On Mon, Jun 28, 2010 at 3:40 AM, raja roy <raja.roy84@xxxxxxxxx> wrote: > Hi All, > I was trying to understand the booting steps, > > BIOS loads the first sector (MBR) of a bootable device(considering disk > only) in RAM (0000:7c00)and then jumps to that addr to execute the primary > boot loader which in turn loads the secondary boot loader(grub) pointed to > by an active partition in the partition table. Expanding it a little... Under x86 Architecture, BIOS starts executing at fff0, that is one paragraph less than 65535. Here its still in real mode. After the BIOS does its 'stuff' , the last instruction executed by BIOS is int 0x19 which causes it to jump to 7c00 where it starts executing code. This code is responsible for loading the operating system or the kernel. Now, the primary and the secondary boot-loader you mention are the same. Grub in this case. Its only because there is just 500 bytes (taking away 55AA) to execute. This 500 bytes is sometimes (most of the time actually) very less to put in substantial amount of code to boot load any Kernel. Grub uses this to jump to stage1.5 and then stage2 to completely load any kernel. > > If i m wrong above,please correct me. > > Now my doubt is that- > > If the partition table has multiple active partitions(in case of multiple > OS) , in that case, are those other partitions are read by the primary boot > loader ,if yes then when?? > how does the secondary boot loader(grub) come to know about the various > active partitions in the disk (since it does not read the partition table in > MBR)?? Grub (any modern bootloader)does not care about active partitions. Once grub is installed you have to register ( let grub know, if not done automatically) where all your multiple OS are in the disk. It is not true grub does not read partition table, once you do something like (hd0,0) it know about first partition 'start / end ' by going through partition table. There is also new version of grub. Which they say is very scriptable. Not tried that though. Thanks > > thanks > Raja > > -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
