Memory Management in Operating Systems: Concepts and Techniques

Memory management is a core component of operating systems (OS) that ensures efficient allocation, protection, and access control of memory resources. This article covers essential concepts like fragmentation, virtual memory, segmentation, and paging, along with the mechanisms that enable multitasking in modern systems.

---

The Basics of Memory Management

The main memory of a computer is divided into storage locations, each identified by a unique physical address. Memory management is responsible for allocating memory, protecting data, and ensuring efficient utilization of resources.

Single-Tasking vs. Multitasking Systems

• Single-Tasking Systems: In these systems, one program runs to completion before another begins. Memory is divided into user address space (for programs) and system address space (for the OS). A boundary register ensures memory access stays within the user address space.
• Multitasking Systems: These systems allow multiple programs to run simultaneously by rapidly switching between tasks (context switching). To manage memory for each process, the OS uses base and bound registers, which define the starting address and size of memory allocated to each process.

---

Challenges of Memory Fragmentation

Fragmentation occurs when memory is inefficiently utilized, leading to wasted space. There are two primary types of fragmentation:

1. External Fragmentation:
   • Occurs when free memory is scattered in small, non-contiguous blocks.
   • Even if total free memory is sufficient, these scattered blocks may be too small to fulfill a process’s memory requirements.
2. Internal Fragmentation:
   • Happens when memory is allocated in fixed-size blocks (e.g., pages), and a portion of each block remains unused.
   • This unused memory within allocated blocks leads to waste.

---

Virtual Memory and Logical Addressing

To overcome fragmentation and memory limitations, modern systems use virtual memory, which creates a large logical address space for programs. This logical address space is mapped to physical memory during runtime, enabling processes to run as if they have access to a larger, contiguous memory space.

How Virtual Memory Works

1. Programs refer to memory locations using virtual addresses.
2. During execution, these virtual addresses are translated into physical addresses using a memory management unit (MMU).
3. Virtual memory relies on paging and segmentation to allocate memory efficiently.

---

Segmentation vs. Paging

Both segmentation and paging are techniques used to manage virtual memory, each with distinct advantages and trade-offs.

Segmentation

• Divides memory into logical units (segments) of variable sizes.
• Each segment corresponds to a specific program component (e.g., code, data, stack).
• Provides clarity and access control, as the boundaries between segments are well-defined.
• Prone to external fragmentation, as variable-sized segments can create scattered free space.

Paging

• Divides memory into fixed-size blocks called pages.
• Pages are mapped to physical memory frames of the same size, simplifying allocation.
• Efficient in reducing fragmentation but lacks logical semantics, making it harder to identify where one process ends and another begins.
• May result in internal fragmentation, as processes may not fully utilize allocated pages.

---

Multitasking and Memory Protection

Context Switching

To enable multitasking, the OS performs context switching, which involves:

• Saving the state (registers, memory mappings) of the current process.
• Loading the state of the next process.
• Updating base and bound registers to reflect the memory space of the active process.

Memory Protection Requirements

Memory management must meet the following criteria to ensure security and stability:

1. Isolation of OS and User Processes:
   • The OS memory must be protected from unauthorized access by user processes.
   • Shared libraries should be readable but not writable by applications.
2. Process Isolation:
   • Each process’s memory should be inaccessible to others, preventing data leaks and unauthorized access.

---

Conclusion

Effective memory management is critical for modern operating systems to handle multitasking, optimize resource usage, and ensure security. Techniques like virtual memory, segmentation, and paging enable systems to overcome physical memory limitations while mitigating fragmentation issues.