5. Paging and Segmentation

Paging vs. Segmentation

Paging divides memory into fixed-size blocks called pages, which simplifies management by treating memory as a uniform structure. In contrast, segmentation divides memory into variable-sized segments based on logical units such as functions, arrays, or data structures.

Both methods offer distinct advantages and are chosen based on the specific needs and complexities of applications and system architectures. Often, modern systems combine both techniques to leverage the benefits of each.

Paging

Paging is a method or technique which is used for non-contiguous memory allocation. It is a fixed-size partitioning theme (scheme). In paging, both main memory and secondary memory are divided into equal fixed-size partitions. The partitions of the secondary memory area unit and main memory area unit are known as pages and frames respectively. 

Features of Paging

• Fixed-Size Division: Memory is divided into fixed-size pages, simplifying memory management.
• Hardware-Defined Page Size: Page size is set by hardware and is uniform across all pages.
• OS-Managed: The operating system handles paging, including maintaining page tables and free frame lists.
• Eliminates External Fragmentation: Paging avoids external fragmentation but can suffer from internal fragmentation.
• Invisible to User: Paging is transparent to programmers and users, simplifying software development.

Paging is a memory management method accustomed to fetching processes from the secondary memory into the main memory in the form of pages. in paging, each process is split into parts wherever the size of every part is the same as the page size.

The size of the last half could also be but the page size. The pages of the process area unit hold on within the frames of main memory relying upon their accessibility. 

Segmentation

Segmentation is another non-contiguous memory allocation scheme, similar to paging. However, unlike paging which divides a process into fixed-size pages segmentation divides memory into variable-sized segments that correspond to logical units such as functions, arrays, or data structures. 

Features of Segmentation

• Variable-Size Division: Memory is divided into logical segments of varying sizes based on program structure.
• User/Programmer-Defined Sizes: Segment sizes are defined by the programmer or compiler, reflecting logical program units.
• Compiler-Managed: Segmentation is primarily managed by the compiler, with OS support for memory allocation.
• Supports Sharing and Protection: Segmentation facilitates sharing code/data between processes and easy implementation of protection.
• Visible to User: Segmentation is visible to programmers, allowing better control over logical memory organization.

In segmentation, both main memory and secondary memory are not divided into equal-sized partitions. Instead, they are split into segments of varying sizes. These segments are tracked using a data structure called the segment table.

The segment table stores information about each segment, primarily:

• Base: The starting physical address of the segment in memory.
• Limit: The length (or size) of the segment.

When accessing memory, the CPU generates a logical address composed of:

• A Segment Number
• A Segment Offset

The MMU (Memory Management Unit) uses the segment number to find the corresponding base and limit in the segment table. If the offset is less than the limit, the address is considered valid, and the physical address is computed by adding the offset to the base.
If the offset exceeds the limit, an error (segmentation fault) occurs, indicating an invalid address access attempt.

The above figure shows the translation of a logical address to a physical address. 

Paging vs. Segmentation

Feature	Paging	Segmentation
Division Unit	Fixed-size pages	Variable-size segments
Managed By	Operating system	Compiler
Unit Size Determined By	Hardware	User/programmer
Address Structure	Page number + page offset	Segment number + segment offset
Data Structure Used	Page table	Segment table
Fragmentation Type	Internal fragmentation	External fragmentation
Speed	Faster	Slower
Programmer Visibility	Invisible to the user	Visible to the user
Sharing	Difficult	Easy
Data Structure Handling	Inefficient	Efficient
Protection	Hard to implement	Easier to apply
Size Constraints	Page = Frame size	No fixed size required
Memory Unit Perspective	Physical unit	Logical unit
System Efficiency	Less efficient	More efficient