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Operating systems provide a mechanism for selectively calling certain functions in the kernel. These select functions are called kernel calls or system calls, and act as gateways into the kernel. These gateways are carefully designed to provide safe functionality. They carefully check their parameters and understand how to move data from a user process into the kernel and back again. We will discuss this topic in more detail in the Memory Management section of the course.

The path in and out of the kernel

The only way to enter the operating kernel is to generate a processor interrupt. Note the emphasis on the word "only". These interrupts come from several sources:

  • I/O devices: When a device, such as a disk or network interface, completes its current operation, it notifies the operating system by generating a processor interrupt.
  • Clocks and timers: Processors have timers that can be periodic (interrupting on a fixed interval) or count-down (set to complete at some specific time in the future). Periodic timers are often used to trigger scheduling decisions. For either of these types of timers, an interrupt is generated to get the operating system's attention.
  • Exceptions: When an instruction performs an invalid operation, such as divide-by-zero, invalid memory address, or floating point overflow, the processor can generate an interrupt.
  • Software Interrupts (Traps): Processors provide one or more instructions that will cause the processor to generate an interrupt. These instructions often have a small integer parameter. Trap instructions are most often used to implement system calls and to be inserted into a process by a debugger to stop the process at a breakpoint.

The flow of control is as follows (and illustrated below):

  1. The general path goes from the executing user process to the interrupt handler. This step is like a forced function call, where the current PC and processor status are saved on a stack.
  2. The interrupt handler decides what type of interrupt was generated and calls the appropriate kernel function to handle the interrupt.
  3. The general run-time state of the process is saved (as on a context switch).
  4. The kernel performs the appropriate operation for the system call. This step is the "real" functionality; all the steps before and after this one are mechanisms to get here from the user call and back again.
  5. if the operation that was performed was trivial and fast, then the kernel returns immediately to the interrupted process. Otherwise, sometime later (it might be much later), after the operation is complete, the kernel executes its short-term scheduler (dispatcher) to pick the next process to run.

Note that one side effect of an interrupt might be to terminate the currently running process. In this case, of course, the current process will never be chosen to run next!

  1. The state for the selected process is loaded into the registers and control is transferred to the process using some type of "return from interrupt" instruction.

The system call path

One of the most important uses of interrupts, and one of the least obvious when you first study about operating systems, is the system call. In your program, you might request a UNIX system to read some data from a file with a call that looks like:

Questions & Answers

what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
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.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
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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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