1. SLC/VER1.0/OS CONCEPTS/OCT'99
Memory Management
Background
Logical Versus Physical Address Space
Swapping
Paging
Virtual Memory
Demand Paging’
Thrashing
SLC/VER1.0/OS CONCEPTS/OCT'99

2. SLC/VER1.0/OS CONCEPTS/OCT'99
Background
Program must be brought into memory and placed within a process for it to be executed
Input queue – collection of processes on the disk that are waiting to be brought into memory for execution.
User programs go through several steps before being executed.
SLC/VER1.0/OS CONCEPTS/OCT'99

3. SLC/VER1.0/OS CONCEPTS/OCT'99
Address Binding
Address binding of instructions and data to memory addresses can happen at three different stages. -Compile Time -Load Time -Execution Time
SLC/VER1.0/OS CONCEPTS/OCT'99

4. SLC/VER1.0/OS CONCEPTS/OCT'99
Dynamic Loading
Routine is not loaded until it is called
Better memory-space utilization
Useful for large amount of code are needed
No special support from the OS is required
SLC/VER1.0/OS CONCEPTS/OCT'99

5. SLC/VER1.0/OS CONCEPTS/OCT'99
Dynamic Linking
Linking postponed until execution time.
Stub used to locate the appropriate memory-resident library routine.
OS needed to check if routine is in processes memory address.
SLC/VER1.0/OS CONCEPTS/OCT'99

6. Logical Vs Physical address space
Logical address are generated by CPU
Unit of memory is commonly referred as a physical addresses
logical and physical address differ at the execution time
The run time mapping from virtual to physical address is done by the MMU, which is a hardware device.
SLC/VER1.0/OS CONCEPTS/OCT'99

7. Logical Vs Physical address space
Relocation
register
1400 +
Logical address
Physical address
CPU
Memory
346
14346
MMU
Dynamic relocation using a relocation register
SLC/VER1.0/OS CONCEPTS/OCT'99

8. SLC/VER1.0/OS CONCEPTS/OCT'99
SWAPING
A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution
swapping can be done on priority bases
A process that is swapped out will be swapped back into the same memory space that is occupied previously
SLC/VER1.0/OS CONCEPTS/OCT'99

9. Schematic View of Swapping
SLC/VER1.0/OS CONCEPTS/OCT'99

10. SLC/VER1.0/OS CONCEPTS/OCT'99
Paging
Physical memory is broken into fixed size blocks called frames
logical memory is broken into blocks of same size called pages
paging is done with the hardware support
SLC/VER1.0/OS CONCEPTS/OCT'99

11. SLC/VER1.0/OS CONCEPTS/OCT'99
Paging
Logical address
Physical
Memory
CPU
P d
F d
Page table f p
Paging hardware
SLC/VER1.0/OS CONCEPTS/OCT'99

12. Paging E.g u a I j k l 1 b 2 c 3 4 3 d m n o p 4 1 4 e f 8 5 1 g 2 6 au a I j k l 1 b 2 c 3 4 3 d m n o p 4 1 4 e f 8 5 1 g 2 6 a b c d h 7 3 2 12 8 i
Page table e f g h 9 j 10 k 16 11 l
Logical Memory
page Example for 32-byte memory with 4byte pages
Physical memory
SLC/VER1.0/OS CONCEPTS/OCT'99

13. SLC/VER1.0/OS CONCEPTS/OCT'99
Virtual Memory
Virtual memory – separation of user logical memory from physical memory.
Only part of the program needs to be in memory for execution.
Logical address space can therefore be much larger than physical address space.
Need to allow pages to be swapped in and out.
Virtual memory can be implemented via:
Demand paging
SLC/VER1.0/OS CONCEPTS/OCT'99

14. SLC/VER1.0/OS CONCEPTS/OCT'99
Demand-paging
Bring a page into memory only when it is needed -Less I/O needed -Less memory needed -Faster response -more users
Page in, Page out and Page fault.
SLC/VER1.0/OS CONCEPTS/OCT'99

15. SLC/VER1.0/OS CONCEPTS/OCT'99
Valid-Invalid Bit
With each page table entry a valid-invalid bit is associated.
Example of a page table snapshot
Frame #
valid-invalid bit 1 1 1 1 M
SLC/VER1.0/OS CONCEPTS/OCT'99

16. SLC/VER1.0/OS CONCEPTS/OCT'99
Demand-paging OS 3 2 1 i
Load M 6
Free frame 5 4
SLC/VER1.0/OS CONCEPTS/OCT'99

17. What happens if there is no free frame?
Page replacement - find some page in memory, but not really in use, swap it out -algorithms -performance-want an algorithm which will result in minimum number of page faults.
Same page may be brought into memory several times.
SLC/VER1.0/OS CONCEPTS/OCT'99

18. SLC/VER1.0/OS CONCEPTS/OCT'99
Thrashing
If process doesn’t have “enough” pages, the page-fault rate is very high. This leads to: -low CPU utilization -OS thinks that it needs to increase the degree of multiprogramming -another process added to the system
Thrashing= a process is busy swapping pages in and out
SLC/VER1.0/OS CONCEPTS/OCT'99

19. SLC/VER1.0/OS CONCEPTS/OCT'99
Thrashing
Causes Of Thrashing: C P U t I l z a o n
Thrashing
Degree of multiprocessing
SLC/VER1.0/OS CONCEPTS/OCT'99