Paging | Memory Management | Operating System

Non-Contiguous Memory Allocation-

 

• Non-contiguous memory allocation is a memory allocation technique.
• It allows to store parts of a single process in a non-contiguous fashion.
• Thus, different parts of the same process can be stored at different places in the main memory.

 

Techniques-

 

There are two popular techniques used for non-contiguous memory allocation-

 

 

1. Paging
2. Segmentation

 

In this article, we will discuss about Paging.

Learn about Segmentation.

 

Paging-

 

• Paging is a fixed size partitioning scheme.
• In paging, secondary memory and main memory are divided into equal fixed size partitions.
• The partitions of secondary memory are called as pages.
• The partitions of main memory are called as frames.

 

 

• Each process is divided into parts where size of each part is same as page size.
• The size of the last part may be less than the page size.
• The pages of process are stored in the frames of main memory depending upon their availability.

 

Example-

 

• Consider a process is divided into 4 pages P₀, P₁, P₂ and P₃.
• Depending upon the availability, these pages may be stored in the main memory frames in a non-contiguous fashion as shown-

 

 

Also Read- Contiguous Memory Allocation

 

Translating Logical Address into Physical Address-

 

• CPU always generates a logical address.
• A physical address is needed to access the main memory.

 

Following steps are followed to translate logical address into physical address-

 

Step-01:

 

CPU generates a logical address consisting of two parts-

1. Page Number
2. Page Offset

 

 

• Page Number specifies the specific page of the process from which CPU wants to read the data.
• Page Offset specifies the specific word on the page that CPU wants to read.

 

Step-02:

 

For the page number generated by the CPU,

• Page Table provides the corresponding frame number (base address of the frame) where that page is stored in the main memory.

 

Step-03:

 

• The frame number combined with the page offset forms the required physical address.

 

 

• Frame number specifies the specific frame where the required page is stored.
• Page Offset specifies the specific word that has to be read from that page.

 

Diagram-

 

The following diagram illustrates the above steps of translating logical address into physical address-

 

 

Advantages-

 

The advantages of paging are-

• It allows to store parts of a single process in a non-contiguous fashion.
• It solves the problem of external fragmentation.

 

Disadvantages-

 

The disadvantages of paging are-

• It suffers from internal fragmentation.
• There is an overhead of maintaining a page table for each process.
• The time taken to fetch the instruction increases since now two memory accesses are required.

 

To gain better understanding about Paging,

Watch this Video Lecture

 

Next Article- Page Table | Page Table Entry

 

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