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  • Dangling pointers: better not recycle storage while it is still being used.
  • Core leaks: Better not "lose" storage by forgetting to free it even when it cannot ever be used again.

Reference Counts: keep track of the number of outstanding pointers to each chunk of memory. When this goes to zero, free thememory. Example: Smalltalk, file descriptors in Unix. Works fine for hierarchical structures. The reference counts must be managed automatically (bythe system) so no mistakes are made in incrementing and decrementing them.

Garbage Collection: storage is not freed explicitly (using free operation), but rather implicitly: just delete pointers. When thesystem needs storage, it searches through all of the pointers (must be able to find them all!) and collects things that are not used. If structures arecircular then this is the only way to reclaim space. Makes life easier on the application programmer, but garbage collectors are incredibly difficult toprogram and debug, especially if compaction is also done. Examples: Lisp, capability systems.

How does garbage collection work?

  • Must be able to find all objects.
  • Must be able to find all pointers to objects.
  • Pass 1: mark. Go through all pointers that are known to be in use: local variables, global variables. Mark each object pointed to, and recursivelymark all objects it points to.
  • Pass 2: sweep. Go through all objects, free up those that are not marked.

Garbage collection is often expensive: 20% or more of all CPU time in systems that use it.

Sharing main memory


  • Want to let several processes coexist in main memory.
  • No process should need to be aware of the fact that memory is shared. Each must run regardless of the number and/or locations of processes.
  • Processes must not be able to corrupt each other.
  • Efficiency (both of CPU and memory) should not be degraded badly by sharing. After all, the purpose of sharing is to increase overall efficiency.

Relocation: draw a simple picture of memory with some processes in it.

  • Because several processes share memory, we cannot predict in advance where a process will be loaded in memory. This is similar to acompiler's inability to predict where a subroutine will be after linking.

  • Relocation adjusts a program to run in a different area of memory. Linker is an example of static relocation used to combine modules into programs.We now look at relocation techniques that allow several programs to share one main memory.

Static software relocation, no protection:

  • Lowest memory holds OS.
  • Processes are allocated memory above the OS.
  • When a process is loaded, relocate it so that it can run in its allocated memory area (just like linker: linker combines several modules intoone program, OS loader combines several processes to fit into one memory; only difference is that there are no cross-references between processes).
  • Problem: any process can destroy any other process and/or the operating system.
  • Examples: early batch monitors where only one job ran at a time and all it could do was wreck the OS, which would be rebooted by an operator.Many of today's personal computers also operate in a similar fashion.

Questions & Answers

find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
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
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?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
is it a question of log
Commplementary angles
Idrissa Reply
im all ears I need to learn
right! what he said ⤴⤴⤴
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?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
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
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
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
what is system testing
what is the application of nanotechnology?
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
anybody can imagine what will be happen after 100 years from now in nano tech world
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
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
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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