How does loader(8) decide where to load the kernel?

Tim Kientzle kientzle at
Sat May 12 23:36:10 UTC 2012

On May 10, 2012, at 5:32 AM, Marcel Moolenaar wrote:

> On May 8, 2012, at 1:32 AM, Tim Kientzle wrote:
>>>> On i386, amd64, powerpc, and arm,  loadimage subtracts
>>>> the dest value from the address declared in the actual ELF
>>>> headers so that the kernel always gets loaded into low memory.
>>>> (there's some intermediate bit-twiddling I'm glossing over, but
>>>> this is the general idea).
>>> The bit twiddling is supposed to be the equivalent of subtracting
>>> KERNBASE from the load address.  On both i386 and amd64, there is
>>> a direct mapping of the kernel text such that KERNBASE maps address
>>> 0, etc.  By default on i386 KERNBASE is 0xc0000000.
>> Exactly my problem.  This all assumes that you're loading
>> the kernel into low memory.
>> On the AM3358, the DRAM starts at 0x8000 0000
>> on boot, so I'm trying to find a clean way to convince
>> the loader's ELF code to put the kernel there.
> Look at what I did for ia64. All that frobbing should be done
> in the machine specific implementation of arch_copyin, arch_copyout
> and arch_readin. It's a kluge to do it in elf_loadimage.

That sounds like a reasonable approach.  I've started
working down that path…  but it looks like I'll have to fix
a lot of the FDT code along the way.


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