3.5 Segmentation and segmented paging
3 min read•august 15, 2024
Memory management is a crucial aspect of operating systems. Segmentation and paging are two key techniques used to organize and allocate memory efficiently. This section explores their concepts, advantages, and trade-offs, as well as the hybrid approach of segmented paging.
Segmentation divides memory into variable-sized segments, aligning with program structure and allowing flexible allocation. Paging uses fixed-size blocks, simplifying allocation but potentially causing internal . Understanding these approaches helps in designing effective memory management systems for modern operating systems.
Segmentation vs Paging
Concept and Advantages of Segmentation
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- Segmentation divides into variable-sized segments representing logical units (functions, arrays, stacks)
- Allows flexible memory allocation based on actual size of program components unlike fixed-size pages
- Provides better support for sharing and protection of memory regions
- Reduces external fragmentation compared to contiguous allocation schemes
- Allows more by allocating only required amount potentially reducing internal fragmentation
- Aligns more closely with programmer's conceptualization of program structure
- Supports dynamic growth of data structures without needing to allocate maximum size upfront
Paging Overview and Comparison
- Paging divides memory into fixed-size blocks called pages
- Simplifies memory allocation and deallocation processes
- Eliminates external fragmentation but may lead to internal fragmentation
- Provides uniform access time to memory locations
- Requires additional memory for tables
- Supports virtual memory implementations more easily
- Allows for easier implementation of memory protection at the page level
Structure of Segmentation Systems
Segment Table and Address Translation
- Uses to map logical addresses to physical addresses
- Segment table entries contain base address and limit information
- Logical address consists of segment number and offset within segment
- (STBR) points to segment table location in memory
- Segment limit register stores length of segment table preventing invalid segment access
- (MMU) performs address translation
- Combines base address of segment with offset to generate physical address
Protection and Fault Handling
- associated with each segment specify read, write, and execute permissions
- Implements access control at segment level
- Raises segmentation fault exceptions for illegal memory access attempts
- Detects access beyond segment limit or violation of protection settings
- Allows operating system to handle memory access violations gracefully
- Supports implementation of memory isolation between processes
- Enables fine-grained control over shared memory regions
Segmented Paging: Benefits
Hybrid Approach Advantages
- Combines benefits of segmentation and paging techniques
- Divides each segment into fixed-size pages for efficient memory allocation
- Reduces fragmentation issues associated with pure segmentation
- Logical address consists of segment number, page number within segment, and offset within page
- Requires two-level address translation process
- Maps segment to its then maps page to physical address
- Allows for fine-grained memory protection at both segment and page levels
Enhanced Memory Management
- Provides better support for shared memory and dynamic linking of libraries
- Potentially reduces size of page tables compared to pure paging systems
- Each segment has its own separate page table
- Improves memory utilization by combining flexible segment allocation with efficient page management
- Supports both logical organization of programs and efficient physical memory usage
- Allows for more efficient implementation of virtual memory systems
- Enables easier implementation of memory compression techniques
Segmentation vs Paging: Trade-offs
Flexibility and Fragmentation
- Segmentation offers better support for variable-sized memory allocations
- Paging provides more efficient management of fixed-size memory blocks
- Paging typically results in less external fragmentation than segmentation
- Segmentation may lead to external fragmentation between segments
- Paging may cause internal fragmentation within pages
- Segmented paging attempts to balance these trade-offs
- Choice depends on specific system requirements and workload characteristics
Performance and Implementation Considerations
- Address translation in segmentation generally simpler and faster than in paging
- Paging relies more on table lookups affecting system performance
- Segmentation requires additional hardware support for bound checking
- Paging systems generally easier to implement and manage from OS perspective
- Segmentation may require more complex memory allocation algorithms
- Paging provides more flexibility in physical memory allocation
- Trade-offs between flexibility, efficiency, and complexity influence design decisions
Key Terms to Review (17)
Address Binding: Address binding is the process of associating a program's logical addresses with physical addresses in memory. This process ensures that a program can access its memory space correctly, regardless of where it is loaded in physical memory. Address binding can happen at different stages, such as compile-time, load-time, or execution-time, and is particularly relevant when considering memory management techniques like segmentation and segmented paging.
Base and Limit Registers: Base and limit registers are key components in memory management, used to protect memory space and facilitate access control in a system. The base register holds the starting address of a process's memory segment, while the limit register specifies the size of that segment. This system helps prevent processes from accessing memory locations that they are not authorized to use, thereby enhancing security and stability in multitasking environments.
Context Switching: Context switching is the process of storing and restoring the state of a CPU so that multiple processes can share a single CPU resource effectively. This mechanism allows an operating system to switch between different processes or threads, ensuring that they can execute concurrently while maintaining their individual states. It's crucial for managing resources in a multi-tasking environment and involves updating the process control blocks, managing the scheduling of processes, and optimizing the performance of threads.
