1. Segmentation Observation: Programmers don’t think in pages!
CSCE 410/611
Segmentation
Observation: Programmers don’t think in pages!
Segmentation
Pro-and-Cons of segmentation; external fragmentation
Solution: Segmented Paging
Segmentation

2. Segmentation: Problem with Paging
CSCE 410/611
Segmentation: Problem with Paging
Observation:
Users think of memory in terms of segments (data, code, stack, objects, ...)
Paging MMU
logical memory
physical memory
Segmentation

3. Segmentation: Problem with Paging
CSCE 410/611
Segmentation: Problem with Paging
Observation:
Users think of memory in terms of segments (data, code, stack, objects, ...)
Segmenting MMU A
< + A B
< + C B C
< +
logical memory
physical memory
Segmentation

4. Segmentation in Hardware
CSCE 410/611
Segmentation in Hardware
segment number
Segmenting Memory Management Unit
CPU
<? + s
logical addr d
offset b s d
limit
base
logical memory
segment table
physical memory
Segmentation

5. Segmentation: Pros and Cons
CSCE 410/611
Segmentation: Pros and Cons
Pros:
Data in a segment typically semantically related
Protection can be associated with segments
read/write protection
range checks for arrays
Data and code sharing
Process 1 s d
<? +
limit
base
Process 2
base
limit + d
<? s
physical
memory
Segmentation

6. Segmentation: Pros and Cons
CSCE 410/611
Segmentation: Pros and Cons
Pros:
Data in a segment typically semantically related
Protection can be associated with segments
read/write protection
range checks for arrays
Data and code sharing
Cons:
External Fragmentation!
Process 1 s d
<? +
limit
base
new segment
Process 2
base
limit + d
<? s
physical
memory
Segmentation

7. Segmented Paging CPU s p d f d s p logical memory physical memory
CSCE 410/611
Segmented Paging
Segmented-Paging Memory Management Unit
segment number
page nr
offset
CPU s
logical addr p d f d
<? s s p
limit
base
frame
logical memory
segment table
segment
page table
physical
memory
Segmentation

8. Segmentation: Summary
CSCE 410/611
Segmentation: Summary
Programmers like to think in terms of memory segments
Segmentation allows for efficient allocation and management of semantically-related data segments
Segmentation re-introduces external Fragmentation
Segmented Paging eliminates fragmentation
Segmentation