1. Chapter 8: Main Memory

2. Chapter 8: Memory Management
Background
Swapping
Contiguous Memory Allocation
Paging
Structure of the Page Table
Segmentation
Example: The Intel Pentium

3. Background
Program must be brought (from disk) into memory and placed within a process for it to be run
Main memory and registers are only storage CPU can access directly
Register access in one CPU clock (or less)
Main memory can take many cycles
Cache sits between main memory and CPU registers
Protection of memory required to ensure correct operation

4. Base and Limit Registers
A pair of base and limit registers define the logical address space

5. Hardware Address Protection with Base and Limit Registers

6. Memory management
Memory management and CPU scheduling are perhaps the two most central tasks of an OS
Memory consists of a large array of words , each with its own address.
Both program and data are stored in memory .
Memory is given an address and it returns value .

7. Logical vs. Physical Address Space
The concept of a logical address space that is bound to a separate physical address space is central to proper memory management
Logical address – generated by the CPU; also referred to as virtual address
Physical address – address seen by the memory unit
Logical and physical addresses are the same in compile-time and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme

8. Memory-Management Unit (MMU)
Hardware device that maps virtual to physical address
In MMU scheme, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory
The user program deals with logical addresses; it never sees the real physical addresses

9. Dynamic relocation using a relocation register

10. Swapping A process needs to be in memory to be executed .
* however , it may be temporarily swapped out of memory into a backing store .
* the process can later be restarted by swapping it back into memory .
* Backing is generally a fast disk. Speed is important .
Swapping example :-
When a process block or timeout:-
1- swap it out to disk
2- swap in anew (ready ) process from disk to memory
3- dispatch the new process .
Another example :- roll out , roll in

11. Schematic View of Swapping

12. Memory Management Requirements
Relocation
Programmer cannot know where the program will be loaded in memory when it is executed
A program may be relocated (often) in main memory due to swapping
Memory references in code (for both instructions and data) must be translated to actual physical memory addresses
Chapter 9

13. Memory Management Requirements
Protection
Processes should not reference memory locations in another process without permission
Cannot do this at compile time, because we do not know where the program will be loaded in memory
address references must be checked at run time by hardware
Chapter 9

14. Memory Management Requirements
Sharing
must allow several processes to access a common portion of data or program without compromising protection
cooperating processes may need to share access to the same data structure
Program text can be shared by several processes executing the same program for a different user
Saves memory over separate copy for each
Chapter 9

15. Memory Management Requirements
Physical Organization
Memory hierarchy: (several types of memory: from slow and large to small and fast.)
main memory for program and data currently in use, secondary memory is the long term store for swapping and paging
moving information between levels of memory is a major concern of memory and file management (OS)
should be transparent to the application programmer
Chapter 9

16. Simple Memory Management
We will look at the following simple memory management techniques :
fixed partitioning
dynamic partitioning
Chapter 9

17. Fixed Partitioning (Single Process)
If only one process allowed (e.g. MS-DOS):
only one user partition
and one for the OS
Not many decisions to make:
program either fits or it doesn’t
I/O and
Interrupt vectors
“OperatingSystem”
(BIOS)
User space
640k
Chapter 9

18. Fixed Partitioning
Partition main memory into a set of non overlapping regions called partitions
Partitions can be of equal or unequal sizes
..both pictures show partitioning of 64 Mmain memory
Chapter 9

19. any program smaller than a partition can be loaded into the partition
Fixed Partitioning
any program smaller than a partition can be loaded into the partition
if all partitions are occupied, the operating system can swap a process out of a partition to make room
a program may be too large to fit in a partition.
Chapter 9

20. Fixed Partitioning
Main memory use is inefficient. Any program, no matter how small, occupies an entire partition.
This leaves holes of unused memory inside the partitions: internal fragmentation.
Unequal-size partitions can help
put small programs in small partitions
but there will still be holes...
Equal-size partitions was used in early IBM’s OS/MFT (Multiprogramming with a Fixed number of Tasks)
Chapter 9

21. Fixed Partitioning
Examples