Complete AMD64 Handbook/Configuring the boot process
Loading the kernel in memory
Introduction to bootloaders
When your system is powered on, it will first perform some sanity checks against its own components. When all tests succeed, the system loads the kernel image which you have built previously in memory. This action is performed by the boot loader.
Since loading files in memory is very architecture specific, you should consult the specific information for your architecture now to find out how to install and configure a boot loader.
Configuring the kernel
Inside the configuration file of your boot loader you can enter specific instructions for the Linux kernel. These parameters allow you to tweak and change the kernel behavior at boot-time.
Root File System Location
The first parameter we'll discuss is the root parameter. This tells the Linux kernel where the root file system of the Linux system is located. You really should provide this parameter as the Linux kernel will otherwise not know where the Linux installation is.
## (The root file system in this example is at /dev/sda3) root=/dev/sda3
However, you can only specify the root file system if you are certain that the kernel image (not through separate kernel modules, but really inside the kernel image) has support for:
- The device controller that governs your disk (for instance the SATA controller if you use SATA disks), and
- The file system that the partition uses (for instance ext3 support)
If this isn't the case, then you have probably made an initialized RAM disk which allows the Linux kernel to load the appropriate kernel modules in memory before it continues with the Gentoo boot-up. Users of the genkernel tool have indeed made such an initrd file, perhaps without their knowledge.
To use such an initrd file, you need to specify /dev/ram0 as the root file system and the root file system with the root= parameter. It is also advisable to inform the kernel about the amount of memory you want to reserve for the RAM disk.
You also need to tell the boot loader where the initrd file is. This is boot loader specific so we don't mention that here - consult the information for your boot loader for more information.
root=/dev/ram0 ramdisk=8192 root=/dev/sda3
The Initial Program
When the kernel has finished its own procedures and mounted the root file system, it hands over the control to the system to the init process. This process then takes care of the rest of the boot sequence. By default, the kernel looks for this tool at /sbin/init. You can however define another initial program if you like using the init= parameter.
For instance, to get a Unix shell immediately, use init=/bin/sh. This is often used to allow you to remount your partitions and make fixes that prevent your system from booting regularly (or when you forgot your root password).
Single User Mode
To inform the system to boot up in the single user mode (which is the single run level we've talked about in the previous chapter), simply add an S.
Not all hardware devices are conform the ACPI specification, even though they think they are. This sometimes results in unstable behaviour of the device, or of other devices influenced by the device.
You can specifically disable ACPI support using the acpi=off parameter.
Disabling IDE Controllers
When one of your IDE disks is broken, your system might not be able to boot even though the system itself is stored on a disk controlled by a different IDE controller. If this is the case, you can explicitly disable a controller using ide0=noprobe.
If you have an SMP system (Synchronous MultiProcessor), you can tell the Linux kernel to only use one CPU by setting nosmp.
Disabling USB Support
To disable USB support, use nousb
More extensive information about available kernel parameters can be found at /usr/src/linux/Documentation/admin-guide/kernel-parameters.txt.
The boot sequence
When the Linux kernel has almost finished with its boot process (where it initializes the memory structures, loads drivers, etc.) it mounts the root file system (given by the root parameter) and then starts the init process (which is default at /sbin/init but can be configured).
The init process is responsible for the rest of the system boot sequence. It looks for the /etc/inittab file which contains the instructions how to further boot the system. At first, it fires up the command that is assigned to the boot and bootwait entries which are, in Gentoo's case:
Where init is rather distribution-independent (and quite simple in its use too), /sbin/rc is quite distribution-specific, especially the rc that Gentoo offers. Its task is to make sure that the scripts in a run level are started well or take appropriate action if they aren't.
Once the boot runlevel has succeeded, the init process goes on by executing the command for the specified runlevel. By default, the runlevel entered at the initdefault part of /etc/inittab is started, but you can ask init to start a different run level by specifying its corresponding number as a boot parameter (entirely similar to how you add kernel parameters).
id:3:initdefault: ## (...) l3:3:wait:/sbin/rc default
When this run level has also finished starting its required scripts, the init process starts the terminal processes at the various ttys (the Alt + F# locations where you get a logon prompt):
c1:12345:respawn:/sbin/agetty 38400 tty1 linux c2:2345:respawn:/sbin/agetty 38400 tty2 linux c3:2345:respawn:/sbin/agetty 38400 tty3 linux c4:2345:respawn:/sbin/agetty 38400 tty4 linux c5:2345:respawn:/sbin/agetty 38400 tty5 linux c6:2345:respawn:/sbin/agetty 38400 tty6 linux
You can manage the runlevels using the rc-update tool. Its syntax is quite simple:
rc-update <add | del> <initscript> <runlevel>
All the init scripts that you can use are located inside /etc/init.d. You will most likely use at least the runlevels boot and default.
- The boot runlevel makes sure that the most important init scripts, which are required for every succesful system boot, are started properly. Any init script that is added to the boot runlevel may not require any service offered by the init scripts in the default runlevel (as it is started later). It may depend on other scripts in the boot runlevel though, Gentoo's rc is smart enough to tackle dependencies.
- The default runlevel contains all init scripts which should be started during normal system operation. This is the runlevel where you will probably add most of the init scripts.
- The root parameter is not really a kernel parameter but is intended for a script inside the initrd file. However, the parameter is used just like kernel parameters which is why we list it here.