The maximum amount of RAM a 64-bit computer can theoretically hold is 8GB.
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?
Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites. Image by Petr Kratochvil.
The Question
SuperUser reader KingNestor is curious about how much RAM a 64-bit computer can hold:
Exabytes you say? Now, now, lets’ not be greedy. We’d be happy to start with a terabyte or two.
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?!
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:
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:
http://en.wikipedia.org/wiki/64-bit#Limitations_of_practical_processors
Finally, David Schwartz notes that even the theoretical limit gets bogged down by current CPU architecture:
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.
Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.
