The following is from the Grub Manual:
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Overview
Briefly, a boot loader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to an operating system kernel software (such as Linux or GNU Mach). The kernel, in turn, initializes the rest of the operating system (e.g. a GNU system).
GNU GRUB is a very powerful boot loader, which can load a wide variety of free operating systems, as well as proprietary operating systems with chain-loading1. GRUB is designed to address the complexity of booting a personal computer; both the program and this manual are tightly bound to that computer platform, although porting to other platforms may be addressed in the future.
One of the important features in GRUB is flexibility; GRUB understands file systems and kernel executable formats, so you can load an arbitrary operating system the way you like, without recording the physical position of your kernel on the disk. Thus you can load the kernel just by specifying its file name and the drive and partition where the kernel resides.
When booting with GRUB, you can use either a command-line interface (see Command-line interface), or a menu interface (see Menu interface). Using the command-line interface, you type the drive specification and file name of the kernel manually. In the menu interface, you just select an OS using the arrow keys. The menu is based on a configuration file which you prepare beforehand (see Configuration). While in the menu, you can switch to the command-line mode, and vice-versa. You can even edit menu entries before using them.
In the following chapters, you will learn how to specify a drive, a partition, and a file name (see Naming convention) to GRUB, how to install GRUB on your drive (see Installation), and how to boot your OSes (see Booting), step by step.
Besides the GRUB boot loader itself, there is a grub shell grub (see Invoking the grub shell) which can be run when you are in your operating system. It emulates the boot loader and can be used for installing the boot loader.
Footnotes
1.
chain-load is the mechanism for loading unsupported operating systems by loading another boot loader. It is typically used for loading DOS or Windows.
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The following is from:
http://home.freeuk.net/foxy2k/disk/disk3.htm
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The boot sector (also called the boot record) is the very first sector on a disk it has two important functions, the first one is to load in your operating system (I'm going to ignore that since it's not relevant for this) and secondly it tells us all kinds of useful(?) information about the disk that its on. A FAT32 boot sector is actually more than one sector long, three to be precise, so I don't see how it can accurately be called a boot sector, the boot record is probably a more accurate term.
Two important things that are stored in the boot sector are the size of each sector (analogous to size of each piece of paper in a notepad), usually 512 bytes (characters in layman's terms) and the total number of sectors (how many bits of paper). With this information we can tell how much data the disk will hold.
For instance a 1.44MB 3 1/2" disk has 2,880 sectors, with 512 bytes in each sector. There are 1024 bytes in a kilobyte and 1024 kilobytes in a megabyte byte, so 2880 sectors multiplied by 512 bytes means 1440KB or 1.406MB. Now as you may know these disks do not store 1.406MB of data, the maximum you can get is 1.38MB the remaining space is used for the file system.
The boot sector also supplies information on where to find the FAT's and root directory, you'll find out what these are later if you don't already know. Other information that the boot sector holds include:
* The name of the operating system that formatted it.
* Sectors per cluster, you'll find out about this when you get to the data area section.
* The maximum number of root directory entries you can have for the disk (this is explained later).
* The volume name, this is what is displayed when you type "VOL" at a DOS prompt or what is shown in "My Computer" for Windows users. (the volume name can also be stored in the root directory.)
* The serial number.
The boot sector is vital to be able to read the disk if this goes the operating system will not know what type/size disk it is dealing with, because of this fact the FAT32 file system stores a copy of the boot sector.
That's it for the none technical area so go on to the next section if you don't want in depth technical details if you do then keep reading.
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Hope this will answer some of your questions.
