Ask Klaus!

Klaus Knopper is the creator of Knoppix and co-founder of the LinuxTag expo. He currently works as a teacher, programmer, and consultant. If you have a configuration problem, or if you just want to learn more about how Linux works, send your questions to:

No DVD Boot


I have an "old" 1 GHz PC with a DVD reader and CD writer, 512MB Ram, and 2 * 80MB HD. Unfortunately, apparently the BIOS is unable to boot from the DVD reader.

So, my question: every month I receive a very valuable Linux DVD from this magazine. How can I convert the DVD into multiple CD's from which I can boot?


I have been thinking about this for quite a while now, especially since there are old computers around that cannot even boot from an existing CD-Rom drive directly.

The problem is really that, when booting from CD or DVD, there is no operating system running at this point, and all steps necessary in order to load a kernel and initial ramdisk from the CD or DVD have to be done by the computer's BIOS. The way this works is called "el torito" for CDs or DVDs, and there are a few different modes that are possible. The way most older BIOSes boot from CD is by virtually switching the CD-ROM drive with the floppy drive and loading a floppy image from CD.

Since this floppy image is only allowed in certain sizes, it is not easy to fit a kernel, an initial ramdisk, plus all boot scripts and programs in it. Knoppix 3.x was the last version with a kernel small enough to fit on a single floppy disk. Now we have to use the so-called "no emulation mode," where a bootloader (isolinux) is loading kernel and initrd directly from CD. This boot mode is not supported by all old computers, so this may apply to your "old" machine as well. By the way. my oldest testing machine for testing Knoppix is a 486 with 100MHz, 28MB of RAM, and I have to boot this with tricks similar to the tricks I am about to describe.

If you try to boot from a floppy disk first, in order to access the CD, and continue booting from there later, you need to include on the floppy all drivers required for all types of CD or DVD-ROM (possibly including SCSI, USB, Firewire or SATA). It's tricky.

Your idea of just "splitting" the DVD into several CDs is not impossible, but it is hard to do. Using Knoppix/unionfs, several CDs could be "joined" into one large file system. But you would need one drive for each CD, and you would need to modify the boot scripts to check each drive for the presence of a unionfs component.

The best/easiest way in your setup would be, for now, to make a bootable CD just with the kernel and the initial ramdisk, and (to be safe) immediately REMOVE this "boot-only" CD from your bootable CD-Drive as soon as the kernel and initrd have been loaded. Then, at least for the Knoppix case, all drives, including the DVD, will be searched for the necessary files to continue starting up the system.

These are the steps to create a bootable CD containing only the necessary boot files from Knoppix (your Knoppix-DVD is mounted at /mnt/dvd in this example):

mkdir /tmp/bootcd
cp -a /mnt/dvd/boot /tmp/bootcd/
chmod -R u+w /tmp/bootcd/
mkisofs -input-charset ISO-8859-1 -pad -l -r -J -no-emul-boot -boot-load-size 4 -boot-info-table -b boot/isolinux/isolinux.bin -c boot/isolinux/ -hide-rr-moved -o /tmp/bootcd.iso /tmp/bootcd

Then you just burn /tmp/bootcd.iso onto an empty CD-R using your favorite burning program that has direct iso-burning support (such as cdrecord). Don't be surprised: this boot CD image is really only about 4-16MB in size.

Since most live CDs have to recheck all drives after loading kernel and initrd, this trick will probably work for a lot of different Live DVDs. You could even make a multiboot CD image that allows you to boot several DVDs using the same initial boot CD.

Boot Options


There are a very large number of boot options in Linux. I assume some are used very often and some are very rarely used. What are the most important boot options to know about, and how are they used? What would be on your "top ten list" of essential boot parameters? Do you have any favorites that you find very useful but most people don't know about?

Figure 1: The Knoppix 4.01 boot screen.


First, we have to distinguish between kernel options, module parameters, and "unofficial" options that are not used anywhere unless a distribution evaluates them inside scripts in the later boot process (like Knoppix does). Everything you type (or list in LILOs or an isolinux "append" parameter) after the name of the kernel image will appear in clear text inside the file /proc/cmdline.

Genuine kernel options are set in the static part of the kernel, for example: noapic or pci=bios. The first example switches off the "advanced interrupt controller," which is a workaround to some problems frequently observed with interrupt routing (or buggy hardware). The second example tells the kernel to stick to the interrupt distribution as set in the BIOS, rather than trying a direct (sometimes "best guess") approach. This can solve problems at a very early stage of the boot process. Unfortunately, there is no way to "undo" these parameters with a followup option, so putting them into the "append" option line in lilo.conf, syslinux.cfg or isolinux.cfg is only recommended if you are sure you won't need apic or pci=direct when booting on a different computer with the same bootdisk. And there are boards that insist on having apic or acpi support, otherwise they won't let Linux come up at all.

