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I recently bought a new ultrabook that came with 4GB of ram. I have seen people talk about how it can be upgraded to 8GB with no trouble. However when I look up the CPU that is inside of the machine it says it can support 16GB of ram. The actual CPU is an Intel Core i5-4202Y.

So my question is this. If Intel says that the CPU can address that much memory and all CPUs now have integrated memory controllers then what would stop me from putting more RAM in the machine beyond what I have seen people add?

Could the manufacturer have tweaked the firmware in some way?

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Actually, since Intel Pentium 686 (I think), there is an upper limit of 64GB of RAM. Likewise, most OSes can address that as well, however it depends on various conditions and has its own limits. Also, as you say memory controller is integrated with CPU, so modern topology of motherboards usually has memory connected directly to CPU, bypassing chipset. Also, it's why most topologies connects at least some of PCI(e)'s to CPU directly as well (giving GPU access to system RAM). This is also, accidentally, why good VGA requires good CPU as well for it to perform satisfactorily.

Anyway, what's really limits the MAX_RAM on the system is motherboard. 4GB requires 32 physical paths for addressing (excluding data, it's just for addressing) from CPU to memory slots. 8GB requires 33, 16GB requires 34 and so on. Intel's CPUs currently have 36 addressing pins (at least).

But laying pathways is expensive. Cutting down on this work lowers costs and in result prices. So basically only limit is in motherboard...

However, sometimes documentation lies (unintentionally). It may be a good idea to try (if you have access to free ram for testing). OTOH, this was more common in days past. I don't know how that works in new/newest products.

EDIT: of course, I talk about POSSIBLE MAX. If manufacturer will not implement them in the chip, they won't be available. That's why there's max amount of memory supported by CPU usually way lower than 64GB. Anyway, what matters is CPU limit (pins and IMC to be exact) and motherboard (physical paths CPU-RAM sockets).

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    "This is also, accidentally, why good VGA requires good CPU as well for it to perform satisfactorily.". Can you explain that a bit more? BEcause it feels wrong to me.
    – Hennes
    Commented Feb 12, 2015 at 10:09
  • What I mean by that is even if you invest in reeeeeally fast VGA it will not give you same result when CPU will be 1st gen i3 as compared to latest gen i7. And by that I mean really GPU-intensive work, not overall system performance. It is, of course, mitigated by giving GPU it's own RAM, but as is well known, no amount of RAM is ever enough. So if it's needed, the GPU often is given access to system RAM. And that is where we're back to CPU. The better CPU the faster it can give access to RAM, as it is not loaded with system management (CPU is called "CENTRAL" not without reason).
    – AcePL
    Commented Feb 12, 2015 at 10:19
  • True. But dropping speed to the dedicated graphics cards does not reduce it all that much. E.g. PCI-e v2 vs PCIe-v3 (same number of lanes) dropped performance by about 4%. Same for going from 16 to 8 lanes. 4% is not nothing, but it is not enough to be noticeable.
    – Hennes
    Commented Feb 12, 2015 at 10:30
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A 32-bit CPU can address a maximum of 4 GB (2^32 bytes), a 64-bit CPU can theorically get to 16 exabytes (2^64 bytes).

However, keep in mind that the CPU use a chipset to address that RAM and it means to create a lot of parallel lines (32 for 32-bit CPU) in the motherboard to connect the chipset to the RAM sockets (usually there are 2 RAM sockets). A simple difference of 1mm in the length from one line to all the others and it won't work. So it isn't cheap to add lines to a working design.

Chipset, mother board and RAM vendors get to an agreement to put the most possible number of lines that doesn't increase a lot the cost of producing them. That's why most of the time your CPU+chipset can be installed only in a few compatible motherboards.

The combination of chipset+motherboard is giving you a maximum of 16 GB. They (vendors) consider that it is a reasonable amount of RAM for a home PC running Windows.

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