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Memory SystemsMemory SystemsVirtual MemoryVirtual Memory

Lecture 25Digital Design and Computer Architecture

Harris & HarrisMorgan Kaufmann / Elsevier, 2007

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But First…But First…

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The Memory HierarchyThe Memory Hierarchy

Cache

Main Memory

Virtual Memory

Capacity

Speed

Technology cost / GB Access time

SRAM ~ $10,000 ~ 1 ns

DRAM ~ $100 ~ 100 ns

Hard Disk ~ $1 ~ 10,000,000 ns

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The Hard DiskThe Hard Disk

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Virtual Memory OverviewVirtual Memory Overview

• Physical Memory acts as a cache for virtual memory.

• Page table maps virtual pages to physical pages.

• This mapping of virtual addresses to physical addresses is called address translation.

• Translation Lookaside Buffers (TLBs) speed up address translation.

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Virtual and Physical AddressesVirtual and Physical Addresses

Physical Memory

Physical AddressesVirtual Addresses Address Translation

Hard Disk

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Cache/Virtual Memory AnaloguesCache/Virtual Memory Analogues

Cache Virtual Memory

Block Page

Block Size Page Size

Block Offset Page Offset

Miss Page Fault

TagVirtual Page Number

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Virtual Address TranslationVirtual Address Translation

Page OffsetPPN

11 10 9 ... 2 1 0

Page OffsetVPN

Virtual Address

Physical Address

Translation

30 29 28 ... 14 13 12

11 10 9 ... 2 1 026 25 24 ... 13 12

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Virtual Address TranslationVirtual Address Translation

Physical Memory

PhysicalPage Number Physical Addresses

Virtual Memory

VirtualPage NumberVirtual Addresses

7FFF 0x7FFF000 - 0x7FFFFFF0x7FFE000 - 0x7FFEFFF

0x0000000 - 0x0000FFF0x0001000 - 0x0001FFF

7FFE

00010000

7FFFA7FFF9

0000600005

7FFFC7FFFB

7FFFE7FFFD

7FFFF

0000100000

0000300002

00004

0x7FFFF000 - 0x7FFFFFFF0x7FFFE000 - 0x7FFFEFFF0x7FFFD000 - 0x7FFFDFFF0x7FFFC000 - 0x7FFFCFFF0x7FFFB000 - 0x7FFFBFFF0x7FFFA000 - 0x7FFFAFFF

0x00005000 - 0x00005FFF

0x00003000 - 0x00003FFF

0x00001000 - 0x00001FFF

0x7FFF9000 - 0x7FFF9FFF

0x00006000 - 0x00006FFF

0x00004000 - 0x00004FFF

0x00002000 - 0x00002FFF

0x00000000 - 0x00000FFF

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The Page TableThe Page Table

001 0x00001 0x7FFE00

001 0x0001001 0x7FFF00

V

VirtualAddress 0x00002 47C

Hit

PhysicalPage Number

1219

15 12

VirtualPage Number

Pag

e T

able

PageOffset

PhysicalAddress 0x7FFF 47C

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Replacement PoliciesReplacement Policies

Page Table

VirtualPage Number

Physical

Page Number

7FFFA

0000600005

001 0x00001 0x7FFE 7FFFC

7FFFB

7FFFE7FFFD

7FFFF

0000100000

0000300002

00004

00

001 0x0001001 0x7FFF00

V

00007

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Translation Lookaside Buffer (TLB)Translation Lookaside Buffer (TLB)

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Example TLBExample TLB

Hit1

V

=

01

15 15

15

=

Hit1Hit0

Hit

19 19

19

VirtualPage Number

PhysicalPage Number

Entry 1

1 0x7FFFD 0x0000 1 0x00002 0x7FFF

VirtualAddress 0x00002 47C

1219

VirtualPage Number

PageOffset

VVirtual

Page NumberPhysical

Page Number

Entry 0

12PhysicalAddress 0x7FFF 47C

TLB

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Virtual MemoryVirtual Memory

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Memory System SummaryMemory System Summary