Segmentation with Paging
Duration: 4 min
This video lesson is available to enrolled students.
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The lecture introduces the concept of Segmentation with Paging, a memory management scheme designed to address the external fragmentation issues found in pure segmentation. The instructor explains that while segmentation divides a process into variable-sized logical units, these segments are further subdivided into fixed-size pages. This hybrid approach combines the logical organization of segmentation with the physical efficiency of paging. The core mechanism involves a two-level address translation process where a logical address is split into a segment number, a page number, and an offset. The instructor contrasts this with multilevel paging, highlighting that segmentation with paging allows for variable partition sizes within segments, unlike the fixed sizes in pure paging. The diagram illustrates the flow from logical address to physical address through segment and page tables.
Chapters
0:00 – 2:00 00:00-02:00
The instructor begins by presenting the slide titled 'Segmentation with Paging.' He explains the motivation behind this scheme, stating that segmentation suffers from external fragmentation, so it is better to divide segments into pages as their size increases. The on-screen text explicitly notes, 'Since segmentation also suffers from external fragmentation, it is better to divide the segments into pages as the segments size increases.' He describes the structure where a process is divided into segments, and segments are further divided into pages. He notes a similarity to multilevel paging but emphasizes a key difference: 'because here when page table is divided the size of partition can be different.' The diagram shows a logical address structure with fields for Segment No, Page No, and Offset, along with a Segment Table Base Register (STBR).
2:00 – 3:50 02:00-03:50
The instructor details the address translation flow using the diagram. He points to the 'CPU' box and the 'Logical Address' fields. He explains that the 'Segment No' is added to the 'STBR' (Segment Table Base Register) to locate the correct entry in the 'Segment Table.' This entry points to a specific 'Page Table.' The 'Page No' is then used to index into this page table to find the 'Frame No.' Finally, the 'Frame No' and 'Offset' are combined to create the 'Physical Address.' He draws on the right side of the screen to visualize how segments map to pages and then to frames in 'Main Memory,' illustrating that different segments can have different page table sizes. The bottom text reads 'Translating Logical Address into Physical Address,' reinforcing the process being demonstrated.
The lesson effectively bridges the gap between segmentation and paging. By using a segment table to point to individual page tables, the system gains the flexibility of variable-sized segments while maintaining the non-contiguous, fixed-size allocation of pages. This structure allows for efficient memory management where the overhead of fragmentation is reduced, and the logical view of the program (segments) is preserved. The visual tracing of the address bits through the tables reinforces the hardware mechanism required for this translation, showing how the CPU uses the STBR and segment number to find the correct page table before calculating the final physical address.