Segmentation in Operating System

Segmentation is a memory management technique where a process is divided into variable-sized chunks called segments. Unlike paging, segmentation matches the user’s logical view of a program (functions, arrays, modules) to physical memory.

It reflects the user’s view of memory rather than the computer’s physical organization, making it easier to manage and protect processes.

Key Features of Segmentation

1. Variable-sized divisions: Segments can have different lengths, depending on the program’s requirements.
2. Logical division of memory: Segments represent meaningful units like code, stack, data, or modules.
3. Two-part address: A logical address consists of:

• Segment number (s): Identifies which segment is being referred to.
• Offset (d): Specifies the exact location within that segment.
  Logical Address = ⟨Segment number, Offset⟩

4. Protection and sharing: Different segments can have access rights (read, write, execute) and can be shared among processes.
5. No internal fragmentation: Since segments are not fixed in size.
6. External fragmentation: Can occur when free memory is divided into small scattered blocks.

Types of Segmentation in Operating Systems

• Virtual Memory Segmentation: Each process is divided into a number of segments, but the segmentation is not done all at once. This segmentation may or may not take place at the run time of the program.
• Simple Segmentation: Each process is divided into a number of segments, all of which are loaded into memory at run time, though not necessarily contiguously.

There is no simple relationship between logical addresses and physical addresses in segmentation. A table stores the information about all such segments and is called Segment Table. 

What is Segment Table?

It maps a two-dimensional Logical address into a one-dimensional Physical address. It's each table entry has:

• Base Address: It contains the starting physical address where the segments reside in memory.
• Segment Limit: Also known as segment offset. It specifies the length of the segment.

Translation of Two-dimensional Logical Address to Dimensional Physical Address. 

 The address generated by the CPU is divided into:

• Segment number (s): Number of bits required to represent the segment.
• Segment offset (d): Number of bits required to represent the position of data within a segment.

Advantages of Segmentation in Operating System

• Reduced Internal Fragmentation: Segments are sized as per program needs, minimizing wasted space.
• Smaller Segment Table: Requires less space compared to page tables.
• Better CPU Utilization: Entire modules are loaded at once, improving performance.
• Closer to User’s View: Programs can be divided into logical modules, matching how users think.
• User-Controlled Size: Segment size is defined by the user, unlike fixed page size in paging.
• Security & Separation: Segments help isolate sensitive data and operations.

Disadvantages of Segmentation in Operating System

• External Fragmentation: Free memory gets scattered, leading to wasted space.
• Overhead: Maintaining segment tables requires extra memory and management.
• Slower Access: Two memory lookups (segment table + main memory) increase access time.
• Complexity: Managing multiple variable-sized segments is harder than paging.
• Segmentation Faults: Errors may occur if a program tries to access memory outside its segment.

Segmentation in Operating System

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