Segmentation
Duration: 9 min
This video lesson is available to enrolled students.
AI Summary
An AI-generated summary of this video lecture.
This educational video provides a detailed lecture on Segmentation as a memory management scheme in operating systems. The instructor begins by contrasting segmentation with paging, emphasizing that segmentation supports the user's view of memory by organizing it into variable-length segments such as subroutines and stacks. The lecture defines the logical address space as a collection of segments, each identified by a name and length. The instructor explains that a logical address consists of a two-tuple: <segment-number, offset>. He illustrates the concept of variable-length segments using whiteboard diagrams, showing how they differ from fixed-size pages. The final section covers the hardware mechanism for address translation using a segment table, which contains segment base and limit registers. The process involves indexing the table with the segment number, checking if the offset is within the limit, and adding the base to the offset to generate the physical address. The lecture concludes by noting that segmentation suffers from external fragmentation.
Chapters
0:00 – 2:00 00:00-02:00
The instructor introduces the concept of Segmentation, contrasting it with paging. The slide text states, 'Paging is unable to separate the user's view of memory from the actual physical memory,' while segmentation supports this user view. He explains that a logical address space is a collection of segments, where each segment has a name and a length. The instructor highlights that addresses specify both the segment name and the offset within the segment. Consequently, the user specifies each address by two quantities: a segment name and an offset. The slide explicitly defines a logical address as a two-tuple: <segment-number, offset>. He notes that segments can be of variable lengths unlike pages and are stored in main memory.
2:00 – 5:00 02:00-05:00
The instructor moves to a whiteboard to visually demonstrate the concept of variable-length segments. He draws a large vertical rectangle to represent physical memory and divides it into smaller rectangles of varying heights to represent segments. He explains that unlike paging, where pages are fixed size, segments can be of different sizes depending on the program's needs. He points to the diagram on the slide which shows a logical address space containing distinct blocks labeled 'subroutine', 'stack', 'symbol table', 'Sqrt', and 'main program'. He draws a corresponding physical memory layout on the board, showing these segments placed in memory with gaps between them, illustrating how they are stored in main memory. He emphasizes that the user sees a logical address space composed of these named segments.
5:00 – 9:30 05:00-09:30
The lecture transitions to the hardware mechanism for segmentation using a 'Segment Table'. The slide text states: 'Each entry in the segment table has a segment base and a segment limit.' The segment base contains the starting physical address where the segment resides, and the segment limit specifies the length of the segment. The segment number is used as an index to the segment table. The offset d of the logical address must be between 0 and the segment limit. If it is not, the system traps to the operating system. When an offset is legal, it is added to the segment base to produce the address in physical memory. The instructor draws a flow diagram showing the CPU generating a logical address (s, d). The segment number s indexes the table to retrieve the base and limit. A comparison checks if d < limit. If valid, base + d yields the physical address. He writes numbers like '109' and '1105' to demonstrate the addition. Finally, he notes that segmentation suffers from External Fragmentation.
The video provides a comprehensive overview of segmentation as a memory management technique. It starts by defining the logical address space as a collection of variable-length segments, contrasting this with the fixed-size pages of paging. The instructor uses diagrams to visualize how segments like stacks and subroutines are organized logically and physically. The lesson culminates in explaining the hardware support required, specifically the segment table containing base and limit registers, which enables address translation and protection by checking offsets against segment limits.