1. Paging Memory Relocation and Fragmentation Paging
CSCE 410/611
Paging
Memory Relocation and Fragmentation
Paging
The Memory-Lookup process in hardware
Paging and Internal Fragmentation
Paging

2. Observation: Naïve relocation causes (external) fragmentation.
CSCE 410/611
Paging
Observation: Naïve relocation causes (external) fragmentation.
simple relocation
Memory Management Unit
limit
register
relocation
register ?
< +
logical memory
physical memory
Paging

3. Solution: Partition memory into equal-size portions.
CSCE 410/611
Paging
Solution: Partition memory into equal-size portions.
Memory Management Unit
< +
< +
< +
< +
logical memory
physical memory
Paging

4. Paging in Hardware CPU page table p d f d p f f logical memory
CSCE 410/611
Paging in Hardware
Memory Management Unit
page number
offset
CPU
logical addr p d
phys. addr f d p f f
page table
logical memory
physical memory
Paging

5. Paging Address Resolution: Example
CSCE 410/611
Paging Address Resolution: Example
Example: PDP-11 (16-bit addresses, page size 8kB => 64kB address space, with 8 pages)
Memory Management Unit
CPU
000
011
011 F0 F1 F2 F3 F4 F5 F6 F7 F8
page table
...
logical memory
physical memory
Paging

6. Paging Address Resolution: Example 2
CSCE 410/611
Paging Address Resolution: Example 2
Example: PDP-11 (16-bit addresses, page size 8kB => 64kB address space, with 8 pages)
Memory Management Unit
CPU
010
101 F0 F1 F2
101 F3 F4 F5 F6 F7 F8
page table
...
logical memory
physical memory
Paging

7. Internal Fragmentation in Paging
CSCE 410/611
Internal Fragmentation in Paging
page size 4kB
3 * 4096 +
1012 Bytes
logical memory
13300 Byte
3084 bytes
wasted!
physical memory
Observation: Last frame may not be completely full.
General Rule: Average internal fragmentation per block is typically half frame size.
Q: Frames: large or small?!
Large frames cause more fragmentation.
Small frames cause more overhead (page table size, disk I/O)
Paging

8. Paging: Summary Paging eliminates External Fragmentation
CSCE 410/611
Paging: Summary
Paging eliminates External Fragmentation
The Memory-Lookup process in the MMU
Paging allows for large address spaces
Paging does not completely eliminate Internal Fragmentation
Paging