1. Allocating Memory

2. Readings
Silberschatz: 8.3, 8.4

3. Outline
Contiguous Memory Allocation
Paging Concepts

4. Introduction
In this section we will discuss approaches to allocating memory to processes
Contiguous
Paging

5. Contiguous Memory Allocation

6. Contiguous Memory Allocation
We will start out with the most basic method used that allows multiple processes to reside in memory
With contiguous memory allocation each process is contained in a single section of memory that is contiguous

7. Fixed Partitioning
Any program, no matter how small, occupies an entire partition.
This represents internal fragmentation.
8 M
Operating System

8. Fixed Partitioning Unequal-size partitions
Lessens the problem with equal-size partitions
Operating System
8 M
12 M
6 M
4 M
2 M

9. Placement Algorithm with Partitions
Equal-size partitions
Since all partitions are of equal size, it does not matter which partition is used
Unequal-size partitions
Each process can be assigned to the smallest partition within which it will fit
There is a queue for each partition or a queue for all partitions
Processes are assigned in such a way as to minimize wasted memory within a partition

10. One Process Queue per Partition
Operating
System
New
Processes

11. One Common Process Queue
When its time to load a process into main memory the smallest available partition that will hold the process is selected
Operating
System
New
Processes

12. Simple MMU for Address Mapping
The base register holds the smallest physical memory address
The limit register specifies the range
These registers can be loaded only by the operating system

13. Simple MMU for Address Mapping
Simple mapping from logical addresses to physical
Relocation register: Value is added to every address generated by a user process

14. Questions Single Queue: Multiple Queues:
What is the problem with taking the first job that fits into the partition?
What is the problem with taking the largest job that fits into the partition?
Multiple Queues:
What is the disadvantage?

15. Multiprogramming with Fixed Partitions
Was used by OS/360 on large IBM mainframes for many years
Incoming jobs were queued until a suitable partition was available
Today no modern OS uses fixed partitions

16. Dynamic Partitioning Partitions are of variable length and number
Process is allocated exactly as much memory as required
Eventually get holes in the memory. This is called external fragmentation
Compaction is required to obtain a large block at the end of memory
Shift processes so they are contiguous and all free memory is in one block

17. Example Dynamic Partitioning
Operating
System
Operating
System
Operating
System
Process 1
320 K
Process 3
288 K
64 K
Process 4
128 K
96 K
320 K
320 K
Process 1
Process 1
224 K
224 K
Process 2
Process 3
288 K
Process 3
288 K
64 K
64 K

18. Example Dynamic Partitioning
Operating
System
Operating
System
320 K
Process 2
224 k
96 K
128 K
128 K
Process 4
Process 4
96 K
96 K
Process 3
288 K
Process 3
288 K
64 K
64 K

19. Dynamic Partitioning Placement Algorithm
Operating system must decide which free block to allocate to a process
Best-fit algorithm
Choose the block that is closest in size to the request
This has the worst overall performance
The smallest block is found for a process
The smallest amount of fragmentation is left
Memory compaction must be done more often

20. Dynamic Partitioning Placement Algorithm
First-fit algorithm
Starts scanning memory from the beginning and chooses the first available block that is large enough.
Next-fit
Starts scanning memory from the location of the last placement and chooses the next available block that is large enough

21. Paging

22. Introduction Previous memory schemes were characterized by
Need for compaction
External fragmentation
Paging is a memory management technique that avoids compaction and external fragmentation

23. Paging
Partition physical memory into small equal-size chunks and divide a process’s logical memory into the same size chunks
The chunks of a process are called pages and chunks of memory are called frames
When a process is to be executed, its pages are loaded (from a file system) into any available memory frames
A page/frame is associated with a set of memory addresses

24. Paging An operating system maintains a page table for each process
Contains the frame location for each page in a process

25. Page Example A0 A1 A2 A3 B0 B1 Page Tables
Frame
Number A0 1 A1 7 2 A2 1 1 1 8 3 A3 2 2 2 9 4 3 3
Process B 5 6
Process A 7 B0 B1 8
Page Tables
Each page table entry consists of a frame number
The contents of the page fill that frame number 9 B2 10 11 12 13 14

26. Address Translation Scheme
Address generated by CPU is divided into:
Page number (p) – used as an index into a page table which contains start address of each page in physical memory
Page offset (d) – combined with start address of the page to define the physical memory address that is sent to the memory unit

27. Address Translation Scheme
Assume that the number of logical address bits is m
Organization of an address is the following
page number
page offset p d
m - n n

28. Address Translation Scheme
If the number of logical address bits is m and the number of bits in the offset is n then
Logical address space is 2m
Page size (number of entries in page) is 2n
Example: m = 4, n = 2
Number of addresses is 16
Number of entries in a page is 4

29. Paging Hardware – Memory Management Unit (MMU)

30. Paging Hardware
The CPU issues a logical address (remember that all addresses are binary)
The hardware extracts the page number, p, and the page offset d
The page number, p, is used to index the page table
The entry in the page table consists of the frame number, f
The actual address is the concatenation of the bits that make up f and d.

31. Paging Example m=4 n=2 Page size of 4 bytes
Physical memory of 32 bytes (8 pages)
Binary representations:
0 00
1 01
2 10
3 11

32. Paging Example
First a little reminder. All addresses are in bits. The example on the previous page gives the base 10 equivalent of binary numbers
0000 1
0001 2
0010 3
0011 4
0100 5
0101 6
0110 7
0111 8
1000 9
1001 10
1010 11
1011 12
1100 13
1101 14
1110 15
1111

33. Example Logical address in binary form is 0000
From the bits we extract the page number as 00 and the offset as 00
Page number 00 corresponds to the first entry of the page table.
There we find that the corresponding frame is 5 (or 0101)
The physical address produced is the concatenation of the bits of the frame number and the offset
010100
In base 10 we find that the physical address is 20

34. Example Logical address in binary form is 0101
From the bits we extract the page number as 01 and the offset as 01
Page number 01 corresponds to the second entry of the page table.
There we find that the corresponding frame is 6 (or 0110)
The physical address produced is the concatenation of the bits of the frame number and the offset
011001
In base 10 we find that the physical address is 25(6*4+1)

35. Exercise
What do you think logical address 13 maps to?

36. Observations Let’s look at logical addresses:
0000 (0), 0001 (1), 0010(2) and 0011(3)
The first two bits (page number) are the same
This means that these logical addresses will be in the same frame.
The position in the frame is determined by the offset
For a process its pages can be in any order
For our example page 1 is in a frame that appears “later” than the frame associated with page 2

37. Observations
The logical address space and the physical address space DO NOT have to be the same size
The logical address space can be larger then the physical address space (more on this later)

38. Paging No external fragmentation
Any free frame can be allocated to a process that needs it
There may be some internal fragmentation
The memory requirements may not coincide with page boundaries
The last frame allocated may not be completely full

39. Paging Larger page sizes means more wasted space
Internal fragmentation is on average one half page per process
Larger page sizes means more wasted space
Smaller page sizes means larger tables

40. Summary
This section introduced the concept of paging in memory