在64位計算機中，理論上最大ram容量是多少？

大多數人從32位計算升級到64位計算，以突破4GB RAM的限制，但一旦進入64位計算機的領域，你又能突破這一限制多遠呢？...

大多數人從32位計算升級到64位計算，以突破4GB RAM的限制，但一旦進入64位計算機的領域，你又能突破這一限制多遠呢？

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。圖片由彼得克拉託奇維爾。

問題

超級使用者讀者KingNestor很好奇一臺64位計算機能容納多少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?!

你說什麼？現在，現在，讓我們不要貪心。我們很樂意從一兩TB開始。

答案

金內斯特的調查答案是一個有趣的混合實際和理論的考慮。馬特·鮑爾跳了進來，給出了理論上的答案：

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插話說，使用當今的技術將理論RAM限制最大化是多麼不切實際：

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.

最後，David Schwartz指出，即使是理論極限也會被當前的CPU體系結構所束縛：

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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