The fundamental difference between paging and segmentation lies in how memory is divided: paging uses fixed-size blocks (pages), while segmentation uses variable-size blocks (segments) based on logical program units.
Let's break down the key differences in more detail:
Paging
- Memory Division: Divides both physical and virtual memory into fixed-size blocks called pages (virtual) and frames (physical).
- Size: Pages are typically small and uniform in size (e.g., 4KB).
- Basis: Based on physical memory organization. Has no logical relationship to the program structure.
- External Fragmentation: Eliminates external fragmentation because all memory blocks are the same size. Internal fragmentation can still occur if a process doesn't completely fill the last page allocated to it.
- Implementation: Simpler to implement due to the uniform size of memory blocks.
- Logical View: Does not directly correspond to the user's logical view of the program. The programmer is generally unaware of paging.
- Address Translation: Uses a page table to map virtual addresses to physical addresses.
- Protection: Provides protection on a page basis.
Segmentation
- Memory Division: Divides memory into variable-size blocks called segments.
- Size: Segments can be of different sizes and correspond to logical units of the program (e.g., code segment, data segment, stack segment).
- Basis: Based on the logical structure of the program. Segments correspond to logical units like procedures, functions, or data structures.
- External Fragmentation: Susceptible to external fragmentation as memory is allocated and deallocated in variable-sized chunks, leading to gaps between allocated segments. Internal fragmentation is typically less of a problem compared to paging.
- Implementation: More complex to implement compared to paging due to the variable size of segments. Requires more sophisticated memory management techniques.
- Logical View: Matches the user's logical view of the program.
- Address Translation: Uses a segment table to map logical addresses to physical addresses.
- Protection: Provides protection on a segment basis, allowing for different levels of access control for different logical units.
Comparison Table
| Feature | Paging | Segmentation |
|---|---|---|
| Memory Division | Fixed-size blocks (pages) | Variable-size blocks (segments) |
| Block Size | Uniform | Variable |
| Basis | Physical memory | Logical program structure |
| Fragmentation | Internal (primarily) | External (primarily) |
| Implementation | Simpler | More Complex |
| Logical View | Not directly related to program structure | Corresponds to logical program units |
| Address Translation | Page Table | Segment Table |
| Protection | Page basis | Segment basis |
In essence, paging manages memory from the operating system's perspective based on physical constraints, while segmentation manages memory from the programmer's perspective based on logical program components. Modern operating systems often use a combination of both techniques, such as paged segmentation, to leverage the benefits of each.
