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in the last lecture we have started
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studying about operating system and we
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also saw the introduction of operating
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system we saw what an operating system
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really is what it really does and we
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have also seen the functions of
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operating systems and we have also seen
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some basic examples of operating system
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now in this lecture we will be seeing
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some of the basic concepts that you need
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to know before we really go into the
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details of operating system so in this
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lecture we will be mainly studying about
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computer system operations so basically
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it is about the structure of computer
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system now why I am doing this is
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because some basic knowledge of the
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structure of computer system is required
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to understand how operating systems work
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so by studying about this structure of
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computer system this will provide you a
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base for understanding operating system
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so instead of directly diving into how
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operating system works we should have a
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base about this so that it will help us
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to understand in a clearer way about
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operating system so after studying this
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when you study each and every concepts
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of operating systems you will not feel
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completely blank but you will have a
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proper idea of why we are doing this or
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why certain things working in that way
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and not only that by having a basic
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introduction to this structure of
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computer system you are getting a basic
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introduction to computer organization
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and architecture which is is an
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important course as far as computer
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science and electronics is concerned all
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right so now let us see what we can
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study from this computer system
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operation in this lecture so here it
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says a modern general purpose computer
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system consists of one or more CPUs and
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a number of device controllers connected
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through a common bus that provides
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access to Shared memory so don't worry
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about this definition it will become
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clear I show you a diagram related to
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this and we will see what this actually
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means so here I have a diagram which we
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can use to explain this sentence that is
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written here so let us break the
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sentence into parts and see what each
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lines mean a modern general purpose
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computer system consists of one or more
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CPUs now what do we mean by this a
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modern general purpose computer it
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consists of one or more central
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processing units now what is a CPU CPU
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is the main part or the it's we can call
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it the brain of the computer system it
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is the processing unit that is where all
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the computations and processing and
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calculations take place in the computer
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now when we mostly talk about CPU what
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comes to our mind usually as an novice
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what comes to our mind is that big box
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beside your desktop that you have and
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that is what we generally think a CPU is
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we just look at it and we call it that
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this is the CPU but that box just not
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just the CPU that box consists of many
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things like your motherboard your CD
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roome your USB ports many things are
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there but CPU is just a small chip that
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is there embedded into your motherboard
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and that small chip is responsible for
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performing all the computations and
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calculations and that is your actual CPU
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so that box itself you can just call it
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a cabinet but the CPU is just a small
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unit that is present in inside that box
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which is there in your motherboard all
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right and then the modern computer
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systems it may have one or more CPUs
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depending on the processing power of
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your computer so that is the meaning of
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the first line and a number of device
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controllers connected through a common
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bus that provides access to Shared
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memory so here if you look in this
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diagram we have some Hardwares like your
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dis which may be your hard dis or
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anything like that and then we have some
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more Hardwares here like your Mouse
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keyboards printers and then you have
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your monitors over here so it is not
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limited to this but this is just an
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example I'm showing you and then each of
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these Hardwares or devices are connected
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to a controller and this controller is
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responsible for the way these devices
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work so here for example we have the
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disk which is connected via the disk
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controller and then these devices here
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like your mouse keyboard and printers
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assuming that they are all USB devices
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they are connected to the USB controller
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because these are connected to the USB
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ports and the USB controllers will take
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care of how these devices perform and
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then you have your monitor that is your
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display and that is connected to a video
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adapter and these adapters or
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controllers are responsible or they are
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in charge of how these devices actually
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work and then the CPU we have it here
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the central processing unit along with
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these controllers are are connected via
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a common bus that we have here this line
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over here it represents the common bus
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this one which we talked about and we
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see that they are all connected to this
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bus it is common to all of them and that
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bus is connected to a shared memory all
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right let us see about this in more
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detail so here I have some points
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written down each device controller is
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in charge of a specific type of device
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so here we have device controllers and
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each of these controllers adapters they
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are in control of a specific kind of
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device as it is shown here and then the
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CPU and the device controllers can
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execute concurrently competing for
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memory Cycles now what do we mean by
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this line so we see that they are all
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connected to a memory controller over
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here and why is that that is because I
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already taught you in the first lecture
