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?