1. Main Memory Management

2. Objectives
To provide a detailed description of various ways of organizing memory
To discuss various memory-management techniques, including paging and segmentation

3. Background
Program must be brought (from disk) into memory and placed within a process for it to be run
Main memory, cache and registers are only storage CPU can access directly
Cache sits between main memory and CPU registers
Protection of memory required to ensure correct operation

4. Binding of Instructions and Data to Memory
Address binding of instructions and data
Compile time: If memory location known, absolute code can be generated; must recompile code if starting location changes
Load time: Must generate relocatable code if memory location is not known at compile time
Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps

5. Multistep Processing of a User Program

6. 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
Compile time & load time binding – same logical and physical addresses
Execution time binding – different logical and physical addresses

7. 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

8. Dynamic Loading Routine is not loaded until it is called
Better memory-space utilization; unused routine is never loaded
Useful when large amounts of code are needed to handle infrequently occurring cases
No special support from the operating system is required implemented through program design

9. Dynamic Linking Linking postponed until execution time
Small piece of code, stub, used to locate the appropriate memory-resident library routine
Stub replaces itself with the address of the routine, and executes the routine
Operating system needed to check if routine is in processes’ memory address
Dynamic linking is particularly useful for libraries

10. Swapping
A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution
Backing store – fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images
Roll out, roll in – swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed
Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped
Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and Windows)
System maintains a ready queue of ready-to-run processes which have memory images on disk

11. Schematic View of Swapping

12. Basic Memory Management Concepts
Mono programming vs multiprogramming
Fixed vs variable size partitions
Contiguous vs. non-contiguous blocks
Cache vs. RAM (main memory) vs External storage

13. Mono vs Multiprogramming
Mono Programming
Single process shares memory with OS
Security – only to protect OS
Very simple implementation
Multiprogramming
N processes share memory with OS
Security – protect all processes from each other
NO FRIENDLY processes
Boundary registers can be used to check processes

14. Mono-programming
Multiprogramming

15. Multiprogramming
Hole – block of available memory; holes of various size are scattered throughout memory
When a process arrives, it is allocated memory from a hole large enough to accommodate it
Operating system maintains information about:
a) allocated partitions
b) free partitions (hole) OS OS OS OS
process 5
process 5
process 5
process 5
process 9
process 9
process 8
process 10
process 2
process 2
process 2
process 2

16. Fixed vs. Variable partitions
Fixed Size partitions
Memory divided into fixed (possibly unequal) sizes
Queuing options for processes
One queue for RAM
Large jobs waiting longer for short jobs using large partitions
Load- time binding can be used
One queue for each partition
Best use of memory
Some partitions empty while processes are waiting
Compile time binding can be used
Fragmentation – internal (inside partitions)

17. Fixed Partitions

18. Fixed vs. Variable partitions
Variable Size partitions
Process given as much memory as needed
Initially better utilization – no gaps
Large processes can use all of memory
No room for growth unless it is added into partition
Fragmentation – external (holes created as processes complete)

19. Variable Size Partitions

20. Variable partitions 5K free P2 terminates: 2k free
Total: 22K free
P6(20k) > 22K free
Cannot load P6(20k)
- coalesce
Could load p7(4k)

21. Variable partitions Coalescing - combining of adjacent free space
don’t have to move any executing jobs
but doesn’t make use of all free space
Compaction - “burping” RAM (garbage collection)
combine all free space at one end of RAM
Requires that processes be moved
Uses CPU time

22. Contiguous vs non-contiguous
Contiguous Blocks
Each process occupies a single contiguous block
Relocation registers used to protect user processes from each other, and from changing operating-system code and data
Base register contains value of smallest physical address
Limit register contains range of logical addresses – each logical address must be less than the limit register
MMU maps logical address dynamically

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

24. Base and Limit Registers

25. Base and Limit Registers

26. Non-contiguous Non-contiguous Blocks
Each process is divided into segments that may not reside next to each other in memory.
Harder to implement (how to divide)
Better utilization of RAM

27. Memory Management Strategies
Fetch Strategies
When to put things into RAM
When to fetch the next piece of data or program to insert into RAM
demand fetching - Next item fetched when referenced
and not resident
anticipatory fetching - Retrieve larger quantities than
referenced to minimize number of fetches.

28. Memory Management Strategies
Placement Strategies
Where to place items in RAM
First-fit: Allocate the first hole that is big enough (fast)
Next fit - Similar, keeps track of where in List left off.
Quick fit - maintains separate lists for common sizes.
Best-fit: Allocate the smallest hole that is big enough
must search entire list, unless ordered by size
Produces the smallest leftover hole
Must search all of RAM
Most extreme external fragmentation (smallest left over hole)
But - leaves larger holes for larger items.

29. Memory Management Strategies
Placement Strategies
Worst-fit: Allocate the largest hole
must also search entire list ( or maintain pointer to largest)
Divides large chunks that are useful for large items
Produces the largest leftover hole (most useful fragment)

30. Memory Management Strategies
Replacement Strategies
Determine which item to replace with the incoming item
Examples:
FIFO
LRU (Least Recently Used)
Variable size partitions – MORE complicated!

31. Memory Usage Strategies
Bit maps
Memory is divided into allocation units (blocks)
Bit map contains a bit for each allocation unit
0 - represents it is free, 1 - represents it is in use.
Linked Lists
Linked list contains nodes sorted by address
Node contains:
type indicator (hole or process),
starting address,
length, and
pointer to previous and next node.

32. End of part 1