大多数人从32位计算升级到64位计算，以突破4GB RAM的限制，但一旦进入64位计算机的领域，你又能突破这一限制多远呢？
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问题
超级用户读者KingNestor很好奇一台64位计算机能容纳多少RAM：
I’m reading through my computer architecture book and I see that in an x86, 32bit CPU, the program counter is 32 bit.
So, the number of bytes it can address is 2^32 bytes, or 4GB. So it makes sense to me that most 32 bit machines limit the amount of ram to 4gb (ignoring PAE).
Am I right in assuming that a 64bit machine could theoretically address 2^64 bytes, or 16 exabytes of ram?!
你说什么？现在，现在，让我们不要贪心。我们很乐意从一两TB开始。
答案
金内斯特的调查答案是一个有趣的混合实际和理论的考虑。马特·鲍尔跳了进来，给出了理论上的答案：
Theoretically: 16.8 million terabytes. In practice: your computer case is a little too **all to fit all that RAM.
http://en.*********.org/wiki/64-bit#Limitati***_of_practical_processors
Conrad Dean插话说，使用当今的技术将理论RAM限制最大化是多么不切实际：
To supplement Matt Ball’s answer, the current largest stick of RAM I can find on one particular online retailer is 32GB. It would take 32 of these to reach 1 terabyte. At about a half inch per stick this brings us to a devoted 16 inches of space on your motherboard for a terabyte of commercial ram. To reach 16.8 million terabytes would require a motherboard 4,242.42 miles. The distance from LA to NYC is about 2141 miles, so the motherboard would stretch across the country and back to accomodate that much RAM.
Clearly this is impractical.
How about we didn’t put our RAM all in one row like on most motherboards, but instead placed them side-by-side. I want to say the average stick of ram is about six inches long, so if we allow a half an inch for width, you can have a square unit of 12 sticks of ram in a 6 inch square. Let’s call this square a RAM-tile. A RAM-tile then holds 384GB of RAM. To reach the required 16.8 million terabytes in 384GB tiles would take 44.8 million tiles. Let’s be messy, and use square root of that to conclude that this will fit in a square of 6693 by 6694 tiles, or 13,386 by 13,388 feet, which is close enough to 2.5 feet squared, enough to cover downtown Seattle in shadow, as if they didn’t already have enough to complain about.
最后，David Schwartz指出，即使是理论极限也会被当前的CPU体系结构所束缚：
Note that no existing x86 64-bit processor can actually do this. Their caches don’t have enough tag bits, their address buses don’t have enough width, and so on. 46-bits (8TB) is the maximum for many modern x86 CPUs.
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