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Describe segmentation and its implementation.

Segmentation in Operating Systems: Definition and Implementation

Segmentation is a memory management technique in operating systems that divides a program’s address space into logical units called segments. Each segment represents a meaningful part of the program—such as code, data, stack, heap, or symbol tables—allowing flexible memory use, protection, and sharing.

What is Segmentation?

  • Segmentation organizes memory based on logical units (modules, procedures, arrays), not fixed-size blocks.
  • Each process has multiple segments, and each segment can have different sizes.
  • Addresses are expressed as a pair: segment number and offset within that segment.

Why Use Segmentation?

  • Logical view of memory matches program structure (modules and functions).
  • Fine-grained protection: different rights (read/write/execute) per segment.
  • Easy sharing of common code/data segments between processes.
  • Supports dynamic growth (e.g., stack segment grows as needed).
  • Efficient relocation: segments can be placed anywhere in physical memory.

Logical Address Format

A logical address in a segmented system has two parts:

Logical Address = (segment_number, offset)

Example:
(segment = 2, offset = 300)

Core Data Structures

  • Segment Table (per process): An array where each entry describes one segment.

Common fields in a Segment Table Entry (STE):

  • Base: Starting physical address of the segment.
  • Limit (Length): Size of the segment (maximum valid offset).
  • Protection bits: Read/Write/Execute permissions.
  • Present/Valid bit: Indicates whether the segment is in memory.
  • Other flags: e.g., shared flag, grow direction (for stack), dirty/accessed bits.
  • STBR (Segment Table Base Register): Holds the physical address of the current process’s segment table.
  • STLR (Segment Table Length Register): Holds the number of valid segments for the process; used for bounds checking.

Address Translation: Logical to Physical

The Memory Management Unit (MMU) converts a logical address to a physical address using the segment table.

  1. Split the logical address into segment_number (s) and offset (d).
  2. Check s against STLR. If s ≥ STLR, raise a segmentation fault (invalid segment).
  3. Fetch STE = SegmentTable[s] using STBR.
  4. Check the Present bit. If not present, trigger a fault/OS handler to bring the segment into memory.
  5. Bounds check: if d ≥ Limit, raise a segmentation fault (offset out of range).
  6. Check permissions (R/W/X) based on the attempted operation.
  7. Compute physical address: Physical = Base + d.
Given:
  STLR = 5, s = 3, d = 120
  STE[3]: Base = 10000, Limit = 512, Prot = R-X, Present = 1

Translation:
  3 < STLR  → ok
  Present   → yes
  120 < 512 → ok
  Write?    → denied (if Prot is R-X)
  Physical  → 10000 + 120 = 10120

Operating System Responsibilities (Implementation Flow)

  1. Program load: The OS identifies segments (code, data, stack), allocates physical memory for each, and creates the segment table entries with Base, Limit, and protection bits.
  2. Context switch: The OS loads STBR and STLR for the new process so the MMU uses the correct segment table.
  3. Growth and resizing: For growing segments (like stack/heap), the OS can extend the Limit and allocate more physical memory (if contiguous space is available).
  4. Sharing: The OS can map the same physical segment into multiple processes’ segment tables with appropriate permissions.
  5. Fault handling: On segment-not-present or protection violations, the OS handles the trap—loading segments, adjusting limits, or terminating the process.
  6. Compaction (optional): To reduce external fragmentation, the OS may relocate segments and update Bases in segment tables.

Protection and Sharing

  • Each segment has R/W/X permissions to control access.
  • Code segments are often read-only and shared across processes to save memory.
  • Data segments can be private or shared. Copy-on-write can be used to delay duplication until a write occurs.

Memory Allocation and Fragmentation

  • External fragmentation: Because segments are variable-sized, free memory may be split into noncontiguous holes.
  • Allocation strategies: First-fit, best-fit, or worst-fit can be used to place segments in memory.
  • Compaction: Periodically moving segments to coalesce free space reduces fragmentation but adds overhead.
  • Segmentation with paging (hybrid): Many systems page each segment to eliminate external fragmentation, keeping segmentation’s logical view with paging’s fixed-size frames.

Advantages and Limitations

Advantages:

  • Natural mapping to program structure.
  • Efficient protection and sharing at module level.
  • Flexible relocation and dynamic growth.

Limitations:

  • External fragmentation in pure segmentation systems.
  • Overhead of compaction and segment management.
  • Hardware complexity in the MMU.

Segmentation vs Paging (Quick View)

  • Segmentation: Variable-size logical units, programmer-visible, supports protection/sharing by module; vulnerable to external fragmentation.
  • Paging: Fixed-size blocks (pages/frames), not logically meaningful; avoids external fragmentation but may cause internal fragmentation.
  • Hybrid (paged segmentation): Combines logical benefits of segments with fragmentation control of paging.

In summary, segmentation in operating systems provides a logical, modular view of memory with strong protection and sharing features. Its implementation revolves around per-process segment tables, hardware registers (STBR, STLR), and MMU checks for bounds and permissions during address translation.