Forgive me, but it's been a while since I looked at CPU architecture but why can't you use cache,at least for the most part, in place of system RAM?
Things are a little foggy but I seem to recall that instructions and data are loaded into RAM then the required instructions and data are loaded into cache and only knocked out of cache when something else needs the space ( determined by some algorithm ). If there is no need to replace the data / instructions, then why is it not possible to run at least a basic system with cache only?
What sort of data can only reside in RAM and if it has to be in RAM is there a problem with it in cache or does the CPU bypass cache completely? Or is it that the OS would require a drastic rewrite to achieve such a thing?