Chapter 9 Virtual Memory Produced by Lemlem Kebede Monday, July 16, 2001.

Chapter 9 Virtual Memory

Produced by Lemlem Kebede

Monday, July 16, 2001

What is Virtual Memory Swaps data in and out of physical

memory. Make physical memory appear to

the CPU that there is more physical memory than is actually present.

The three main reasons for using Virtual Memory1. To free programmers from the need to carry out storage

allocation and to permit efficient sharing of memory space among different users.

2. To make programs independent of the configuration and capacity of the memory systems used during their execution. 3. To achieve the high access rates and low cost Per bit that is possible with a memory hierarchy.

What does Virtual Memory really do? Virtual memory places open

applications that are not active and portions of active applications on your hard drive and only keeps what is necessary for functionality in real RAM.

Memory Management Unit (MMU)

Moves data between physical memory and some slower storage device, usually a disk.

It translates the CPU address (logical address) to physical memory address.

It moves data from the swap disk to physical memory and from physical memory back to the swap.

How the MMU convert logical address output from the CPU to a physical address

CPU MMUPhysicalMemory

Swap Disk

Controlsignal

Logical

address

Physical

address

Two primary methods to implement virtual memory

Paging Segmentation

Paging and Frames In paging, the entire range of logical

addresses is divided into contiguous blocks called pages.

Each pages is the same size and each logical address is resides in exactly one page.

Physical memory is divided into non-overlapping frames

The size of each frame is the same as the size of one page

paging Moves pages from the swap disk to

frames of the physical memory so data can be accessed by the processor. Any page can occupy any frame. This leads to several issues must be resolved by a paging system.

Paging issues that are handled

by the MMU

1. When should a page be moved into physical memory?

2. How does the CPU find data in physical memory?

3. What happens when all the frames have page and the CPU needs to access data from a page not currently stored in physical memory?

Paging Example: Simple CPU

Physical memory space –16K Logical address space –64k Page size –4k 4 frames in physical memory 4k

Page F

Page 0

Page 1

Page 2

FFFFH F000H

2FFFH 2000H 1FFFH 1000H 0FFFH 0000H

Frame 3

Frame 2

Frame 1

Frame 0

3FFFH 3000H 2FFFH 2000H

1FFFH 1000H 0FFFH 0000H

Logical memory Address space

Physical memory...

Page Table The MMU uses a page table to

keep track of which pages are stored in which frames.

The MMU uses a page table to calculate the required physical addresses.

Example of Page table and corresponding Physical Memory

0 1 1 1 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

1 1 1 0 1 1

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 1 0 0 0

Frame Valid Count Dirty

0

1

2

3

45

6

7

8

9A

B

CD

E

F

Page 4

Unused

Page 0

Page F

3FFFH3000H

2FFFH2000H

1FFFH1000H

0FFFH0000H

Physical Memory

Page Table

Page Fault The page is not located in physical

memory. The Memory Management unit

would cause the page to be copied from the swap disk to a frame in physical memory.

Demand Paging Memory Management unit would

clear a page out of physical memory and move the newly required page into the vacated frame if all frames in physical memory already contained pages.

BeginProcess fetched

word

Compute Physical address X of nextWord required Fetch required word

From M

Is X in M?

Is there a freePage frame in

M?

Select page frame F in M to replace

Has F been altered?

Transfer pageIn F to Secondary

memoryTransfer page containing XForm secondary memoryTo free page frame in M

Update page Address tables

YesNo

SuspendNormalexecution

Resume normalexecution

Yes

No

Yes

No

OPERATION

OF

A

DEMAND

PAGINGSystem

Translation Lookaside Buffer (TLB) Because of a time delay associated

with reading the frame number from the page table, and the system may read the same value hundreds or thousands of times consecutively as it fetches contiguous instructions, MMU usually contain a TLB.

Conversion of Logical Address to Physical Address Using the TLB

Offset

Logical address

Page

Page Frame Valid

0000 01 10100 11 1

1111 00 1

0101 10 1

OffsetFrame

Physical address:

To physical memory

Segmentation Another method of allocating

memory that can be used instead of or in conjunction with page.

Program is divided into several segments.

Segments can vary in size. Segmentation have a segmented

MMU.

Segments compared to paging Segments has advantage (internal

fragmentation), and disadvantage (external fragmentation)

Internal fragmentation A program of size 4k + 1 would

require the MMU to allocate two pages of memory, even though the second page would use one of its 4K locations.

External Fragmentation There are three segments resident

in memory and 8K of free space.

However, the free spaces is partitioned

such that no segment greater than 3K

can be loaded into memory with out

moving or removing one of the currently

loaded segments.

Unused

Segment 3

Unused

Segment 2

Unused

Segment 1

16K –1 13K13K –1 9k

9k –1 6k 6k –1 5k

5k –1 3k 3k –1 0

External fragmentation in physical memory caused by segmentation

Memory Protection

Why need Memory Protection?1. A multitasking operating system may have several different program, or parts

of programs, resident in memory simultaneously.

2. These program components may belong to one user or to different users.

3. The operating system may also have components.

Advantages of Virtual Memory You can run more

applications at once. You can run larger applications with less

real RAM. Applications may launch

faster because of File Mapping. You don't have to buy

more memory.

Disadvantages of Virtual Memory Applications run slower . It takes more time to switch

between applications. Less hard drive space for

your use. Reduced system stability.

Thanks

The End

Do you have any Question ?