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
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA
http://crashcourse.ca
========================================================================
