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I recently had my first foray into dealing with bad RAM modules and struggled to get the system to boot far enough to allow me to run memtest. Two of the three RAM modules had severe damage and it brought to light a question that I couldn't seem to think of answer for: where does memtest run from that it can access 100% of the RAM?

Is it small enough to fit entirely in the processor cache? Does it block off a pretested good section? What happens when all of the RAM modules are bad, can it still run?

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If litterally ALL the RAM is bad you will get a BIOS level memory error, and nothing can be loaded not even MEMTEST. MEMTEST and it's totally stripped down OS, needs a little bit of RAM to load into, if it loads it will start testing RAM and moveing were is it loaded to test other blocks.

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I may or may not have inadvertently shorted out the good stick and experienced an unPOSTable machine. Whoops. I knew that it's an extremely small program so I guess it doesn't effect the results by any significant margin. –  Jake Wharton Sep 15 '10 at 0:54
    
Don't put too much stock in Memtest anyway...memtest is like asking your brain to diagnose it's own neurosis. If it tells you there is bad RAM, it is bad. If it tells you all is fine, it might be bad. Only way to really tell is to swap out modules and/or use a hardware memory tester. –  Bart Silverstrim Sep 15 '10 at 12:16

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