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COSC 3P92

Cosc 3P92

Week 9 & 10 Lecture slides

Violence is the last refuge of the incompetent.

Isaac Asimov, Salvor Hardin in "Foundation"

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COSC 3P92

Virtual Memory• The main idea is to allow programs to address

more memory locations than are physically available.

• first: overlays programmer manually divide programs into sections, and read, write them out during execution.

• virtual memory: automatic transparent overly mgmt

• virtual (logical) address: generated by program physical address: actual memory address during execution

• Addresses generated by programs are called virtual addresses, and which are mapped into physical addresses during execution time.

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Virtual Memory

• Three techniques:– paging system

– segmentation system

– paged segmentation system

program

virtualaddress

VMalgorithm

physicaladdress Main

memory

secondarystorage

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IF PageTable[page #]. residence-bit =1 THEN

RETURN Frame[ PageTable[page #].frame ]+offset

ELSE

page fault

Virtual Memory: PagingPaging system: The virtual address space is divided into equal-sized

pages and the physical memory is divided into frames of the same size. Physical memoryFrame

01234567

(4K per frame)

(32K total)

Page TableVirtual Memorypage 0page 1page 2page 3page 4page 5page 6page 7page 8page 9page 10page 11page 12page 13page 14page 15

0110010010100001

0650010000700003

page 2

page 5page 8

page 10page 1

page 15

page # offsetVirtual address

(4K per page)

(64K total)

3-bit

4-bit 12-bit

1-bit

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Example

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Paging • page mapping mechanism is often done via special hardware

• page fault: page not resident

- read from secondary storage into main memory

- update page table with physical memory address

- repeat instruction that caused fault

• demand paging: get page only when asked for

vs. algorithms which evaluate page usage and do predictive page fetches

• working set: the finite set of pages which a program will use during execution (ie. progrms don't use infinite memory)

• when working set size > # page frames, thrashing likely

totalpagesused

time --->

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programmemoryspace

virtual mapping

Paging• Page replacement:

– random (not recommended)

– least recently used

– first-in-first-out (least recently paged in)

• can use dirty bits: write back pages only when modified

• fragmentation: page sizes fixed, so can have unused page portions (and therefore unused memory)

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Paging

• Large page sizes: – maximal use of slow secondary storage

– less frequent page reads

– smaller tables

– more apt to have fragmentation

• Small page sizes:– less fragmentation

– good for programs that use small spread out memory references

– need more IO calls, larger tables

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COSC 3P92

Virtual Memory: Segmentation• Segmentation system: A program is divided into a collection of logical

segments (e.g., procedures, arrays, stacks) which may be of different sizes. Each segment has a separate virtual address space.

segment B

segment A

segment C

segment D

0

1

2

3

Virtual Memorysegment B

segment A

Physial Memory

1 10K

1

0

0

7K

4K

5K

Segment Table

seg # offsetVirtual address

2-bit N-bit

unused

unused

IF SegmentTable[seg #].valid-bit = 1 THEN

IF offset < SegmentTable[seg #].length THEN

RETURN SegmentTable[seg #].base + offset

ELSE

memory violation

ELSE

segment fault

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COSC 3P92

Segmentation

• Segments: can be allocated for different program parts, data structures, users

• Different modes, access privileges can be assigned– (permits memory violation checking, shared memory,...)

• Sizes are alterable during execution.

• The basis for multitasking multi-user systems

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Segmentation

• Garbage collection: best fit, first-fit, hole compaction, ...

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Paged segmentation system• Merges ideas of previous 2 approaches

• Segments are divided into groups of pages

• Virtual address is a triple <s,p,d>

• e.g MULTICS

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[fig next page]

Paged segmentation• Address mapping:

1. segment numbers leads to index in segment table

» gives base address of page table

2. page no. p indexed into page table, and frame p' is found

3. physical address computed by adding displacement d to p'

• Speeding up virtual memory access– address translation look aside buffer

– like a page table cache

– associative memory contains most recently used page info

– common in mainframes

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Physical Address

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Paged vs Segmentation

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Example: Pentium II VM support• A paged-segmentation system

• MMU: memory mgmt unit on CPU

• PII has segment registers: DS (data), CS (code), ...• [6.12, 6.13, 6.14]

• Scheme:– selector: 16-bits, 1 per segment, loaded into approp. segment register

– local: application; global: OS, system

– corresp. segment descriptor loaded for that segment

– offset is checked to see if it is beyong segment bounds (TRAP - software interrupt)

– seg size: G = 0 (seg size up to 1 Mb); G=1 (pages)

– base added to offset to get a linear address (base split for back-compatibility with 286)

– if paging off, this address is physical address

– if paging on: --> it’s a virtual address

» must be mapped: page directory (1K entries) --> page table (1K page entries) --> phys address [6.15]

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Example: ultraSparc VM support

• Paged virtual memory supported

• only 44 bits of 64-bit addr space used for VM• varying page sizes: [6.17]

• TLB: translation lookaside buffer [6.18]– maps virtual page # to physical page frame #

– 64 most recently used virt. page #’s for each of instns, data

– also a context -- process ID

– if not in TLB, a trap called, and OS software must fix

– not same as a page miss! page may be in memory.

– OS must also maintain a TSB (trans. storage buffer) - like a cache of pages

– otherwise, if a TSB miss, then OS does whatever it wants to implement it --> no H/W support!

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Comparing PII and UltraSparc

• PII: – 32-bit segments, fixed 1K segment/page table sizes

– max 1 million pages per segment

– can do pure segmentation, pure paging, or paged segmentation

– only 1 segment per process in Windows, Unix

• UltraSparc: – huge address space

– 2 billion pages: conventional page tables unworkable

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The end