Segmentation

Segmentation

• In segmentation, a program's data and instructions are divided into blocks called segments. A segment is a logical entity in a program.

• Fig. 4.7.1 shows programmer's view.

• Logical view: A process consists of a set segments.

• Physical view: It consists of non-adjacent areas of memory allocated to segments.

• All segment size may be equal or may not be equal. Segmentation support user view of memory. Fig 4.7.1 shows programmer`s view of a program.

• Segmentation can be implemented with or without paging.

• Collection of segment is called as logical address space. Each segment is identified by its name.

• Processor generates logical addresses. These addresses consists of a segment number and an offset into the segment. Segment number is used as an index to segment page table.

• Fig 4.7.2 shows logical address

• Segment names are normally symbolic names.

• Operating system maintains a segment table for each process. It is usually stored in main memory as a segment that is not to be loaded as long as the process can

• Each entry in the segment table has a segment base and a segment limit. The base field contains the segment relocation register for the target segment. The segment limit field contains the length of the segment.

• Fig. 4.7.3 shows an address translation in segmentation.

• Segment table contains the physical address of the start of the segment. Then add the offset to the base and generate the physical address.

• If the required reference is not found in one of the segment registers, then error is generated. At the same time, operating system does lookup in segment table and loads new segment descriptor into the register.

Protection and Sharing

• Sharing of segment provides less overhead than the pure paging segment. Multiple processes can share a segment

• Two processes share a segment when their segment table entries point to the same segment in main memory. Operating system maintains global segment table for everyone and also used by OS.

• Access rights for segment are usually included in table entry. An illegal access of memory is protected by memory mapping hardware. It checks the protection bits associated with each segment table.

• Fig. 4.7.4 shows sharing in segmentation. Access rights for segments are : Read, write, append and execute.

• Segmentation suffer from external fragmentation because segment sizes vary. Entire segment is either memory or on secondary storage disk.

Advantages

1. It provides virtual memory.

2. Allows dynamic segment growth.

3. Segmentation assists dynamic linking.

4. Segmentation is visible.

Disadvantages

1. Maximum size of a segment is limited by the size of main memory.

2. Difficulty to manage variable size segments on secondary storage.

3. Fragmentation and complicated memory management.

Difference between Segmentation and Paging

Example 4.7.1 Describe a mechanism by which one segment could belong to the address space of two different processes. AU May-16, Marks 8

Solution: Segment tables are a collection of base-limit registers, segments can be shared when entries in the segment table of two different jobs point to the same physical location. The two segment tables must have identical base pointers, and the shared segment number must be the same in the two processes.

University Questions

1. Draw the diagram of segmentation memory management scheme and explain its principle. AU Dec.-17, Marks 13

2. Explain why sharing a reentrant module is easier when segmentation is used than when pure paging is used with example.  AU May-18, Marks 13