Paging in Operating Systems: A Comprehensive Guide

SPaging is a method of gaining access to data more quickly. When a program requires a page, it is available in the main memory because the OS copies a set number of pages from the storage device into the main memory. Paging permits a process’s physical address space to be noncontiguous. Paging refers to a memory management strategy that does away with the need for the allocation of contiguous physical memory.

What is Paging in the OS?

Paging is a storage mechanism used in OS to retrieve processes from secondary storage to the main memory as pages. The primary concept behind paging is to break each process into individual pages. Thus the primary memory would also be separated into frames.

One page of the process must be saved in one of the given memory frames. These pages can be stored in various memory locations, but finding contiguous frames/holes is always the main goal. Process pages are usually only brought into the main memory when they are needed; else, they are stored in the secondary storage.

The frame sizes may vary depending on the OS. Each frame must be of the same size. Since the pages present in paging are mapped on to the frames, the page size should be similar to the frame size.

 

Example

Assuming that the main memory is 16 KB and the frame size is 1 KB, the main memory will be partitioned into a collection of 16 1 KB frames. P1, P2, P3, and P4 are the four processes in the system, each of which is 4 KB in size. Each process is separated into 1 KB pages, allowing one page to be saved in a single frame.

Because all of the frames are initially empty, the pages of the processes will be stored in a continuous manner. The graphic below depicts frames, pages, and the mapping between them.

Consider the case when P2 and P4 are shifted to the waiting state after a period of time. Eight frames are now empty, allowing other pages to be loaded in their stead. Inside the ready queue is the process P5, which is 8 KB (8 pages) in size.

Given that we have 8 noncontiguous frames accessible in memory, paging allows us to store the process in many locations. As a result, we can load the process P5 page instead of P2 and P4.

Memory Management Unit

The Memory Management Unit (MMU) is responsible for converting logical addresses to physical addresses. The physical address refers to the actual address of a frame in which each page will be stored, whereas the logical address refers to the address that is generated by the CPU for each page.

When the CPU accesses a page using its logical address, the OS must first collect the physical address in order to access that page physically. There are two elements to the logical address:

• Page number
• Offset

The OS’s memory management unit must convert the page numbers to the frame numbers.

Examples

Let’s say the CPU requests the 10th word of the 4th page of process P3 in the image above. Because page number 4 of process P1 is stored at frame number 9, the physical address will be returned as the 10th word of the 9th frame.

Let’s consider another example:

• If the physical address is 12 bits, then the physical address space would be 4 K words
• If the logical address is 13 bits, then the logical address space would be 8 K words
• If the page size is equal to the frame size, which is equal to 1 K words (assumption),

Then:

The address generated by the CPU is divided into the following:

• Page offset(d): It refers to the number of bits necessary to represent a certain word on a page, page size in Logical Address Space, or page word number or page offset.
• Page number(p): It is the number of bits needed to represent the pages in the Logical Address Space or the page number.

The Physical Address is divided into the following:

• Frame offset(d): It refers to the number of bits necessary to represent a certain word in a frame, or the Physical Address Space frame size, the word number of a frame, or the frame offset.
• Frame number(f): It’s the number of bits needed to indicate a frame of the Physical Address Space or a frame number.

Dedicated registers can be used to implement the page table in hardware. However, using a register for the page table is only useful if the page table is tiny. We can employ TLB (translation look-aside buffer), a particular, tiny, fast look-up hardware cache if the page table has a significant number of entries.

• The TLB is a high-speed, associative memory.
• LB entries are made up of two parts: a value and a tag.
• When this memory is accessed, an item is compared to all tags at the same time.
• If the object is located, the value associated with it is returned.

m = main memory access time

In case the page table is kept in the main memory,

then the effective access time would be = m(page table) + m(page in page table)

what is Paging in the OS?

Before directly delving into the concept of paging, it is essential to understand the memory management in an operating system. In brief, memory management is a method used by operating systems to manage and organize the memory resources of a computer. This includes memory allocation/deallocation, mapping, protection, and fragmentation.

Having an idea of the meaning of memory management, now we can better understand the paging in OS. So, paging is a memory management method used in an operating system. We all know computers use several programs to run a variety of tasks. And what paging does is that it tells the program how to use the memory.

