On Sun, 22 Mar 2015 01:20:47 AM Colin Fee wrote:
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.
The amount of engineering work required to add an extra address line will usually scale linearly with the number of address lines. Once you have 32 address lines adding a few more isn't going to add much extra silicon or motherboard space.
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.
The more DIMM sockets you have the further away from the CPU and memory controller they have to be which decreases the maximum clock rate through longer signal propagation delays and greater capacitance in the lines. This can be partially addressed through greater buffering which gives new different problems. The consumer motherboards with 2 DIMM sockets fit a real market need, they are simple to design and cheap to manufacture.
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.
Yes.
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.
There are server motherboards that take many DIMMs (16 or more). But they are expensive, don't fit desktop cases, and have special cooling requirements. That said Intel has a long history of not supporting as much RAM as can be physically installed. For example I have a P3 router that has 2*256M DIMMs but the 3rd DIMM socket is empty because the Intel chipset only supports 512M. It's quite possible that Intel is deliberately reducing the lifetime of it's products as Robert suggests. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/