1213-1216.html

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

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DEL
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<A NAME="PAGENUM-1216"><P>Page 1216</P></A>


<P><B>
HISTORY
</P></B>

<P>An ascii manual page appeared in version 7 AT&amp;T UNIX.
</P>
<P><B>
SEE ALSO
</P></B>

<!-- CODE SNIP //-->
<PRE>
iso_8859_1(7)
</PRE>
<!-- END CODE SNIP //-->

<P>Linux</P>

<H3><A NAME="ch07_ 4">
bootparam
</A></H3>
<P>bootparam&#151;Introduction to boot-time parameters of the Linux kernel.</P>
<P><B>
DESCRIPTION
</P></B>
<P>The Linux kernel accepts certain command-line options or boot-time parameters at the moment it is started. In general,
this is used to supply the kernel with information about hardware parameters that the kernel would not be able to determine
on its own, or to avoid or override the values that the kernel would otherwise detect.
</P>
<P>When the kernel is booted directly by the BIOS (say, from a floppy to which you copied a kernel using
cp zImage /dev/fd0), you have no opportunity to specify any parameters. To take advantage of this possibility, you have to use software that is
able to pass parameters, such as LILO or loadlin. For a few parameters, one can also modify the kernel image itself, using
rdev; see rdev(8) for further details.
</P>
<P>The LILO program (LInux LOader) written by Werner Almesberger is the most commonly used. It has the ability to
boot various kernels and stores the configuration information in a plain text file. (See
lilo(8) and lilo.conf(5).) LILO can boot DOS, OS/2, Linux, FreeBSD, and so on and is quite flexible.
</P>
<P>The other commonly used Linux loader is loadlin,
which is a DOS program that has the capability to launch a Linux
kernel from the DOS prompt (with boot args) assuming that certain resources are available. This is good for people who want
to launch Linux from DOS.
</P>
<P>It is also very useful if you have certain hardware that relies on the supplied DOS driver to put the hardware into a
known state. A common example is SoundBlaster-compatible sound cards that require the DOS driver to twiddle a few
mystical registers to put the card into a SB-compatible mode. Booting DOS with the supplied driver and then loading Linux from
the DOS prompt with loadlin avoids the reset of the card that happens if one reboots instead.
</P>
<P><B>
THE ARGUMENT LIST
</P></B>
<P>Most of the boot args take the form of
</P>
<!-- CODE SNIP //-->
<PRE>
name[=value_1][,value_2]...[,value_11]
</PRE>
<!-- END CODE SNIP //-->
<P>name is a unique keyword that is used to identify what part of the kernel the associated values (if any) are to be given
to. Multiple boot args are just a space-separated list of the preceding format. Note the limit of 11 is real because the present
code handles only 11 comma-separated parameters per keyword. (However, you can reuse the same keyword with up to
an additional 11 parameters in unusually complicated situations, assuming the setup function supports it.)
</P>
<P>Most of the sorting occurs in linux/init/main.c. First, the kernel checks to see if the argument is any of the
special arguments root=, ro, rw, or debug. The meaning of these special arguments is described later in the document.
</P>
<P>Then, it walks a list of setup functions (contained in the
bootsetups array) to see if the specified argument string (such as
foo) is associated with a setup function
(foo_setup()) for a particular device or part of the kernel. If you passed the kernel the
line foo=3,4,5,6, then the kernel searches the
bootsetups array to see if foo is registered. If it is, it calls the setup
function associated with foo (foo_setup()) and hands it the arguments
3, 4, 5, and 6 as given on the kernel command line.
</P>
<P>Anything of the form foo=bar that is not accepted as a setup function as described is then interpreted as an
environment variable to be set. A (useless?) example is to use
TERM=vt100 as a boot argument.
</P>


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