Segmentation in Operating Systems: A Memory Management Technique

Explore segmentation, a memory management scheme that divides a program into variable-sized segments. This tutorial contrasts segmentation with paging, highlighting its advantages in aligning with program structure and improving memory access efficiency, especially for logically related code components.

Segmentation in Operating Systems: A Memory Management Technique

What is Segmentation?

Segmentation is a memory management scheme that divides a program's address space into variable-sized blocks called segments. Each segment typically corresponds to a logical division of the program, such as a code segment, data segment, or stack segment. This contrasts with paging, which divides memory into fixed-size blocks (pages), regardless of program structure. Information about each segment (its starting address and size) is stored in a segment table.

Why Use Segmentation?

Segmentation offers advantages over paging, particularly when it comes to aligning with how programmers view their code. Paging, while efficient in terms of memory usage, can divide a program's logically related components across multiple pages. This can lead to performance inefficiencies as related parts of a program might need to be fetched from different locations in memory.

Segmentation addresses this by allowing logically related parts of a program to reside within the same segment, improving memory access efficiency.

Address Translation in Segmentation

In segmentation, a logical address consists of two parts:

  • Segment Number: Identifies the segment.
  • Offset: Specifies the location within the segment.

The segment number is used to index into the segment table. The segment table contains the base address and limit (size) of each segment. The operating system checks if the offset is within the segment's bounds (less than the limit). If it is, the physical address is calculated by adding the base address of the segment to the offset. If the offset is out of bounds, an error occurs.

Diagram illustrating address translation in segmentation

Advantages of Segmentation

  • No Internal Fragmentation: Segments are variable-sized, so there's no wasted space within a segment.
  • Efficient Memory Usage (on average): Average segment size tends to be larger than page size in paging, reducing the number of memory accesses.
  • Relocation Simplicity: Easier to move a segment in memory than an entire program.
  • Smaller Segment Tables (compared to Paging): Potentially fewer entries in the segment table than a page table for larger programs.

Disadvantages of Segmentation

  • External Fragmentation: Free space can become fragmented into small, unusable pieces.
  • Complex Memory Allocation: Finding contiguous space for variable-sized segments is more challenging.
  • Overhead of Memory Management Algorithms: Managing variable-sized partitions can be computationally more expensive than managing fixed-size pages.