Linux boot process

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usual Linux boot process

The PC starts up and runs LILO or some other bootloader
LILO knows where the kernel image is and starts to load it
The kernel runs. When its finished doing all its checks, it attempts to mount the root filesystem. The major and minor numbers for this device are usually encoded in the kernel itself or passed to it as arguments from LILO
Once the file system is mounted, /sbin/init is executed and your system starts up as per your inittab … and your /etc/rc*.d scripts

CD boot process

The CD boot process is somewhat different. Again we need a boot loader, but we don't necessarily know what device our CDROM is. It could be /dev/hdb, /dev/hdc, /dev/hdd. Even if we told the boot loader where the kernel is, we would still need to tell the kernel where its root filesystem is. ISOLINUX helps us get around this, by working out where the CD is. This allows us to boot the kernel, but it doesn't really help us to load the initial root filesystem. Many boot disks use a thing called an initrd (Initial ram disk) to get around this. initrd is an initial root filing system running in RAM. It loads prior to when the kernel attempts to mount the 'real' root file system. Yes this is odd. The idea is is that your initrd starts up, loads some critical modules, then mounts your real root file system.

my CD startup process

The CD is installed with ISOLINUX, so it boots first.
ISOLINUX loads the kernel from the /isolinux directory on the CD.
ISOLINUX now loads the initrd.gz compressed ext2 file system. Its important to note that ISOLINUX loads this, and not the kernel. The kernel will grab it later.
The kernel starts up and eventually decompresses the initrd.gz to ram (in /dev/ram0 actually) and mounts it as root. You have to enable INITRD and Ram disk support in the kernel to have this happen.
The kernel tries to execute the /linuxrc file in the new root filesystem (because its an 'initrd').
The linuxrc program tries to mount the CDROM (It has to make a few guesses to work out where it is), then copies a compressed 'real' root filesystem from the CD into /dev/ram1. We mount the new filesystem just so we can add a softlink in its /dev directory for the CD (so we don't have to work out where the CD is again).
When the linuxrc script finishes, control returns to the kernel and it attempts to mount its configured root filesystem. In this case, I've rdev'd the kernel to make it use /dev/ram1 as the root filesystem. This will mount our newly created ram based root file system. In this file system I have an /sbin/init and the system starts up running entirely in RAM.

So why does it have to be a two phase process? Linux doesn't really know how to boot off a CD yet. The beauty of the initrd phase is that the initrd filesystem is loaded by the bootloader (ie. not the kernel). It means that we can effectively boot off any device, so long as the bootloader is able to read from it.