Segmentation is a memory management technique that divides memory into logical segments, improving organization and access control. It allows for memory protection and addresses external fragmentation issues by structuring memory in non-contiguous segments with associated permissions. Segmentation can be used alongside paging to manage segments in virtual address space, further optimizing memory use and process isolation.

1. What is segmentation
Segmentation is a technique to break
memory into logical pieces where each
piece represents a group of related
information.

2. Why is Segmentation used?
• Segmentation is one of the most common ways to achieve memory
protection
• Because internal fragmentation of pages takes place ,the user’s view of
memory is lost
• The user will view the memory as a combination of segments
• In this type, memory address used are not
contiguous.
• Each memory segment is associated with specific length and set of
permission
• When a process tries to access the memory it is first checked to see
whether it has the required permission to access the particular memory
segment by the particular memory

3. Segmentation
• Process are allocated with
segments
• Segment that the process
needs like
heap,stack,code,data are
divided into segments
• Segments make the
process more organized
• Logical view- multiple
separate segments
• Typical- code,data,stack
• Others-memory sharing etc

4. Memory Segmentation in 8086

5. Segmentation Architecture
• Each segment has
contiguous memory
• Logical address consists of
a two tuple: <segment-
number, offset>,
• Segment table – maps
two-dimensional physical
addresses; each table
entry has:
• 1. base – contains the
starting physical address
where the segments
reside in memory.
• 2. limit – specifies the
length of the segment.

6. Segmentation Hardware

7. Segmentation Hardware
• Let’s first assume no paging in the system
• User generates logical addresses
• These addresses consist of a segment number and an
offset into the segment
• Use segment number to index into a table
• Table contains the physical address of the start of the
segment
– often called the base address
• Add the offset to the base and generate the physical
address
– before doing this, check the offset against a limit
– the limit is the size of the segment

8. Segmentation with paging

9. Segmentation with Paging

10. Segmentation with Paging
• Most architectures support segmentation and paging
• Basic idea,
– segments exist in virtual address space
– base address in segment descriptor table is a
virtual address
– use paging mechanism to translate this virtual
address into a physical address
• Now an entire segment does not have to be in
memory at one time
– only the part of the segment that we need will be
in memory

11. Segmentation with Paging

12. Example of
Segmentation

13. Segment Protection
Protection: How does one process ensure that no
other process can access its memory? OS make sure
that it never creates a segment table entry that
points to same physical memory

14. Segment Sharing

15. Segmentation issue
• Entire segment is either in memory or on disk
• Variable sized segments leads to external
fragmentation in memory
• Must find a space big enough to place
segment into
• May need to swap out some segments to
bring a new segment in

17. Segmentation Over Paging
• Paging:
Fixed Size
Transparent to programmer (system allocates memory)
No separate protection
No separate compiling
No shared code
Internal Fragmentation take paces
• Segmentation:
Involves programmer (allocates memory to specific function inside code)
Separate compiling
Separate protection
Share code
Variable Size
Internal l Fragmentation does not take place

18. Thank You