On 22 March 2015 at 10:17, Rick Moen <[email protected]> wrote: > Quoting Craig Sanders ([email protected]): > > > funnily enough, i have a similar reaction to most intel motherboards - > > their CPUs can be quite good, but the PCIe lines available and the I/O > > is minimal compared to AMD AM2/3/3+ CPUs and motherboards. > > Here's a question that stumps me just a bit: Why are so many x86_64 > motherboard / CPU combinations limited to what seem like arbitrarily low > ceilings on total RAM? > > I don't want to seem like a spoiled child in this. Even 8GB (most Atom) > is nice. But why not more? > > Back when x86_64 was new (and was AMD64, technically), my recollection > was that we heard about the glorious new 16 exabyte (2^64) _theoretical_ > linear address space that for reasons of practicality would be limited > to 256 terabytes (2^48). Yet, we've never seen that, right? Mind you, > I'm not talking about machines shipping _with_ 256 terabytes of RAM, > only ones that could address that amount if it were available in > real-world hardware. > > AIUI it's an economic compromise made by CPU manufacturers and mobo manufacturers. I'm no hardware engineer either however I did study electronic engineering at uni in the halcyon days of the 8086/186.
Whilst 64 bit CPUs have a theoretical 64 bit address space available to them for performance and other functionality reasons the CPU designers limit the number address lines i.e pins available to memory. For example the Kabini CPUs featured in this thread have 40-bit address space, whereas some the higher end CPUs will have 42 and 43 bit address lines available to them. Of course this is larger than some of the 32Gb limits we see on mobos. Here we hit the economic compromises made by the mobo makers. They have to balance what they can physically fit on a mobo with how easy they can be physically wired to the CPU and with how much the market is willing to pay versus their bottom line. Given the clock speeds that modern mobos and CPUs operate at, balancing the arrival of bits from RAM to the CPU etc is a critical factor in making sure it all works as expected. I'm sure if you looked around you could find a mobo that maxed out the RAM limit but I expect it would be expensive, power hungry and large (well full-sized anyway). Not the focus of some of this thread I'm sure you'll agree. Again a compromise has to be made to made amongst physical size, speed, power consumption etc. Which all contributes to the less than theoretical RAM limit. -- Colin Fee [email protected]
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