Demand paging: Demand paging is a memory management scheme that loads pages into memory only when they are needed, rather than loading the entire program at once. This technique allows systems to use memory more efficiently by minimizing the amount of memory required for running processes and improving overall system performance. By delaying the loading of pages until they are accessed, demand paging enhances the use of virtual memory and works alongside paging techniques to optimize resource usage.
Efficient memory utilization: Efficient memory utilization refers to the effective management and allocation of memory resources in a computing system to maximize performance and minimize waste. This involves using memory in such a way that it reduces fragmentation, optimizes data access speed, and allows for flexible allocation strategies. In systems that implement advanced techniques like segmentation and segmented paging, efficient memory utilization becomes crucial as it balances the need for organized memory structures with the demands of running multiple processes simultaneously.
Fragmentation: Fragmentation refers to the inefficient use of memory that occurs when storage space is allocated and deallocated in such a way that it leaves small, unusable gaps. This phenomenon can significantly impact performance by making it difficult for the system to find contiguous blocks of memory for new processes. In contexts like segmentation and segmented paging, fragmentation can arise as different segments may vary in size and can lead to both internal and external fragmentation issues.
Logical address space: Logical address space refers to the range of addresses that an application can use to access memory, as viewed from the perspective of that application. It acts as a virtual map of memory that allows processes to think they have access to a contiguous block of memory, regardless of the actual physical memory layout. This abstraction is crucial for enabling features like segmentation and segmented paging, where memory can be divided into different segments or pages to manage memory allocation and protection efficiently.
Memory management unit: A memory management unit (MMU) is a hardware component that handles the translation of virtual memory addresses to physical memory addresses, enabling efficient memory utilization and protection. The MMU plays a crucial role in segmentation and segmented paging by allowing programs to utilize memory in a more flexible manner, optimizing both performance and security by isolating different processes in their respective address spaces.
Page: A page is a fixed-size block of memory used in a virtual memory system that allows the operating system to manage how data is stored and retrieved. By dividing the logical address space into pages, systems can efficiently map these pages to physical memory, facilitating better memory management and process isolation. This concept is essential in understanding how segmentation and segmented paging work together to optimize memory usage and access times.
Page Table: A page table is a data structure used in computer operating systems to manage virtual memory. It maps virtual addresses to physical addresses, allowing the system to track which pages are currently in memory and where they are stored in physical RAM. The page table is essential for implementing virtual memory and paging, as it helps the system efficiently manage memory allocation and retrieval.
Protection Bits: Protection bits are specific binary indicators used in memory management to control access rights for segments or pages in an operating system. They dictate the type of operations (read, write, execute) that a process can perform on a segment or page, ensuring that processes do not interfere with each other's memory space and maintain system stability and security.
Segment Fault: A segment fault, often referred to as a segmentation fault, occurs when a program tries to access a memory segment that it is not allowed to, resulting in an error. This is a critical issue in segmented memory systems where memory is divided into segments, and the operating system uses this structure to manage memory protection and allocation. When a program exceeds its allocated memory space or tries to access a restricted area, it triggers a segment fault, which can lead to program crashes or abnormal behavior.
Segment selector: A segment selector is a 16-bit value used in segmentation to identify a specific segment in memory. It plays a crucial role in managing memory by referencing the base address of a segment and its associated permissions, ensuring that programs can access the correct memory locations securely. By utilizing segment selectors, systems can support multiple segments efficiently, allowing for better organization and isolation of data and code.
Segment Table: A segment table is a data structure used in segmentation memory management to keep track of the various segments of a process. Each entry in the segment table contains information about a segment, such as its base address in physical memory and its length, allowing for efficient memory access and protection. By using a segment table, the operating system can enable logical segmentation of processes, facilitating easier management of memory and support for user-defined data structures.
Segment Table Base Register: The segment table base register is a crucial component in a computer's memory management system, specifically used in segmentation. It holds the starting address of the segment table in memory, which contains information about the segments allocated to a program. This register allows the system to access the segment table quickly, facilitating the translation of logical addresses to physical addresses while supporting features like protection and sharing among processes.
Segmented memory: Segmented memory is a memory management technique that divides a program's address space into different segments based on the logical organization of data and functions. This method allows for easier management of complex programs by separating them into manageable parts, such as code, data, and stack segments, and enhances the protection and sharing of memory among processes.
Swap space: Swap space is a designated area on a hard drive or other storage medium that the operating system uses as an extension of physical memory (RAM). When the system runs low on RAM, it can move inactive pages of memory to this swap space, allowing more active processes to utilize the available physical memory. This mechanism plays a critical role in managing memory efficiently, particularly in systems that implement strategies like segmentation and paging.