MTRRs question

["Bryan K." <ef057@hotmail.com>]

I have an athlon-tbird cpu with 768MB and I am doing some experiments with 
MTRRs. I want to have the first 512 MB of ram write-back and the rest 256 
uncachable. So i (would like to) do:

echo "base=0x00000000 size=0x20000000 type=write-back" > /proc/mtrr
echo "base=0x20000000 size=0x10000000 type=uncachable" > /proc/mtrr

But after the execution of the first command and before I complete the 
typing of the second the system hangs.
What is really a mystery for me is that if i do the echos in the opposite 
order, meaning

echo "base=0x20000000 size=0x10000000 type=uncachable" > /proc/mtrr
echo "base=0x00000000 size=0x20000000 type=write-back" > /proc/mtrr

everyting works as expected.
I would be grateful if someone cound give me some explanation of this.
Thank you in advance.

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