1. Lecture 11: Memory Management

2. Example memory configuration before and after allocation of a 16KB block
First Fit (16K)
22K 6K
Best Fit (16K)
18K 2K
Last allocated block (14K) 8K 8K 6K 6K
14K
14K
Next Fit (16K)
36K
20K

3. Hardware support for relocation
Relative Address
Process Control Block
Base register
Program
Adder
Absolute Address
Data
Bounds register
Comparator
Stack
Interrupt to Operating System

4. Logical addresses
Logical Address:
Page# = 1, Offset = 478
Logical Address:
Segment# = 1, Offset = 752
Relative Address = 1502
Page 0
Segment bytes
478
752
Page 1
User process 2700 bytes
Segment bytes
Page 2

5. Paging
6-Bit page
10-Bit offset
Process Page Table
000101
000110
011001 1 2
16-Bit physical address

6. Segment
4-Bit segment
10-Bit offset
Process Segment Table
Length Base 1 +
16-Bit physical address

7. Memory Management Techniques
Description
Strengths
Weaknesses
Fixed Partioning
Main memory is divided into a number of static partitions at system generation time. A process may be loaded into a partition of equal or greater size
Simple to implement; little operating-system overhead
Inefficient use of memory due to internal fragmenation; number of active processes is fixed
Dynamic Partitioning
Partitions are crerated dynamically, so that each process is loaded into a partition of exactly the same size as that process
No internal fragmentation; more efficient use of main memory
Inefficient use of processor due to the need for compaction to counter external fragmentation
Simple paging
Main memory is divided into a number of equal size frames. Each process is divided into a number of equal size pages of the same length as frames. A process is loaded by loading all of its pages into available, not necessarily contiguous frames
No external fragmentation
A small ammount of internal fragmentation
Simple segmentation
Each process is divided into a number of segments. A process is loaded by loading all of its segments into dynamic partitions that need not be contiguous
No internal fragmentation
Need for compaction

8. Paging Technique Overlays Paging
When a program is too big to fit in the available memory, the programmer has to split it into pieces
The swapping is done by the OS
Paging
The physical memory is partitioned into small equal fixed-size chunks (called frames)
The logical memory is also divided into small and equal fixed-size chunks (called pages)

9. Paging Technique (ctd)
Features
Internal fragmentation: only a fraction of the last page of a process
No external fragmentation
In order to load the pages of a process into non contiguous frames the OS needs page table for each process
Logical address consists of a page number and an offset within the page
Translation logical-to-physical address is done by the processor hardware

10. Paging Hardware P d F d P CPU Physical Memory Logical Address Physical
Page table

11. Paging Hardware CPU P d Physical Memory F d P Logical Address Address
Page table
P d
F d P
Logical Address
Address
TLB hit
TLB miss

12. Analysis Definition Performance
Hit ratio: percentage of times that a page number is found in the associative registers
Performance
Effective Access Time:
Hit ratio x TLB access time + Miss ratio x Memory access time

13. Segmentation Technique
The logical memory is divided into a number of segments of different length
Logical address consists of two parts: segment number and an offset
Features
With segmentation a process may occupy more than one partition
Internal fragmentation: solved
External fragmentation: Not solved

14. trap: addressing error
CPU
Physical
Memory
Segment table
S d S +
<
limit
Base
Yes No
trap: addressing error

15. Virtual Memory

16. Real Programs
An examination of real programs shows as that a program can be divided into two parts:
part of code rarely or never executed
code handling unusual error conditions
code handling certain options and features rarely used
allocation of much more memory needed (arrays, lists, tables)
part of code usually executed
Benefit of executing a program that is partially in physical memory

17. Virtual Memory Characteristics of paging and segmentation:
Process may be broken up into a number of pieces (pages or segments)
Memory references within a process are logical addresses
Logical addresses are dynamically translated into physical addresses
Pages or segments of a process do not need to be contiguously located in main memory by using page or segment table
With these characteristics it is not necessary that all pages or all segments of a process be in main memory during execution

18. Virtual Memory (ctd) Definition
Virtual memory is an illusion supported by system hardware and software that a user has a vast linear expanse of useful storage
In fact, a much smaller real memory is used to hold portions of a user’s program during execution

19. Virtual Memory (ctd) Advantages
A program is not constrained by the physical memory space available
Users can write an extremely large virtual address space
The increase in CPU utilisation and throughput
Less I/O would be needed to load or swap each user program into memory

