Segmentation and Paging | Segmented Paging

Segmentation and Paging-

 

Before you go through this article, make sure that you have gone through the previous articles on Paging and Segmentation.

 

We have discussed-

• Paging and Segmentation are the non-contiguous memory allocation techniques.
• Paging divides the process into equal size partitions called as pages.
• Segmentation divides the process into unequal size partitions called as segments.

 

Segmented Paging-

 

Segmented paging is a scheme that implements the combination of segmentation and paging.

 

Working-

 

In segmented paging,

• Process is first divided into segments and then each segment is divided into pages.
• These pages are then stored in the frames of main memory.
• A page table exists for each segment that keeps track of the frames storing the pages of that segment.
• Each page table occupies one frame in the main memory.
• Number of entries in the page table of a segment = Number of pages that segment is divided.
• A segment table exists that keeps track of the frames storing the page tables of segments.
• Number of entries in the segment table of a process = Number of segments that process is divided.
• The base address of the segment table is stored in the segment table base register.

 

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 three parts-

1. Segment Number
2. Page Number
3. Page Offset

 

 

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

 

Step-02:

 

• For the generated segment number, corresponding entry is located in the segment table.
• Segment table provides the frame number of the frame storing the page table of the referred segment.
• The frame containing the page table is located.

 

Step-03:

 

• For the generated page number, corresponding entry is located in the page table.
• Page table provides the frame number of the frame storing the required page of the referred segment.
• The frame containing the required page is located.

 

Step-04:

 

• The frame number combined with the page offset forms the required physical address.
• For the generated page offset, corresponding word is located in the page and read.

 

 

Diagram-

 

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

 

 

Advantages-

 

The advantages of segmented paging are-

• Segment table contains only one entry corresponding to each segment.
• It reduces memory usage.
• The size of Page Table is limited by the segment size.
• It solves the problem of external fragmentation.

 

Disadvantages-

 

The disadvantages of segmented paging are-

• Segmented paging suffers from internal fragmentation.
• The complexity level is much higher as compared to paging.

 

Next Article- Practice Problems On Segmented Paging

 

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