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(The last two alternatives are kludges used for machines with such small addresses that there is not room for both a segmentnumber and an offset)

 Segment table holds the bases and bounds for all the segments of a process.

 Show memory mapping procedure, involving table lookup + add + compare. Example: PDP-10 with high and low segments selected byhigh-order address bit.

Segmentation example: 8-bit segment number, 16-bit offset.

  • Segment table (use above picture -- all numbers in hexadecimal):
  • Code in segment 0 (addresses are virtual):
  • 0x00242:mov 0x60100,%r1
  • 0x00246:st %r1,0x30107
  • 0x0024A:b 0x20360
  • Code in segment 2:
  • 0x20360:ld [%r1+2],%r2
  • 0x20364:ld [%r2],%r3
  • ...
  • 0x203C0:ret

Advantage of segmentation: segments can be swapped and assigned to storage independently.

Problems:

  • External fragmentation: segments of many different sizes.
  • Segments may be large, have to be allocated contiguously.
  • (These problems also apply to base and bound schemes)

Example: in PDP-10's when a segment gets larger, it may have to be shuffled to make room. If things get really bad it may benecessary to compact memory.

Paging

Goal is to make allocation and swapping easier, and to reduce memory fragmentation.

  • Make all chunks of memory the same size, call them pages. Typical sizes range from 512-8k bytes.
  • For each process, a page table defines the base address of each of that process' pages along with read/only and existence bits.
  • Page number always comes directly from the address. Since page size is a power of two, no comparison or addition is necessary. Just do tablelookup and bit substitution.
  • Easy to allocate: keep a free list of available pages and grab the first one. Easy to swap since everything is the same size, which is usually thesame size as disk blocks to and from which pages are swapped.
  • Problems:
    • Internal fragmentation: page size does not match up with information size. The larger thepage, the worse this is.
    • Table space: if pages are small, the table space could be substantial. In fact, this is a problem even for normal pagesizes: consider a 32-bit address space with 1k pages. What if the whole table has to be present at once? Partial solution: keep base and bounds for pagetable, so only large processes have to have large tables.
    • Efficiency of access: it may take one overhead reference for every real memory reference (page table is so big it has to be kept in memory).

Two-level (multi-level) paging

Use two levels of mapping to make tables manageable.

Segmentation and paging

Use two levels of mapping, with logical sizes for objects, to make tables manageable.

  • Each segment contains one or more pages.
  • Segment correspond to logical units: code, data, stack. Segments vary in size and are often large. Pages are for the use of the OS; they arefixed size to make it easy to manage memory.
  • Going from paging to P+S is like going from single segment to multiple segments, except at a higher level. Instead of having a single pagetable, have many page tables with a base and bound for each. Call the material associated with each page table a segment.

Questions & Answers

how can chip be made from sand
Eke Reply
is this allso about nanoscale material
Almas
are nano particles real
Missy Reply
yeah
Joseph
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
Lohitha
where is the latest information on a no technology how can I find it
William
currently
William
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
has a lot of application modern world
Kamaluddeen
yes
narayan
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Operating systems. OpenStax CNX. Aug 13, 2009 Download for free at http://cnx.org/content/col10785/1.2
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