Re: during fedora boot, what picks up the initrd.img from /boot?

[Robert P. J. Day]

On Fri, 30 Nov 2007, Mikkel L. Ellertson wrote:

>  I don't believe the kernel is ever told the location of the file
> on
>  the disk. The initrd option tells grub the file to load. Grub then
>  loads the file into system memory. I am not sure, but I believe the
>  address that Grub loaded it to in memory is passed in a register.
>  But it may be in a fixed place in the block of data passed to the
>  kernel, along with the options in the kernel line. I suspect that
>  this is covered in the Grub info page if you want to know exactly
>  how it is managed. I have never cared enough about the exact method
>  to dig into it. It is also possible to boot a kernel without using
>  an initrd file if you build all the drivers needed to access the
>  root file system into the kernel.
>
>  You may also want to take a look at the initrd.txt file in the
>  kernel documentation tree. 
i've learned the hard way not to trust the in-kernel documentation,
but i did eventually track down what i was after.

for interested parties, in the kernel source tree,
include/asm-x86/bootparam.h:

/* The so-called "zeropage" */
struct boot_params {
	...
	struct setup_header hdr;    /* setup header */
	...
}

and at the top of that very same file, we have:

struct setup_header {
	...
        __u32   ramdisk_image;
        __u32   ramdisk_size;
	...
}

in the 32-bit case, that info is extracted during system boot in
arch/x86/kernel/setup_32.c thusly:

#ifdef CONFIG_BLK_DEV_INITRD
        if (boot_params.hdr.type_of_loader && boot_params.hdr.ramdisk_image) {
                unsigned long ramdisk_image = boot_params.hdr.ramdisk_image;
                unsigned long ramdisk_size  = boot_params.hdr.ramdisk_size;
                unsigned long ramdisk_end   = ramdisk_image + ramdisk_size;
                unsigned long end_of_lowmem = max_low_pfn << PAGE_SHIFT;
...

etc, etc.

  so i'm good.

rday
--
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Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://crashcourse.ca
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