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Say it can only use 2GB's, and is provided 4, does it only use half, or not use any of the ram?

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    \$\begingroup\$ If you provide 4 GB then there is an assumption that the device can use 4 GB. If your device can only address 2 GB then no matter how hard you try, you are not providing it with 4 GB. \$\endgroup\$ – Andy aka Feb 27 '17 at 11:36
  • \$\begingroup\$ So if it's connected to 4gb, will it only use 2? \$\endgroup\$ – AlexDoe Feb 27 '17 at 11:37
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    \$\begingroup\$ It CANNOT be connected to 4 GB if it only supports 2 GB. It can only be connected to the first 2 GB but even then, I'm generalizing and there may be specific examples where nothing is possible. Be specific about the SoC and memory chip. \$\endgroup\$ – Andy aka Feb 27 '17 at 11:45
  • \$\begingroup\$ Okay, I was just asking in general. \$\endgroup\$ – AlexDoe Feb 27 '17 at 11:47
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    \$\begingroup\$ It's a fair question Alex, deserves a friendly answer. The SoC FPGA may have 4GB connected to it but it depends on the circuit actually implemented in the FPGA. If that is designed to address less RAM than 4GB, it will behave as if the extra RAM doesn't exist. \$\endgroup\$ – TonyM Feb 27 '17 at 11:55
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Say it can only use 2GB's, and is provided 4, does it only use half, or not use any of the ram?

A "generalized" microprocessor or SoC has typically three buses and the relevant one to this question is the address bus. If it has a 16 bit width (like the 8085) then it can only "talk-to" 65,536 (\$2^{16}\$) memory locations. This means you cannot force it to be connected to more. Buses on an 8085: -

enter image description here

If you have some RAM that is bigger than that implied by an address bus of 16 lines then you should ensure that unused lines (on that memory) are tied to either logical 1 or 0.

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  • \$\begingroup\$ Apologies, that was me trying to upvote on a tiny mobile screen...only just noticed I had an inexplicable -1 on my score. I can't fix it now, tells me it's over 10 hours - edit it to remove it if you want. \$\endgroup\$ – TonyM Feb 27 '17 at 23:41
  • \$\begingroup\$ @TonyM OK, done. \$\endgroup\$ – Andy aka Feb 28 '17 at 8:16
  • \$\begingroup\$ That's OK and I'll assume a 'thanks for the upvote you were trying to give'. \$\endgroup\$ – TonyM Feb 28 '17 at 9:46
  • \$\begingroup\$ Hey I'm not desparate LOL! \$\endgroup\$ – Andy aka Feb 28 '17 at 9:50
  • \$\begingroup\$ Just got those eyes on the scoreline :-) :-) All the best \$\endgroup\$ – TonyM Feb 28 '17 at 9:54

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