 Dear students, we have enough talked about operating system and now the time has come that we should learn that how the operating system is loaded in the main memory to start the computer. You might have heard about this term, the booting. So when you start your computer, so someone says, please hold on, please wait, your computer is booting. So before starting this module, I have two questions. First of all, where the OS is stored. So you know that operating system like Microsoft Windows is basically stored on mass storage and where the program should be to be executed in computer. So you know that in computer, the program should be available in the main memory which is called RAM and if the program is available in the RAM, then that program can be executed. So now your operating system is not available in the RAM when computer starts because RAM is empty, it's a volatile memory and when you start your computer, there is nothing in the RAM and your operating system is actually stored on the mass storage. So now the process of taking or converting the operating system from mass storage to your main memory is known as booting. So it is a short form of bootstrapping and this procedure transfers the operating system from mass storage to main memory and you know, we have discussed that in the start your main memory is empty and how the operating system then will be loaded into the main memory. So this is our next question. So first of all, to answer this question, we should recall module number 37 in which we discussed that this is the architecture of a computer system which has a main memory, a bus and then central processing unit. So the program which we want to execute should be available in the main memory and from where the program is going to be started that register address or the address of the main memory should be available into a specialized register known as program counter and the instruction will be loaded into instruction register. So in the start our all of the main memory is empty. It doesn't have anything here. So we need someone which could load our operating system which is residing on mass storage to this main memory and this process is known as booting. So small portion of main memory is made from non volatile memory cell where the CPU finds its way to transfer the control from or transfer the operating system from mass storage to the main memory. So such a small non volatile memory is known as ROM or OM read only memory. So it is basically the read only memory where we can only perform the reading operation. However we are not able to write on the ROM but the today's computer we construct ROM through flash memory. So the content could be altered at some later stage for some special circumstances. So there is a boot loader which is permanently stored in ROM and load the OS into the main memory and in smartphones it can load from flash non volatile memory. However in some small workstation it can load an operating system which may be residing on an other machine over the network physically somewhere far away. So here is the process of booting. So in the first step machine starts by executing the boot loader. The boot loader is a program which is able to initiate the process of booting and this boot loader is available into the main memory that particular slot which we call ROM which is non volatile memory. And this boot loader program which is already in this non volatile memory operating system is stored in mass storage and in step 2 this boot loader program directs a transfer of operating system into the main memory and then transfer control to it. So this boot loader has basically migrated the operating systems instructions from mass storage to the main memory and now the operating system has started its functionality. So how this will be handled you know that the first instruction of operating system will be stored at the first address which is for example A0 in our previous example and that would be stored that instruction would be stored in the instruction register. So if we summarize today's module we have learned very basic phenomena of how computers start working by following the booting process and mainly we have discussed that our operating system is residing into the mass storage and then it should be available into the RAM for execution and the boot loader is a program that is permanently stored into a small non volatile memory of RAM which transfers the control to operating system by transferring operating system from mass storage to the main memory.