How PCs Boot: Explained A classroom lecture Legacy BIOS

biases job the firmware on your computer one of his jobs is to locate a bootable device once it runs it will then run when you hit power it runs automatically and its job is to find a bootable device and in most cases how you configure your bias you tell it what device to look for first yes so we'll say look for the hard drive first then look for the dvd or blu-ray then maybe the flash drive and then at home you probably never use the nic but in an enterprise environment many times we want to boot to the net so this is the most common bootable so the bias will actually look in the configuration and say okay let me go find a hard drive all right so what does it find when it finds the hard drive let's take a look so here's the structure of the hard drive once the bias does find a hard drive on a sata cable the first thing that it does is it looks for an important structure this is found on sector one and i would also in the way that hard work hardware is hardware always starts with zero software always starts with one so i'm going to call it sector zero in other words this is the very first sector on the hard drive this is called your master boot record very important this is called your master boot record so bias locates the hard drive finds a on the other end of a sata cable finds a sata hard drive the first sector is red and it is what we call the master boot record inside the master boot record are basically two elements one a some executable code there's actually some assembly language code written in there and there is also the partition table and it reads that partition table this is very important bias needs to find the bootable partition bias needs to find the bootable partition when it finds the npr it runs and executes the code that is in there that executable code reads the partition and says oh i see how they partition this hard drive they just created one big c drive okay so in this will read this code reads the partition table this is very small amount of data this is only this sector is only i think 512 bytes not a lot of data it's not like it's got a big program in it it's just one sector 512 bytes but in 512 bytes we're able to read the partition table and we can understand exactly how you partition the hard drive in the partition table as we look at the partition table it basically says this we know that sector 0 is reserved for the master record so the partition table will say and i'm just using arbitrary terms just so you can understand from sector one from sector one let's say the last sector is ten thousand on the hard drive from sector one to sector ten thousand represents the active partition that's what your partition will say if you had two partitions it would say from sector one to five thousand or whatever and it would say okay that's the active partition and then you've got another partition so the partition table is basically going to say it's going to tell exactly where you start what sector starts your partition and where does it end it's just that simple and you must have an active partition this is going to be the partition that will boot the operating system okay so my active partition is going you may have four partitions you may have 10 partitions but one partition must be active that's the partition that's going to boot the operand system all right so let's let's say that this is the active partition master boot record is not included so from sector 1 to sector 10 000 represents the partition that we're going to boot to the next sector okay inside now we know that starting at sector one and it ends at ten thousand yes this is the active partition this is the partition that we're going to boot and this is typically what we call what c drive yeah this is what we call c drive all of you know this is c drive but listen to me very carefully i'm going to blow this out of the water in a minute because microsoft this is the way we did it all the way through xp all the way we did it historically all operating systems did it like this but when microsoft brought in vista listen to me carefully windows 7 windows 8 microsoft is going to change this so hang on we're going to talk about it so the first sector in the active partition now i'm inside everyone this one's outside yes this one's inside this one is called the boot sector this is very important this is where virus is used to love to go so if i was a virus writer and i wanted to infect you every time you boot it up what i would do is write a small amount of code i would take what was in here override it put my slick little code in there and i would add my virus to the operating system every time you boot it up was that fun these were called boot sector viruses in the boot sector is basically the information to boot the operating system listen carefully when you the boot sector this both the master boot record and the boot sector are created when you install windows when you install windows those two sectors are created that's the job of the installation program yes so inside of the active partition there are there are other parts partition you could in other words in this case i just said i'm joe blow at home i just take all my hard drive and make it c drive okay is that is that what we normally do we typically do that the the person responsible for creating these special sectors on the hard drive are the the installation of windows now what happens when i install so what happens when i install linux what does linux do it also creates those so no matter what operating system installs on the hard drive if it's going to be bootable the operating system that you install is going to create the boot sector and the master boot record yes yes so that's that's who's responsible for creating this now here's the difference the master boot record that linux creates listen carefully and the master boot record that windows creates will always be exactly the same but the boot sector that linux creates and the boot sector that windows creates is going to be different this has to be the same for both operating systems but this one will be customized so that it installs what and if i'm linux linux will customize this so it will boot up okay does that make sense yes all right everybody comfortable these are two very important you must understand the boot sector and the master boot record these are two critical structures the nvr is not is not a part of the active partition