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Hii dude,
Same problem happens with me, I am Android developer while testing one of my application it shows error as -
"Unable to allocate memory" and Screen goes completely black, It didn't restor after resetting also. And I have to run towords Sony Customer Care Center.
RAM works almost same as a laptop does, let's say you got a windows laptop with intel hd gpu and 3GB ram then you will also se RAM: 3GB (2.75GB availabe for use) that's because some of it goes to intel HD graphics, so it's something what android does i guess it takes some for it's gpu or something else.