Segmentation

Introduction

Segmentation is a memory management scheme in operating systems that divides a program's logical memory into variable-sized segments. Each segment represents a logical unit such as a function, array, or data structure. This segmentation enables easier sharing, protection, and logical addressing compared to linear memory models.

"Segmentation reflects the programmer’s view of memory as a collection of logical units rather than a linear sequence of bytes." -- Abraham Silberschatz et al.

Concept and Definition

Logical Segments

Memory divided into segments: code, stack, heap, data. Each segment has a name, length, and purpose. Segments are variable-sized and logically meaningful.

Segment Addressing

Addresses specified as (segment number, offset). Offset within segment bounds checked during access. Enables modular access and logical separation.

Segmentation Model

Process memory represented as a set of segments. Segment table maintains base and limit for each segment. Enables dynamic allocation and protection.

Address Translation

Logical to Physical Mapping

CPU generates (segment number, offset). Segment table provides base physical address and segment limit. Physical address = base + offset.

Bounds Checking

Offset compared with segment limit. If offset > limit, trap or exception generated. Prevents illegal memory access.

Translation Process

1. Extract segment number and offset. 2. Validate offset < limit. 3. Add offset to segment base. 4. Access physical memory at computed address.

if offset < segment_limit then physical_address = segment_base + offsetelse generate_memory_access_exception()

Segment Table Structure

Entries

Each entry contains: base address (physical start), segment limit (length), protection bits (read/write/execute).

Storage

Segment table stored in memory or special registers. Fast access critical for performance. Sometimes cached in associative memory.

Example Table

Advantages of Segmentation

Logical View of Memory

Matches programmer's view: functions, data structures as segments. Simplifies modular programming and debugging.

Protection and Sharing

Segments can have different protection rights. Enables safe sharing of code segments between processes.

Efficient Memory Utilization

Variable segment sizes reduce internal fragmentation compared to paging. Segments grow or shrink dynamically.

Disadvantages and Limitations

External Fragmentation

Variable-sized segments cause external fragmentation. Requires compaction or complex allocation algorithms.

Complex Management

Segment tables and dynamic allocation require more hardware and OS support than simple paging.

Overhead in Address Translation

Additional steps in translation compared to simple linear addressing or paging. May impact performance.

Segmentation vs Paging

Segmentation

Variable-sized logical units. No fixed-size blocks. Matches program structure.

Paging

Fixed-size blocks (pages). Simplifies memory allocation. No logical program structure.

Hybrid Approaches

Many systems combine segmentation with paging. Segments divided internally into pages for efficient management.

Protection and Sharing

Protection Mechanisms

Segment table entries include protection bits. Controls access rights: read, write, execute. Prevents unauthorized access.

Sharing Segments

Common code segments shared among processes. Reduces memory usage. Data segments typically private.

Fault Handling

Invalid access triggers segmentation fault or exception. OS handles security violations and memory errors.

if (access_type & ~protection_bits) != 0 then raise_segmentation_fault()

Fragmentation in Segmentation

External Fragmentation

Free memory split into small holes due to variable segment sizes. Leads to inefficient memory use.

Compaction Techniques

Memory compaction reclaims fragmented space. Moves segments to create contiguous free memory blocks.

Allocation Strategies

First fit, best fit, worst fit algorithms used for segment allocation. Tradeoff between speed and fragmentation.

Implementation Considerations

Hardware Support

Segment registers store current segment descriptors. MMU performs translation and protection checks.

Operating System Role

Maintains segment tables. Handles allocation, deallocation, compaction, and protection enforcement.

Performance Optimization

Use of associative registers (TLBs) to cache segment table entries. Reduces translation latency.

Real-world Usage Examples

Multics Operating System

Early system implementing segmentation for modular and secure memory management.

Intel x86 Architecture

Supports segmentation with segment registers (CS, DS, SS, ES, FS, GS). Mostly used for legacy and protection.

Modern Systems

Hybrid models combining segmentation and paging in virtual memory management.

References

• Silberschatz, A., Galvin, P.B., Gagne, G., Operating System Concepts, 9th Ed., Wiley, 2012, pp. 185–220.
• Tanenbaum, A.S., Modern Operating Systems, 4th Ed., Pearson, 2014, pp. 307–335.
• Stallings, W., Operating Systems: Internals and Design Principles, 9th Ed., Pearson, 2018, pp. 245–275.
• Bach, M.J., The Design of the UNIX Operating System, Prentice Hall, 1986, pp. 145–165.
• Hennessy, J.L., Patterson, D.A., Computer Architecture: A Quantitative Approach, 6th Ed., Morgan Kaufmann, 2017, pp. 489–515.