Most people upgrade from 32-bit computing to 64-bit computing to blow through the 4GB RAM limit, but how far can you blow through that limit once you've entered into the realm of 64-bit computers?
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The Question
SuperUser reader KingNestor is curious about how much RAM a 64-bit computer can hold:
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?!
Exabytes you say? Now, now, lets' not be greedy. We'd be happy to start with a terabyte or two.
The Answer
The answers to KingNestor's inquiry are an interesting blend of practical and theoretical considerations. Matt Ball jumps right in with the theoretical answer:
Theoretically: 16.8 million terabytes. In practice: your computer case is a little too small to fit all that RAM.
http://en.wikipedia.org/wiki/64-bit#Limitations_of_practical_processors
Conrad Dean jumps in with a note about how entirely impractical it would be to max out the theoretical RAM limit using today's technology:
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.
Finally, David Schwartz notes that even the theoretical limit gets bogged down by current CPU architecture:
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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