now consider the following virtual address.

Process ID = P1

Virtual Page = 5

Offset = 120

Solution

The operating system searches the table.

Therefore,

Physical Address = Frame 2 + Offset 120

The requested data is located in Frame 2.

Advantages of an Inverted Page Table

An Inverted Page Table provides several benefits.

1. Saves Memory

Only one page table is maintained for the entire system, significantly reducing memory usage.

2. Suitable for Large Virtual Memory

It efficiently supports systems with large virtual address spaces, such as 64-bit operating systems.

3. Reduces Page Table Size

The number of entries in the page table depends on the number of physical frames rather than the number of virtual pages.

4. Supports Multiple Processes

A single page table can manage pages belonging to multiple processes.

Disadvantages of an Inverted Page Table

Despite its advantages, the Inverted Page Table also has some limitations.

1. Slower Searching

The operating system must search the table to locate the required page.

2. Requires Hashing

To improve search speed, most systems use hash tables, increasing implementation complexity.

3. More Complex Design

An Inverted Page Table is more difficult to implement than a traditional page table.

Traditional Page Table vs Inverted Page Table

Applications of an Inverted Page Table

The Inverted Page Table is commonly used in:

• 64-bit operating systems

• IBM Power Systems

• Large enterprise servers

• High-performance computing systems

• Systems with large physical memory

Tips to Remember

• An Inverted Page Table has one entry per physical frame.

• It maintains a single page table for the entire system.

• Each entry stores the Process ID, Virtual Page Number, and Physical Frame Number.

• It reduces memory usage compared to traditional page tables.

• Hashing is commonly used to speed up address translation.

Frequently Asked Questions (FAQs)

What is an Inverted Page Table?

An Inverted Page Table (IPT) is a memory management structure that stores one entry for each physical memory frame instead of one entry for each virtual page.

Why is an Inverted Page Table used?

It is used to reduce the memory required for page tables, especially in systems with large virtual address spaces.

What information is stored in an Inverted Page Table?

Each entry typically stores the Physical Frame Number, Process ID (PID), Virtual Page Number (VPN), and control bits.

What is the main advantage of an Inverted Page Table?

The main advantage is reduced memory usage, as only one page table is maintained for the entire system.

What is the main disadvantage of an Inverted Page Table?

Searching for a page can be slower, so operating systems often use hash tables to improve lookup performance.

Conclusion

The Inverted Page Table in Operating System is an efficient memory management technique that reduces the memory overhead of traditional page tables by maintaining one system-wide page table with one entry per physical frame. Although it requires more complex searching and often relies on hashing for fast lookups, it is widely used in modern operating systems with large virtual address spaces because of its excellent memory efficiency and scalability.