the boot sector is now listen carefully microsoft doesn't want you to see or touch or modify or change the mood sector so you say well mr vanderbilt i've been using windows forever i've never seen that that's right and you won't either because microsoft doesn't want joe grandma to see the boot sector why would it be a disaster for grandma and grandpa to see the boot sector thank you it wouldn't take but just a few minutes and grandma would hose that boot sector so just to give you an idea this is wikipedia's dissertation on the boot sector it is very complex don't think that these are are any way simple or easy this is the master boot record this is actually the structure inside the nvr it's 512 bytes it is listen it is very complex i'm just giving you the general overview so that you understand it but if you go to wikipedia and start diving into mbr you will fall asleep in about three seconds okay it will just absolutely you'll go okay sure that i understand everything you say so when we talk about these we're talking about some really complicated stuff okay everybody there this is not simple stuff little kids didn't grow up five years old and start writing stuff this is very complicated now the food sector is very important because in the boot sector it is going to listen very carefully the boot sector is very important because in the boot sector will be the name listen carefully the name of the first file that begins windows doesn't have to be a first file does the windows have to start somewhere there is there is always the first file that has to be run in order to get the whole thing going yes yes yes so guess what you need to know those names so let's talk about those file names there must be one file whether it's linux unix the mac windows that starts the operating system yes and it's in the boot sector that we have that name so let's take a look uh do i have enough room i need to change screens here let's see if i have enough room to write i think i do so with windows xp we'll call these boot files these are the files that boot the operating system then we'll call vista 7 and 8. very very different the first file that boots the operating system for xp is called n t l d r no extension that's the name of the file right there anti-loader it's called anti-loader ntldr no extension it's not dot doc it's not dot executable it's just it has no extension in vista it's called boot mgr that's the name of the file these are the two files that launch vista windows 7 windows 8 and 8.1 so if you look inside the boot sector you will actually see that finally if you look in the boot sector you'll actually see that file name and by looking in the booth sector and seeing the file name you know what operating system is going to be started so every one of you need to know what is the name of the file that launches the operating system anti-loader is what starts xp boot manager is what starts windows 7 8 and 8.1 so this is how operating systems boot the bios initializes the motherboard it then looks into how what you have set up in your uh non-volatile ram it'll say okay what does he want to boot up to first and it'll say you've chosen a hard drive yes because you had to do that or you just left it default it'll say okay let's go into memory oh he wants to move from the hard drive is there a hard drive at the end of the cable yep there it is let's read the sector zero zero we're gonna read sector 0 we're going to read the master boot record the master boot record that will then say look he's only got one partition it's uh from sector 1 to sector 10 000. then it's going to say okay let's go to sector 1 that's where this active partition begins that first sector is called the boot sector it will then read that sector and it will look for the name of the food file once it knows the name then it's going to expect that file to be only one place now hang on here because i'm going to blow your mind here okay is it getting fun this is very cool it expects these two files to always be what is that any more technically correct when i put that in front of you you should have a very technically correct answer the root of c drive it expects those files to be in the root of c so if these files are very very important very very critical in order for windows to run what do you think microsoft is going to do with that file when it puts it in the root directory it's going to hide it it's going to make its system it's going to make it read only it doesn't want grandma joe blow billy bob from kentucky to mess with the file yes and so microsoft it's not going to be easy to find that file because microsoft right off the bat is going to hide the file make a system and make it read only because if joe can see it he'll probably what do we do we've and then he'll wonder why his machine doesn't move now keep in mind when we get into operating systems like windows 7 listen carefully windows 7 windows 8 windows 8.1 microsoft secures the route and it doesn't allow users in other words when you log on and you are a user you cannot mess with this directory yes you can now if you log on as an administrator with different rights then you can add files to the root you can rename files but if you're logged on as a user not an administrator then you do not have rights to add files rename files change files do anything because microsoft is protecting that root directory because in it are critical files so let's pop into windows 8.1 i'm going to first of all go to c drive and i'm at the root and in my case i've already went to view and notice i have allowed what notice up here so what have i allowed myself to see [Music] so i want to see extensions and i've also asked to see what 109's so if that was unchecked i probably now can go to options and i can go to view and this is where this is where you as a tech need to be able to go so if i want to see these system files these very critical things then i'm going to have to say do what and i'm going to also say have to uncheck okay so if this is checked it they will be hidden and this is the way we want it for joe grandma auntie uncle yes but many times you're going to have to go in here and uncheck that because we need to see these critical files they're not going to be seen normally so this is a this is the first thing that you have to do in order to see these now i'm going to i'm

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Length: 19min 39sec (1179 seconds)

Published: Thu Jan 30 2014