20. Virtual Memory Features
Usually virtual memory systems use paging technique
OS must maintain a page/segment table for each process
OS must maintain a free frame list
A page or segment number and the offset are used to calculate absolute address
Not all pages or segments of a process need to be in main memory frames for the process to run
Reading a page or segment into main memory may require writing a page or segment out to disk (called page fault or segment fault)

21. Page Table Structure N. Page Offset N. Frame Offset Pointer + F.No.
Page number
Page table
Process
Main Memory
Register
Virtual Address
Page
Frame
Offset

22. Page Fault
Three major components of the page-fault service time are required:
Service the page-fault interrupt
Read in the page
Restart the process

23. Page Fault (ctd) A page fault causes the following sequence to occur:
Trap to the OS
Save the user registers and process state
Determine that the interrupt is a page fault
Check if the page reference is legal and determine its location in the disk
Issue a read from the disk to a free frame
While waiting, allocate the CPU to another process
Interrupt form the disk (I/O completed)

24. Page Fault (ctd)
Save the registers and the state of the current process
Determine that the interrupt is from the disk
Update the page table and other tables to show that the desired page is now in memory
Wait for the CPU to be allocated to this process again
Restore the process context (registers, state) and the new page table, then resume the interrupted instruction

25. NO LECTURE ON THURSDAY 20/02/2003 – SCIENCE DAY LECTURE ON FRIDAY 11-12 INSTEAD

26. Lecture 12: Memory Management

27. PAGING ONLY Page Number Offset P M Other Control Bits Frame Number
Virtual Address
Page Number Offset
Page Table Entry
P M Other Control Bits Frame Number

28. SEGMENTATION ONLY Segment Number Offset
Virtual Address
Segment Number Offset
Segment Table Entry
P M Other Control Bits Length Segment Base

29. COMBINED SEGMENTATION & PAGING
Virtual Address
Segment Number Page number Offset
Segment Table Entry
P M Other Control Bits Length Segment Base
Page Table Entry
P M Other Control Bits Frame Number

30. Address translation in a segmentation system
Virtual Address
Real Address +
Segment Number S# Offset d
Base + d d
Register
Seg. Table Ptr
Segment length + S
Length Base
Segment Table
Main Memory

31. Address translation in a segmentation/paging system
Virtual Address
Segment # Page # Offset
Frame # Offset d
Register
Seg. Table Ptr
Segment length P + + S
Page Table
Segment Table
Main Memory

32. Page size Page Fault Rate Page Fault Rate P
Number of page frames allocated
Page size

33. Protection relationships between segments
Main Memory
20 K
All accesses not allowed
Dispatcher
35 K
Branch instruction not allowed
50 K
Reference to data (not allowed)
Process A
Reference to data (allowed)
80 K
90 K
Process B
140 K
Process C
190 K

34. Replacement Policy Memory management concepts:
Deals with the selection of a page in memory to be replaced when a new page must be brought in
Memory management concepts:
The number of frames to be allocated to each active process
The replacement can be
limited to those of the process that caused the page fault
encompass all frames in main memory
Frame Locking: Much of the kernel and control structures of the OS is held on locked frames
the particular page that should be selected for replacement
The first 2 concepts are called “resident set management”

35. Basic Algorithms Optimal
replaces the page that will not be used for the longest period of time
LRU (Least recently used)
replaces the page in memory that has not been referenced for the longest time
FIFO
the pages are removed in round-robin style
Clock policy
When a page is referenced after a page fault its use bit is set to 1. When a page is replaced the pointer is set to indicate the next frame in the buffer. For replacement, the OS scans the buffer to find a frame with a use bit to 0

36. Example
Consider the following page address stream: and a fixed frame allocation of 3 frames 2 4 3 1 5
Optimal 2 3 5 1 4
LRU

37. Example (ctd) 2 5 3 1 4
FIFO 2 2* 3 5 5* 1 4
CLOCK

38. Fetch policy: Process pages can be brought in on demand, or a pre-paging policy can be used; the latter clusters the input activity by bringing in a number of pages at once
Placement policy: With a pure segmentation system, an incoming segment must be fit into an available space in memory
Replacement policy: When memory is full, a decision must be made as to which page or pages are to be replaced
Resident set management: The operating system must decide how much main memory to allocate to a particular process when that process is swapped in. This can be a static allocation made at process creation time, or it can change dynamically.
Cleaning policy: Modified process pages can be written out at the time or replacement, or a pre-cleaning policy can be used; the latter clusters the output activity by writing out a number of pages at once
Load control: Load control is concerned with determining the number of processes that will be resident in main memory at any given time

39. WEEK 8: EXAMPLES OF MEMORY MANAGEMENT
System/370 and MVS
Windows NT
Unix System V
LINUX