Shubha Paging Segmentation

8/8/2019 Shubha Paging Segmentation

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Paging and Segmentation

BY,

Shubha B.

1PI08LDS16


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Logical vs. Physical Address Space

Logical address generated by the CPU; also referred to as

virtual address.

Physical address address seen by the memory unit

Logical and physical addresses are the same in compile-time

and load-time; logical (virtual) and physical addresses differ in

execution-time.

The user program deals with logical addresses; it never sees

the real physical addresses.


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Paging

Memory-management technique that permits the physicaladdress space of a process to be non-contiguous.

Each process is divided into a number of small, fixed-sizepartitions called pages or Divide logical memory into blocks of

same size called pages.

Physical memory is divided into a large number of small, fixed-size partitions called frames.

When a process is to be executed, its pages are loaded into anyavailable memory frames.

Some internal fragmentation , but no external fragmentation.


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Address Translation Scheme

Operating system maintains apage table for each process

Contains the frame number foreach process page

To translate logical to physical

addresses

A logical address is divided into:

Page number (p) used as anindex into a page table which

contains base address of eachpage in physical memory.

Page offset (d) combined withbase address to define thephysical memory address that issent to the memory.


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Paging Model of memory is shown:

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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Address translation in paging


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The following steps are needed for address Translation:

Given a (page #, offset), find (frame #, offset).

Assuming the size of a page/frame is 2m bytes , m bits todetermine the offset within a page

2n > number of pages in process => n bits to hold the pagenumber

2k > number of frames in memory => k bits to hold the frame

number

Logical address is nm

Physical address is simply km where k is the frame that stores

page n


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Paging Example


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Paging Example

Page size 4 bytes.

Physical memory 32bytes.

Logical address 0 maps to physicaladdress 20 (=(5*4)+0).

Logical address 3 maps to physicaladdress 23 (=(5*4)+3).

Page1 is mapped to frame6.It maps tophysical address 24(=(6*4)+0)

Logical address 13 maps to physicaladdress 9.


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Page Table Implementation

Implemented as a set of registers.(if Page table is less).

Problem: If page table is large, it is not applicable.

Page table kept in main memory. PTBR register points to the

Page table. Page-table length register (PRLR) indicates size of

the page table.

Problem: In this scheme every data/instruction access

requires two memory accesses. One for the page table and

one for the data/instruction.

Using Translation look aside buffer or associative registers.

Very fast, very expensive.


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Memory Protection

Memory protection implemented by associating protection bitwith each frame.

Valid-invalid bit attached to each entry in the page table:

valid indicates that the associated page is in the process

logical address space, and is thus a legal page

invalid indicates that the page is not in the process logical

address space


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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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Two-Level Page Table

A logical address (on 32-bit machine with 4K page size) isdivided into:

a page offset consisting of 12 bits.

a page number consisting of20 bits; further divided into:

a 10-bit level-2 page number.

a 10-bit level-1 page number.

Thus, a logical address look likes

p1 is an index into the outer page table

p2 is the displacement within the page of the outer page

table

Address translation scheme


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Advantages

Easy to allocate memory keep a list of available frames, andsimple grab first one thats free.

Easy to swap pages, frames, and often disk blocks as well,

all are same size.

No external fragmentation

Any page can be placed in any frame in physical memory

Sharing of the common code is possible.


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Disadvantages

Internal fragmentation: Page size may not match size neededby process

Wasted memory grows with larger pages

Page tables are fairly large , so page tables are too big to fit in

registers, so they must live in physical Memory.

This table lookup adds an extra memory reference for every

address translation.


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Segmentation

A segment can be defined as a logical grouping of instructions,such as a subroutine, array, or a data area.

A program is a collection of segments.

Segmentation is t technique for managing these segments.

Divide a process into unequal size blocks called segments.

Each segment has a name and a length.

Memory Management Scheme that supports user view ofmemory.


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A segment is a logical unit such as

main program, procedure, function

local variables, global variables, common block

stack, symbol table, arrays

Protect each entity independently

Allow each segment to grow independently

Share each segment independently

Each of these segments are of variable

length and length will be defined in the

program.


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Segmentation Architecture

Segment Table Maps two-dimensional user-defined addresses into one-

dimensional physical addresses. Each table entry has

Base - contains the starting physical address where the

segments reside in memory. Limit - specifies the length of the segment.

A logical address consists of two parts.

A segment number, s, and an offset, d. Segment number used as index into the segment table.

The offset d is between 0 and the segment limit.

The offset is added to the base to produce the physical

address.


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If the offset is not within the limit, trap to the operating

system.(logical address beyond end of segment).

Segmentation Example:


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Address Translation


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Sharing of Segments

Segments are shared when entries in the segment tables oftwo different processes point to the same physical locations.

It is possible to share only the parts of the programs.

Subroutine packages are shared

among many users.


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Advantages and Disadvantages Advantages:

Each segment can be

located independently

separately protected

grow independently

Segments can be shared between processes.

Eliminate the fragmentation.

Disadvantages:

Allocation algorithms as for memory partitions External fragmentation, back to compaction problem.

Solution: combine segmentation and paging.


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Segmentation vs. Paging

Segment is good logical unit of information sharing, protection

Segmentation

Break process up into logical segments

Segments are generally of different sizes

Page is good physical unit of information

simple memory management

Paging

Break process up into physical pages

Pages are the same size


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Paging Segmentation

The main memory partitioned into frames

or blocks.

The main memory partitioned into

segments.

The logical address space divided into

pages by compiler or MMU

The logical address space divided into

segment specified by the programmer.

Suffers from internal fragmentation or

page break

Suffers from external fragmentation.

Operating system maintains the page

table.

Operating system maintains the segment

table.

Does not supports the user view of

memory

Supports the user view of memory

Processor uses the page number anddisplacement to calculate absolute

address.

Processor uses the segment number anddisplacement to calculate absolute

address.

Multilevel paging is possible Multilevel segmentation is possible. But

no use.


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Similarity:

Address space can exceed size of real memory.

Differences:

Programmer is aware of segmentation. Paging is hidden.

Segmentation maintains multiple address spaces per

process. Paging maintains one address space.

Segmentation allows procedures and data to be separatelyprotected. This is hard with paging.

Segmentation easily permits tables whose size varies.

Segmentation facilitates sharing of procedures between processes.This is hard with paging.

Pure segmentation suffers from memory fragmentation.


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Thank you

Shubha Paging Segmentation

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