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that whenever something has to be
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executed or whenever something has to be
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loaded it has to be loaded into to your
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main memory your main memory is like
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your RAM Random Access Memory that you
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have and don't worry if you don't know
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about it in detail I will do another
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lecture where I will be explaining about
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the storage and memory that we have so
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just know that whenever something has to
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be executed it has to be loaded into
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your main memory and your main memory is
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not unlimited or infinite but it is
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limited it is a finite amount of memory
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that you have like in your laptops or in
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your desk Toops you may have 2 GB of RAM
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or 8 GB of RAM so on and so forth so
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depending on the size of your RAM that
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is the amount of memory or the main
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memory that you have and whenever you
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execute something that has to be loaded
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into your main memory and now we see
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that all these devices in order to work
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they need to be loaded into the main
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memory for any execution that has to be
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performed and then the dis controllers
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or the adapters are taking care of this
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and this has to be Lo loaded into the
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main memory and we see here that all
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these device controllers along with the
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CPU can execute concurrently now what do
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we mean by concurrently concurrently
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means that all these devices or
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controllers can execute all at the same
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time now let's take an example suppose
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you are watching something like let's
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say a video or a movie in your monitor
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and at the same time you can minimize
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and type something using your keyboard
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onto your Microsoft Word now when you do
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this is you don't experience a lag you
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never experience a lag like when you are
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typing something your video get paused
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or something like that you never
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experience why because they are all
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working concurrently they are all
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working at the same time that is how
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powerful your system is and that is
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where you have to appreciate the beauty
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and power of your system now in order to
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make all this work seamlessly without
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having any lag or having any problems
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what do we need to ensure orderly access
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to the shared memory memory controller
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is provided whose function is to
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synchronize access to memory so as I
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already told you all these devices they
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need access to the memory because they
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all have to be loaded to the memory in
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order to work and now we need to ensure
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that all of these devices gets their
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required share of memory so that they
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can be loaded correctly and they can be
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executed in the right way without having
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any problem for the user so what we have
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is a memory controller so we have a
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memory controller here which ensures
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that every controller or every other
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controller or devices gets the proper
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access to the memory that they need so
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that they can perform or work smoothly
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without having any problems so the
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memory controller synchronizes
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everything and makes everything work in
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a very smooth way so that is what we can
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understand by this diagram and from this
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diagram we can understand the basic
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structure of how things are arranged so
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first we have have the CPU and then the
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CPU along with the disc controllers
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which are responsible for the working of
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each and every devices like I have shown
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here they are all connected via a common
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bus this line represents the common bus
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and that is connected to the memory
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controller which takes care of how the
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memory has to be shared between each and
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every devices so that it's working is
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proper and smooth all right now I want
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to introduce some important terms to you
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you have already seen the structure of a
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computer system just the basic structure
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using that diagram and now I want you to
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know these terms which we are going to
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study here right now the first one is
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known as a bootstrap program so what is
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a bootstrap program the initial program
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that runs when a computer is powered up
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or rebooted is known as the bootstrap
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program so bootstrap program it is the
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first program that is executed or that
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runs when you power up or reboot your
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computer system
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so when you go and press the power
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button of your computer the first
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program that loads that helps in
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powering up the system is known as the
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bootstrap program now let us see what
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are the features of this bootstrap
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program and what are its functions so
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the bootstrap program it is stored in
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the Rome Rome stands for readon memory
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which is a kind of secondary memory so
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your bootstrap program is stored in your
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Rome and then it must know how to load
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the OS and start executing that system
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system so as I told you bootstrap is the
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first program that loads when you power
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on your system and that program what is
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its function it must load your operating
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system your operating system is the
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interface between you and the physical
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Hardware so your operating system also
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is a software it is kind of a system
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software so that operating system it is
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already residing or it is stored
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somewhere in your secondary memory and
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then the bootstrap program must know how
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to load that operating system and start
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it for you and also I told you that
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operating system is stored somewhere in
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the secondary memory so the bootstrap
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program must know where your operating
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system is stored and it must go there
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and it must invoke the operating system
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and it must load the OS kernel into your
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main memory it must locate and load into
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memory the OS kernel now what do we mean
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by kernel kernel is like the heart of
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the OS it is the main part of the
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operating system so that has to be