To understand this, let’s consider an example of a computer program that is used to edit the images. Now, before performing any further task, the program is first divided into different sections called pages. These pages are used by the OS to manage its memory. In order to do so, the OS assigns frames to its pages. 

What is Paging Protection?

Paging protection is a technique used for the purpose of security and isolation of applications and processes running on the computer. It also aids in managing virtual memory, thereby allowing the use of memory resources efficiently. That is to say, when a process attempts to access a memory location, the associated protection bits in the page table entry for the requested address are checked by the hardware. If the process is trying to perform an unpermitted operation by the protection bits, an interrupt is generated.

Memory Management Unit

Memory Management Unit, also widely known as MMU, is a component found in the central processing unit of a computer that manages virtual memory and physical memory. When a program runs, it creates virtual addresses to access memory locations without knowing the actual physical location of the information. When the CPU tries to access memory using a virtual address, the MMU translates it to the physical address. So, it is primarily used to translate virtual memory addresses to physical memory addresses. 

This can be clearly understood with the help of the following example. Imagine you're using a web browser that's running multiple tabs with various websites. Each tab represents a different program. As you switch between tabs, the operating system and MMU work together to manage memory. When you click on a tab, the MMU translates the tab's virtual memory addresses into actual physical memory locations. If another program is using a lot of memory, the MMU might move some of its data to disk to make room for the active tab's data in RAM. This swapping happens transparently, allowing your tabs to run smoothly even

Advantages and Disadvantages of Paging

Paging in operating systems offers streamlined memory management but presents challenges like internal fragmentation and the overhead of managing page tables. Here are some of the key advantages and disadvantages of paging in the OS.

Advantages of Paging

·       Paging offers simplified memory management so that the programs need not worry about the physical memory addresses.

·       It allows efficient memory usage.

·       Aids in eliminating external fragmentation.

·       Provides memory protection by preventing unauthorized access.

·       The mapping between virtual and physical addresses is quite simple.

Disadvantages of Paging

·       Since the pages are of fixed size, there is a possibility for internal fragmentation.

·       Page table overhead arises in systems that require large memory.

·       Complex page replacement.

·       An additional layer of memory access is created by paging.

·       Too many pages in a physical memory at the same time lead to thrashing.

 

 

 

 

 

 

 

 

Practice Problems on Paging in OS

1. The logical memory gets broken into various blocks of similar sizes known as _______.

a) pages

b) backing store

c) frames

d) none of the above

Answer- (a) pages

2. The physical memory gets broken into various fixed-sized blocks known as _________.

a) pages

b) backing store

c) frames

d) none of the above

Answer- (c) frames

3. Which of these is used in the form of an index in a given page table?

a) page offset

b) frame offset

c) frame bit

d) page number

Answer- (d) page number

4. The smaller page tables can be implemented in the form of a set of which of these?

a) registers

b) counters

c) stacks

d) queues

Answer- (a) registers

5. Paging helps in increasing the ______ time.

a) execution

b) context – switch

c) waiting

d) all of the above

Answer- (b) context – switch

Frequently Asked Questions on Paging in OS

Q1

Why is paging used?

Paging is a method of gaining access to data more quickly. When a program requires a page, it is available in the main memory because the OS copies a set number of pages from the storage device into the main memory. Paging permits a process’s physical address space to be noncontiguous.

Q2

What are paging and frames?

Paging is a method of memory management. External Fragmentation is removed using the concept of paging. The process is divided into ‘Pages’, and the Main Memory is divided into ‘Frames’ so that a part of the process (a page) can be contained in a frame (part of the main memory).

Q3

What is the difference between a page and a frame?

A page (often known as a memory page, a logical page, or a virtual page) is a contiguous virtual memory block with a set length. The frame (often known as a memory frame, page frame, or physical page) is basically a fixed-length RAM block, i.e. physical memory.

Q4

What is a page in OS?

A page, also known as a memory page or virtual page, refers to a contiguous block of virtual memory with a defined length that is described by one single entry in a page table. In a virtual memory OS, it is the smallest data unit for memory management.