Booting the Kernel

This article is a description of the steps required to boot the Linux kernel. While this kind of information is not relevant to the system's functionality, it is interesting to see how the different architectures bring up the system.
Using LILO

Most Linux-x86 users don't boot the raw kernel image from a floppy; instead they boot LILO from the hard disk. LILO replaces part of the process outlined above so that it can load a Linux kernel that is scattered throughout a disk. This capability allows the user to boot a kernel file from a file system partition without using the floppy.

In practice, LILO uses the BIOS services to load single sectors from the disk, and then it jumps to setup.S. In other words, it arranges the memory layout in the same way as bootsect.S; thus, the usual booting mechanism can complete painlessly. LILO is also able to handle a kernel command line, and this is a good reason by itself to avoid booting the raw kernel image.

If you want to boot a bzImage with LILO, you must use LILO version 18 or later. Earlier versions of LILO are not able to load segments into high memory, an ability that is needed when loading big images in order for setup.S to find the expected memory layout.

The main disadvantage of LILO is that is uses the BIOS to load the system. This forces the kernel and other relevant files into the first 1024 cylinders of disks to be accessible to the BIOS. When using the PC firmware, you discover how old-fashioned the architecture actually is.

Even if you don't run LILO, you can enjoy the documentation files distributed with LILO's source code. They document the boot process on the PC and explain how to handle (almost) every conceivable situation.

Using Loadlin

If you want to boot your operating system from another operating system, Loadlin is the tool for you. This program is similar to LILO in that it loads the kernel from a disk partition and then jumps to setup.S. It is different from LILO in that it not only faces the BIOS restrictions, but also must dispose of an established memory layout without compromising the system's stability. On the other hand, Loadlin is not restricted to a half kilobyte length because it is a complete program file, not a boot sector. Version 1.6 and later of Loadlin are able to load big images.

Loadlin can pass a command line to the kernel and is, therefore, as flexible as LILO. Most of the time, you'll write a linux.bat file to pass a full-featured command line to Loadlin when calling the linux command.

Loadlin can be used to turn any networked PC into a Linux box. All that is needed is a kernel image equipped for mounting the root partition via NFS, Loadlin and a linux.bat containing the correct IP numbers. You need a properly configured NFS server as well, but any Linux machine can fill that job. For example, the following command line turns a PC ( into a workstation:

loadlin c:\zimage rw nfsroot=/usr/root/alfred \
More of It

The code is not as easy as I described—it must deal with a lot of details, such as bringing around the kernel's command line, keeping an eye on the boot technique being used, and so on. The curious reader can look in the source file to learn more and to read the authors' comments. There's a lot of information in the comments, and they are often funny to read.

I personally feel most users will never need to touch the boot code, because things are much more interesting when the system is up and running. At those times you can exploit all the features of your processor and all the available RAM without going mad with processor-level issues.

Booting an Alpha

The Alpha platform is much more mature than the PC, and its firmware reflects this maturity. My experience with Alpha is limited to the ARC firmware, which is the most widely used.

After performing the usual detection of devices, the firmware displays a boot menu that lets you choose which file to boot. The firmware can read a disk partition (though only a FAT partition), so you actually boot a “file” without the need to hack boot sectors and build maps of disk blocks.

The file booted is usually linload.exe, which in turn loads MILO (the “Mini Loader”). In order to boot Linux through the ARC firmware, you must have a small FAT partition on your hard drive to store linload.exe and milo files. The Linux kernel doesn't need to access the partition unless you upgrade MILO, so FAT support can be safely left out of your Alpha kernel.

Actually, the user can exploit different options. The ARC boot menu can be configured to boot Linux by default, and MILO can be burnt in flash memory in order to get rid of the FAT partition. However, whatever you do, you end up with MILO running.

The MILO program is a stripped-down version of the Linux kernel. It has all of the Linux device drivers and a file system decoder; unlike the kernel it doesn't have process control and does include Alpha initialization code. This tool can set up and enable virtual memory and can load a file from either an ext2 partition or an iso9660 device. The “file” in question is loaded to virtual address 0xfffffc0000300000 and then executed. This virtual address is also the one where the Linux kernel runs; however, it's unlikely you'll ever load anything but Linux. One exception is the fmu (“flash management utility”) program used to burn MILO in flash ROM—fmu is compiled to execute from the same virtual address whence the kernel runs, and it is distributed with MILO.

It's interesting to note that MILO also includes a small 386 emulator and some of the PC BIOS functionality. This is needed in order to execute self-initialization code found on many ISA/PCI peripheral boards (PCI boards, though claiming to be processor-independent, use Intel machine code in their ROM images).

Since MILO does all of this, what is left to the Linux kernel?—very little, actually. The first kernel code to execute in Linux-Alpha is arch/alpha/kernel/head.S, and all it does is set up a few pointers and jump to start_kernel(). Actually, kernel/head.S for Alpha is much shorter than the equivalent x86 source file.

If, for some reason, you don't wish to run MILO there is an alternative, though not a practical one. In arch/alpha/boot you'll find the source for a “raw” loader that is compiled by issuing make rawboot from the top level Linux source directory. This utility can load a file from a sequential region of a device—either floppy or hard disk—using the firmware's “call backs”.

In practice, the raw loader accomplishes a task similar to the one bootsect.S performs for the PC platform—it forces a copy of the kernel to either a raw floppy or a raw hard disk partition. There's no real reason to use this technique—it is quite hairy and lacks the flexibility MILO offers. I personally don't know if it still works; the “PALcode” used by Linux is exported by MILO and is different from the one exported by the ARC firmware. The PALcode is a library of low-level functions used by Alpha processors to implement low-level hardware management like paging; if the current PALcode implements different operations than the software expects, the system won't work.



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Good one

Anonymous's picture

Good one