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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

Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
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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