A quite large but not complete list of kernel options can be found in the documentation that comes with each source release of the linux kernel, in the file Documentation/kernel-parameters.txt.

Module parameters, the second type of options, are parameters passed on to specific kernel modules that are compiled into the kernel. These module parameters sometimes contain the module name in the first part, and the options (or a list thereof) following as a (comma-separated, if there are more than one) list, like this:


This example would tell the psmouse module (ps/2 mouse port, sometimes referred to as /dev/psaux), which, in this example, is compiled directly into the kernel, to use the scrollwheel-enabled imps-protocol rather than the autodetection or plain ps/2 protocol. This is sometimes necessary for notebooks when you cannot get parts of a touchpad or buttons to work correctly (frequently used for Synaptics touchpads that don't respond well to the original Synaptics driver), or for cases when the mouse seems to jump over the screen for no apparent reason.

The third type of option relates to arbitrary, space-separated texts that are just ignored by the kernel, and by the kernel modules. If these options have the form name=value, you can evaluate them directly as shell variables inside /linuxrc or an initrd (initial ramdisk), which is very practical for configuration scripts that don't implement their own enhanced /proc/cmdline parser. Knoppix uses this for activating DMA when the user types knoppix dma at the boot command line. dma is not a (current) kernel option, but it is used for a shell conditional inside the /linuxrc hardware detection part.

Here is my short list of "favorite" kernel parameters:

Most of these problems with interrupts, I/O addresses, buses, etc. could have been avoided in the first place if PC hardware hadn't started life with a quick and dirty, if not broken, design, that is just dynamically extended nowadays, but not replaced.

The combination acpi=off noapic pci=bios pnpbios=off solves the majority of cases (for me), when Linux (especially full-featured kernels designed for live CDs) hangs during boot for no apparent reason.

Actually, there is another type of boot option that isn't really a boot option. It is used by the specific bootloader to find and relocate an initial ramdisk, change the verbosity level, or choose the location of the kernel image. Sometimes these options (like initrd=..., which tells the bootloader where to read the initial ramdisk BEFORE the kernel is started) are also passed through as if they were real kernel options.

Sata Problems


I have just purchased a HP Pavilion a1250n computer with a SATA hard drive. I have been using Linux full-time since 1997, and I am not considered a beginner by any means. However, I have come across a problem that I cannot seem to resolve.

Linux does not seem to recognize the sata hard drive that is in this HP Pavilion. It hangs just after the boot sequence shown in Listing 1.

Listing 1: Boot Sequence Before the Problem
01 Lading sata_sil
02 Ata1: SATA max udma/100 cmd OxF880E080 ct1 OxF88oeo8a bmdma oxF880E000 irq 11
03 Ata2: SATA max udma/100 cmd )xF880Eoco ct1 OxF88oEoca bmda OxF880E008 irq 11
04 Input: 1mEX ps/2 Logitecch explorer mouse on isa0060/serial01
05 Ata1: Dev o ata, max udma/100 488397168 sectors: lba48

At this point, the system will not load the hard drive controller. I have tried using other SATA drivers but none will work. However, I can boot Linux in safe mode. Any help would be greatly appreciated since HP can't help me. I want my Linux back.


For a definitive answer to that specific controller problem, I would have to do some more research, but I can give you some general hints about controller/SATA problems.

First, I would see if anything changes when using my "favorite" options from the previous question, or using certain combinations of those options, such as (acpi=off noapic pnpbios=off pci=bios), since they usually do not cause any performance loss.

There have been reports that some SIL controllers also start working with the irqpoll kernel option, though this solution is not really recommended since it can cause side effects for other devices and slow down the system.

Some combinations of acpi/apic/bios settings also seem to influence the way the SATA controller works, and the full specifications may not have been reverse engineered yet in order to fix the driver itself. Thinking about it, you should really try booting with irqpoll first.

The best recommendation for non-working SATA controllers (and you may not like this recommendation...) is to wait for the next kernel release, which may provide support for new controllers or fixes for broken ones.

In your case, if the SATA controller works in "safe mode" (by the way, what is "safe mode", a special boot option in your distribution?), you may want to check which kernel is running in that mode, and which options have been enabled. Try the command uname -a ; cat /proc/cmdline, and maybe you will find a quick solution.