Memory: Paging Summer 2011 Cornell University 1 Today What is - - PowerPoint PPT Presentation

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CS 4410 Operating Systems

Memory: Paging

Summer 2011 Cornell University

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Today

• What is paging and why do we need it?
• Paging
• Address translation scheme
• Page table
• Implementation of page table
• TLB
• Structure of page table in memory

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Paging

• Memory management scheme.
• Noncontiguous physical address space of a

process.

• The process is allocated physical memory wherever

the latter is available.

• It avoids external fragmentation.

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Paging

• Divide physical memory into fixed-sized blocks called

frames (size is power of 2, between 512 bytes and 8,192 bytes)

• Divide logical memory into blocks of same size called

pages

• Keep track of all free frames.To run a program of size

n pages, need to find n free frames and load program

• Set up a page table to translate logical to physical

addresses

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Paging

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Address Translation Scheme

• Address generated by CPU is divided into:
• Page number (p) – used as an index into a page table

which contains base address of each page in physical memory

• Page offset (d) – combined with base address to define the

physical memory address that is sent to the memory unit

p d m - n n page number page

• ffset

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Address Translation Scheme

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Address Translation Scheme

0000 a 0001 b 0010 c 0011 d 0100 e 0101 f 0110 g 0111 h 1000 i 1001 j 1010 k 1011 l 1100 m 1101 n 1110

• 1111

p

logical memory

00 101 01 110 10 001 11 010

page table

00000 00100 i j k l 01000 m n

• p

01100 10000 10100 a b c d 11000 e f g

physical memory

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Paging

• User's view: memory is one single space
• Actual view: the program is scattered

throughout physical memory

• The OS manages physical memory.
• Frame table: Availability and owner of every frame

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Hardware Support

• The OS allocates one page table per process.
• A pointer to the page table is stored in the PCB.
• The dispatcher defines the correct hardware

page-table values.

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Implementation of page table

• Set of dedicated registers.
• Fast
• Small
• In memory
• Page-table base register (PTBR)
• Slow
• Large
• TLB + memory
• Translation look-aside buffer
• Special, small, fast-lookup hardware cache
• Contains a few of the page-table entries
• If page_number is in TLB get frame_number out

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Paging Hardware With TLB

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Memory Protection

• Implemented by associating protection bit with

each frame.

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Shared Pages

• Shared code
• One copy of read-only (reentrant) code shared among

processes (i.e., text editors, compilers, window systems).

• Shared code must appear in same location in the logical

address space of all processes

• Private code and data
• Each process keeps a separate copy of the code and data
• The pages for the private code and data can appear

anywhere in the logical address space

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Structure of the Page Table

• Hierarchical Paging
• Hashed Page Tables
• Inverted Page Tables

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Hierarchical Paging

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Hierarchical Paging

• Two-Level Paging Example
• A logical address (on 32-bit machine with 4K page size) is divided into:
• a page offset of 12 bits
• a page number of 20 bits
• Since the page table is paged, the page number is further divided into:
• a 10-bit page number
• a 10-bit page offset
• Thus, a logical address is as follows:

page number page offset p1 p2 d 10 10 12

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Hierarchical Paging

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Hashed Page Tables

• Common in address spaces > 32 bits
• The virtual page number is hashed into a page table.

This page table contains a chain of elements hashing to the same location.

• Virtual page numbers are compared in this chain

searching for a match. If a match is found, the corresponding physical frame is extracted.

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Hashed Page Tables

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Inverted Page Table

• One entry for each real page of memory.
• Entry consists of the virtual address of the page stored

in that real memory location, with information about the process that owns that page.

• Decreases memory needed to store each page table,

but increases time needed to search the table when a page reference occurs.

• Use hash table to limit the search to one — or at most

a few — page-table entries.

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Inverted Page Table

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Today

• What is paging and why do we need it?
• Paging
• Address translation scheme
• Page table
• Implementation of page table
• TLB
• Structure of page table in memory