about real-base, real-size

Tony Breeds tony at bakeyournoodle.com
Wed Oct 19 09:38:55 EST 2011

On Tue, Oct 18, 2011 at 09:22:21PM +0800, Ryan Wang wrote:
> Hi,
> I do not really understand real-base/real-size and have some questions:
>       1) Is "real-base + real-size" the memory seen by client OS, AIX or
> Linux?
>           Or it's just for the kernel image, e.g vmlinux for Linux?
>       2) What about yaboot? Where is is loaded to in the memory?
>           If yaboot uses "real-base + real-size", where is the kernel image
> loaded?

real-base is the address that OpenFirmware (OF) is loaded at, and real-size
is the ammount of menory that (OF) is using.  The memory seen by the
boot loader varies.  If you're in real mode (for example on an IBM
pSeries system) You only see the first LMB (typically 128 or 256MB)  If
you're in virtual mode, then the boot loader sees all the memory in the

The client OS (Linux or AIX) can interrogate the device-tree to
determine the full system memory and then control the MMU to acces it.

Another envirnoment var similar to real-base and real-size is load-base
which is where images loded by OF will be placed until the ELF headers
are processed.

So on a pSeries system booting yaboot the memory layout will look
something like

128MB of Real Mode Addressable memory (RMA)

    0 ->  16KB	Interrupt vectors
 16KB ->  14KB  yaboot ELF image
  1MB ->   3MB  yaboot text and data
 -------------  Small ammounts of memory claim()ed by yaboot, eg the
                Malloc space
 12MB ->  28MB  OF
 28MB ->  40MB  vmlinux ELF image
 64MB ->  96MB  vmlinux text and data
 96MB -> 110MB  initrd

Once the kernel is booted this all changes. but I hope that helps a
little.  Netbooting makes this even tighter.

Yours Tony
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