[coreboot] How to test RAM beyond 640 kByte boundary

llandre r&d2 at dave-tech.it
Wed May 21 12:44:15 CEST 2008

>> 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?


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