What Is Segmentation In OS (Operating System)?

Segmentation in an operating system (OS) is a memory management scheme that divides the program’s memory into different segments based on logical divisions. Unlike paging, which breaks memory into fixed-size blocks, segmentation divides memory into segments of varying sizes, with each segment representing a different part of a program or data. These segments typically include:

1. Code segment: Contains the executable code of the program.
2. Data segment: Holds global and static variables.
3. Heap segment: Used for dynamic memory allocation (e.g., with malloc in C).
4. Stack segment: Stores local variables, function parameters, and return addresses.

Key features of segmentation:

• Logical Division: Each segment represents a logical entity (like code, data, stack), making the system more intuitive in terms of managing memory.
• Variable Size: Segments can vary in size, unlike pages in paging, which are of fixed size.
• Protection and Sharing: Different segments can be given different protection levels (e.g., read-only for code), and sharing of segments between processes is possible.

Addressing in Segmentation:

• The logical address in a segmented system is a combination of a segment number and an offset (or displacement) within that segment.
• Segment table: A data structure used by the OS to keep track of the base address and length of each segment.

Example:

Suppose a program has three segments: code (1000 bytes), data (500 bytes), and stack (200 bytes). The OS will allocate each of these segments separately and track their locations. When the program accesses a particular segment, the OS translates the logical address (segment number and offset) into a physical address.

Advantages:

• Flexibility: It allows a more natural and flexible way of organizing memory.
• Protection: Different segments can have different access rights.
• Ease of Sharing: Segments can be shared between processes if needed.

Disadvantages:

• External Fragmentation: Since segments are of variable size, unused gaps (fragments) can appear between allocated segments.
• Complexity: Managing segments requires more complex hardware and software support compared to paging.

In modern systems, segmentation is often used in conjunction with paging to combine the benefits of both techniques (segmented paging).