Re: [coreboot] How to test RAM beyond 640 kByte boundary

[llandre]

> > I have a question about the ram_check function.
> > All the board specific codes invoke it like this:
> >
> > ram_check(0x00000000, 640 * 1024);
> >
> > I had a look with JTAG debugger and indeed the RAM beyond 640 KByte
> > boundary is not accessible. Why? Is it possible to enable and test
> > all the RAM available?
>
> I would like to remove this function from our codebase.
>
> Please note that this is not a useful RAM test.
>
> Please build coreboot with a memtest86 payload in order to test your
> RAM setup.
>
> ram_check() is ONLY useful for _very early_ testing of the RAM setup.
>
> Even successful ram_check() does not indicate that RAM is completely
> correctly set up. You have to run a burn-in test like memtest86 for
> a long time to have reliable results.

I see.
Why is the RAM physically unaccessible beyond 640 KByte at this stage? 
Is it a specific (legacy) memory controller mode?