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loaded into your memory and then the
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bootst program as you switch on your
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computer it is the first program that is
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run and it goes and finds where is your
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operating system and then the kernel of
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your operating system it takes it and it
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loads into the memory so that the
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operating system will come live and
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after that the rest of the things the
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operating system will take care of so
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that was about rootstrap program now now
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the next thing that I want to introduce
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to you is something known as an
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interrupt so let us see what an
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interrupt is so what comes to your mind
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when you hear the word interrupt so in
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English we know what is an interrupt
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when you are doing something if someone
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comes and disturbs you or if someone
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comes and says please stop doing this
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work and do something else that is an
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interrupt we are interrupted so think of
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it in the same context this is almost
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the same even in case of a computer
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system and let's see what or how we can
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Define this and what it actually is so
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the occurrence of an event is usually
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signaled by an interrupt from the
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hardware or software so we know that the
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CPU is always working it is doing some
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work and when the CPU is doing that work
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sometimes the hardware or software May
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interrupt the CPU it may come and tell
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the CPU that wait just wait with
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whatever you are doing and please
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execute this task that I am giving you
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this is more important so the CPU has to
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top and it has to execute that task
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which comes up and that is known as an
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interrupt so the hardware May trigger an
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interrupt at any time by sending a
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signal to the CPU usually by the way of
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the system bus so we have different
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Hardwares in our system and then that
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Hardware can trigger an interrupt it can
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interrupt and how does it do that it
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sends a signal to the CPU and how can
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you send signal to the CPU by the way of
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a system bus through the system bus I
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showed you in our previous diagram what
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I mean by bus so here this line it
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represents the common bus we have so
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similarly using a system bus the
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hardware can send the interrupt to the
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CPU and the CPU have to stop whatever it
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is doing and it has to execute the
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interrupt so we'll see how the CPU
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responds to the interrupt later so let
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us understand what is meant by interrupt
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in that way now the next term I want to
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introduce to you is known as a system
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call also known as a monitor call let's
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see what it says a software May trigger
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an interrupt by executing a special
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operation called a system call now as I
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told you when a hardware triggers the
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interrupt we usually call it as
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interrupt if the software is what that
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is triggering the interrupt if the
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software is causing the interrupt then
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that is known as a system call or a
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monitor call so if it is a hardware we
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usually call it an interrupt or if it is
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triggered by a software then we usually
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call it a system call or a monitor call
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so I hope that made clear to you what an
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interrupt actually means now let us see
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how does the CPU respond when it
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receives an interrupt so here it says
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when the CPU is interrupted it stops
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what it is doing and immediately
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transfers execution to a fixed location
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so I already told you how interrupt can
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arise either from Hardware or software
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so CPU is already doing some work and
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then immediately it gets an interrupt
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from either a hardware or a software so
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when when the CPU is interrupted in that
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way what does it do it stops what it is
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doing it stops what it was doing and
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then it immediately transfers execution
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to a fixed location so whatever the CPU
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was doing it just stops it and then it
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transfers his execution to a fixed
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location now what is that fixed location
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that we are talking about the fixed
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location usually contains the starting
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address where the service routine of the
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interrupt is located now we have a new
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term here which is called service
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routine now what is a service routine
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service routine is nothing but it is
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something where what the interrupt
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actually wants to do is written so let's
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not go into the technical details I'll
00:15:42
just tell you basically what it means so
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when an interrupt arises there is
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something that the interrupts wants to
00:15:48
do so there is something that the
00:15:50
interrupts want to do something it wants
00:15:51
to execute so what actually has to be
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executed or what actually the interrupt
00:15:57
wants to do that is written in the
00:15:59
service routine so every interrupt has
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its service routine known as the
00:16:03
interrupt service routine sometimes it
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is known as ISR so when the CPU is
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interrupted it stops what it is doing
00:16:12
and it immediately transfers its
00:16:14
execution to the starting address of the
00:16:18
interrupt service routine so the
00:16:21
interrupt service routine may be located
00:16:23
somewhere and it has a starting address
00:16:25
from where it should start its execution
00:16:27
so the CPU it stops what it was doing
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and it transfers its execution to the
00:16:32
starting address of the interrupt
00:16:34
service routine so that is the first
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thing that the CPU does when it is
00:16:38
interrupted and then what happens the
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interrupt service routine executes so
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whatever was written in the interrupt
00:16:44
service routine gets executed completely
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and then what happens on completion the
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CPU resumes the interrupted computation
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so we know that the CPU was already
00:16:54
doing something before so when the
00:16:56
interrupt arose what it did it
00:16:59
transferred its execution to the
00:17:01
interrupt service routine and then it
00:17:03
executes it and when it is done once the
00:17:06
interrupt service routine executes
00:17:08
completely then the CPU can go back from
00:17:12
where it came from the execution of the
00:17:14
CPU goes back to where it came from and
00:17:18
it resumes whatever it was doing so this
00:17:21
is how the CPU responds to interrupts
00:17:25
and I hope that made it clear to you how
00:17:28
an interrupted is handled by a CPU at
00:17:30
least the basics so this was just a
00:17:32
basic introduction to computer system
00:17:34
operations so I will not go into great
00:17:37
detail because this is just the basics
00:17:39
that you need to know before you really
00:17:41
understand the details of operating
00:17:43
system so I hope this was clear to you
00:17:45
in the next lecture we will be seeing
00:17:47
About Storage structures and about
00:17:49
memory which is also an important topic
00:17:51
that you need to be knowing for
00:17:53
understanding your operating system so
00:17:55
thank you for watching this and see you
00:17:56
in the next one
00:18:00
[Music]
