As operating systems and software applications get bigger and bigger, we often find that we need to install more system memory (RAM) in our PCs.  Worse still, we may find that our PC motherboard won't support any more, so we need to upgrade that, too. So we head down to our local computer store and the salesman convinces us to buy 8Gb of RAM, and maybe a new motherboard in which to install it. Back home, upgrade completed, we power up, expecting our operating system and software to luxuriate in the relative ocean of 8Gb of memory. But wait!  What's this?  Windows tells me I have only 4Gb ...