Operating Systems

Unit 7:– Virtual Memory organization

Operating Systems

COP 5994 - Operating Systems 2

Evolution of memory organization

COP 5994 - Operating Systems 3

Virtual Memory concept

• Solves problem of limited memory space– Creates illusion of more memory than exists

• Creates 2 types of addresses• Virtual addresses

– Referenced by processes

• Physical addresses– Describes locations in main memory

• Memory management unit (MMU)• Dynamic address translation • Translates virtual addresses to physical address

COP 5994 - Operating Systems 4

Two-level Storage

• backing store:– maintained on hard drive– contains complete virtual memory

• real memory:– portion of virtual memory that is currently

in use

• Process can only access real memory– Management of mapping is essential

COP 5994 - Operating Systems 5

Two-level Storage

COP 5994 - Operating Systems 6

Address Translation

• Mapping virtual addresses to real addresses

• need address translation map per process

• Address translation maps– Indicate which regions of a process’s

virtual address space are currently in main memory and where they are located

COP 5994 - Operating Systems 7

Mapping based on block

COP 5994 - Operating Systems 8

• address translation map is table with – block number– actual location of block in memory

• typical implementation– block number is offset from table origin– table entries contain block location in

memory

Concept: block map table

COP 5994 - Operating Systems 9

• structure of virtual address:

•b is the block number in virtual memory•d is the displacement from the start of block

bat which the referenced item is located

Concept: virtual address

COP 5994 - Operating Systems 10

Block Mapping Process

COP 5994 - Operating Systems 11

Blocks ?

• Pages– Blocks are fixed size– Technique is called paging

• Segments– Blocks maybe of different size– Technique is called segmentation

COP 5994 - Operating Systems 12

Paging

• Paging uses fixed-size block mapping:– Virtual address in paging system: v =

(p, d)•p is the number of the page in virtual

memory on which the referenced item resides

•d is the displacement from the start of page p at which the referenced item is located

COP 5994 - Operating Systems 13

Page Frame: page in real memory

• Fixed-size block of main memory

• Aligned with page size multiple

COP 5994 - Operating Systems 14

Page to Frame correspondence

COP 5994 - Operating Systems 15

Page Table Entry (PTE)

• maps virtual page p to page frame p´– Contains bit to indicate if page is in real memory

– Term: Page Fault• access to memory location on page that is not in real

memory

COP 5994 - Operating Systems 16

Page Address Translation

• Direct Mapping• Associative Memory• Translation Lookaside Buffer

• Concerns:– memory access speed– size of page table

COP 5994 - Operating Systems 17

Address Translation by Direct Mapping

COP 5994 - Operating Systems 18

Page Table size

• address size: 4 bytes bytes virtual memory amount: 4GB

• Page size: 4k pages in virtual memory

• PTE size: 8 bytes bytes maximum page table size: 8MB

• Page table will not fit into processor cache

202

322

232

COP 5994 - Operating Systems 19

Page Table size

• address size: 8 bytes bytes virtual memory amount

• Page size: 4k pages in virtual memory

• PTE size: 16 bytes bytes maximum page table

size

• Page table will not fit into processor cache

522

642

562

COP 5994 - Operating Systems 20

Address Translation by Associative Mapping

COP 5994 - Operating Systems 21

Combination: Direct/Associative Mapping

Experience shows:TLB size of

64 or 128

Can reach 90% of associative mapping performance

COP 5994 - Operating Systems 22

Page Address Translation: concerns

• memory access speed– consider cache speed

• size of page table– 32 bit vs. 64 bit

• use of memory space– locality of memory references– sparseness

• advanced approaches– multi-level page tables– inverted page table

COP 5994 - Operating Systems 23

Multilevel Page Tables

• Hierarchy of page tables– Each level containing a table that stores

pointers to tables in the level below– Bottom-most level comprised of tables

containing address translations

• Can reduce memory overhead compared to direct-mapping system– page table does not have to be

contiguous

COP 5994 - Operating Systems 24

Multilevel Page Tables

COP 5994 - Operating Systems 25

Inverted Page Tables

– One inverted page table stores one PTE in memory for each page frame in the system

– Inverted relative to traditional page tables

– Uses hash functions to map virtual page to inverted page table entry

COP 5994 - Operating Systems 26

Inverted Page Tables

COP 5994 - Operating Systems 27

Inverted Page Tables

• Hash collisions increase memory accesses

• Collisions can be reduced by increasing the range of the hash function– Cannot increase the size of the inverted

page table because it must store exactly one PTE for each page frame

• trick: hash anchor table– one more level of indirection

COP 5994 - Operating Systems 28

Inverted Page Tables with hash anchor table

COP 5994 - Operating Systems 29

Also: Page Sharing

• programs may share common pages– data and/or

instructions

• pages marked as sharable or nonsharable

COP 5994 - Operating Systems 30

Segmentation

• based on variable-size segments– contiguous block of process’ address

space• data, text, stack

• process can runif current segment(s) is in main memory

COP 5994 - Operating Systems 31

Segmentation

COP 5994 - Operating Systems 32

Segmentation

• virtual memory address v = (s, d)– s is the segment number in virtual

memory– d is the displacement within segment s

at which the referenced item is located

COP 5994 - Operating Systems 33

Address Translation by Direct Mapping

COP 5994 - Operating Systems 34

Segment map table entry

• maps segment s to real memory address s´– has resident bit to indicate segment is

in memory• If so, it stores the segment base address• Otherwise, it stores the location of the

segment on secondary storage

– has segment length field• useful to protect memory outside of

segment

COP 5994 - Operating Systems 35

Segment map table entry

COP 5994 - Operating Systems 36

Segment map table entry: protection

COP 5994 - Operating Systems 37

Also: Segment Sharing

Caution: segment protection

COP 5994 - Operating Systems 38

Combination: Segmentation/Paging Systems

• Segments occupy one or more pages

• All pages of segment need not be in main memory at once

• Pages contiguous in virtual memory need not be contiguous in main memory

COP 5994 - Operating Systems 39

– s is segment number– p is page number within segment– d is displacement within page at which

desired item located

Virtual memory address

COP 5994 - Operating Systems 40

Address Translation

COP 5994 - Operating Systems 41

System-wide Table Structure

COP 5994 - Operating Systems 42

Sharing and Protection

• Page replacement requires updates to several tables

• Protection checking is complex

COP 5994 - Operating Systems 43

Agenda for next week:

• next week:– Chapter 11: Virtual Memory

Management– Read ahead !