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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

find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
I know this work
salma
The given of f(x=x-2. then what is the value of this f(3) 5f(x+1)
virgelyn Reply
hmm well what is the answer
Abhi
how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
hmm
Abhi
is it a question of log
Abhi
🤔.
Abhi
I rally confuse this number And equations too I need exactly help
salma
But this is not salma it's Faiza live in lousvile Ky I garbage this so I am going collage with JCTC that the of the collage thank you my friends
salma
Commplementary angles
Idrissa Reply
hello
Sherica
im all ears I need to learn
Sherica
right! what he said ⤴⤴⤴
Tamia
hii
Uday
hi
salma
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris 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
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele 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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