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Most boot arguments take the form:
name[=value_1][,value_2]...[,value_11]``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 above format. There is a limit of 11 parameters that can be associated with each keyword, though you can reuse a keyword for additional parameters, if the setup function supports it.
First, the kernel checks to see if the argument is any of the special arguments `root=', `ro', `rw', or `debug'. Then it walks a list of setup functions (contained in the bootsetups array) to see if the specified argument string (such as `foo') has been 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,bar then the kernel would search the bootsetups array to see if `foo' was registered. If it was, then it would call the setup function associated with `foo' (foo_setup()) and hand it the integer arguments 3, 4, 5 and 6 as given on the kernel command line, and also hand it the string argument bar.
Anything of the form `foo=bar' that is not accepted as a setup function above is then interpreted as an environment variable to be set. An example would be to use TERM=vt100 or BOOT_IMAGE=vmlinuz.bak as a boot argument. These environment variables are typically tested for in the initialization scripts to enable or disable a wide range of things.
Any remaining arguments that were not picked up by the kernel and were not interpreted as environment variables are then passed onto process one, which is usually the init program. The most common argument that is passed to the init process is the word single which instructs init to boot the computer in single user mode, and not launch all the usual daemons.