kernel position

kernel position

[Any Anderson]

Hi,
I wann know where in the physical memory is kernel
loaded by the loader (such as lilo) and does this
position has any significance in mm system. If that
location is to be changed which files should be
changed. Lets assume we are talking for x86 platform.
Thanks in advance for your time.

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Re: kernel position

[Stephen C. Tweedie]

Hi,

On Mon, May 14, 2001 at 02:22:19AM -0700, Any Anderson wrote:

> I wann know where in the physical memory is kernel
> loaded by the loader (such as lilo)

Look at the Technical_Guide.ps documentation file in the lilo
distribution: it contains a lot of detail on exactly what gets loaded
where during boot.

> and does this
> position has any significance in mm system. 

Not really, no.  The only impact is that the architecture-specific
parts of the MM's initialisation need to be aware of a few details of
the boot process.  In particular, they need to know where any initrd
ramdisk has been loaded so that they don't try to reuse that memory
until the initial ramdisk has been freed.  Apart from that, the MM
system really knows nothing about booting.

Cheers,
 Stephen
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Re: kernel position

[Kunaal Mahanti]

On Mon, 14 May 2001, Any Anderson wrote:

Hi,
> I wann know where in the physical memory is kernel
> loaded by the loader (such as lilo) and does this
> position has any significance in mm system. If that

The kernel is loaded loaded beyond 0x1000000 (1MB), this is a h/w
constraint as most DMA devices cannot address beyond that, so we leave if
free, although there should'nt be any problem using a lower address for
kernel.

> location is to be changed which files should be
> changed. Lets assume we are talking for x86 platform.
> Thanks in advance for your time.

I think all we need is to use -Ttext flag while loading the kernel to
modify the load address. If I am correct the other things will fall in
place. mem_init() will take care of marking the remaining memory as
dynamic.
-

Kunaal Mahanti
Senior Software Engineer,
IBM NUMA-Q IDC,
Wipro Technologies.
Ph : +91-80-5732293/96 Ext: 5242
Fax: +91-80-5732296
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Re: kernel position

[Stephen C. Tweedie]

Hi,

On Mon, May 14, 2001 at 03:19:54PM +0530, Kunaal Mahanti wrote:

> > I wann know where in the physical memory is kernel
> > loaded by the loader (such as lilo) and does this
> > position has any significance in mm system. If that
> 
> The kernel is loaded loaded beyond 0x1000000 (1MB)

Not necessarily.  A zImage kernel is loaded by lilo at 0x10000; only
bzImage is loaded at 0x100000.  The setup part of the kernel is loaded
at 0x90000 in either case.

*After* lilo has done its loading, the kernel setup code may
relocate the kernel image if necessary to its final desired address
in physical memory.

Also look at linux/arch/i386/boot/setup.S, which is the kernel setup
header where the relocation of the kernel image is performed; and
linux/arch/i386/kernel/head.S which is the starting point of the
relocated image, where we finally make the switch to protected mode.

> this is a h/w
> constraint as most DMA devices cannot address beyond that

Not true --- even ISA DMA can access up to 16MB of physical memory,
and all modern PCI or AGP devices can access up to 4GB of physical
memory (except for a few buggy devices which don't wire the high
address lines correctly and which are limited to 2GB or 1GB).  With
PCI64 or PCI DAC (Dual Address Cycle), the full 64GB of physical
memory on an Intel PAE box can be addressed over PCI.

> > location is to be changed which files should be
> > changed. Lets assume we are talking for x86 platform.
> > Thanks in advance for your time.
> 
> I think all we need is to use -Ttext flag while loading the kernel to
> modify the load address.

Be careful: the text address used for the bulk of the kernel is
designed to have an origin in virtual memory (usually 0xc0000000),
which has nothing to do with the physical location of the kernel.  

Cheers,
 Stephen
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