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The "SPD" tab in CPU-Z show several sets of settings in the timings table:

SPD tab in CPU-Z

According to the SPD article in Wikipedia, a single set of timing settings is stored in the SPD data (e.g., CAS latency in byte 16). There's no mention of multipe sets of settings being stored there.

So where does the data in the timings table come from?

Sam Morris
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2 Answers2

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I've seen other screenshots where XMP profiles appear in the timings table. DDR3 XMP data is stored in the 'customer use' bytes at the end of the SPD data.

But these aren't XMP profiles. Here's someone else's screenshot of what XMP data looks like in CPU-Z:

enter image description here

Note that the 4th profile is labelled XMP-2988, whereas the other three (and all four of mine) are labelled JEDEC #n.

decode-dimms outputs a "timings at standard speeds" section that calculates the DIMM timings based on the supported CAS latencies from the SPD data.

So perhaps CPU-Z is doing the same thing, and the data I'm seeing isn't stored in the SPD data at all?

Sam Morris
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The SPD (Serial Presence Detect) is a chip found on each RAM module. It allows your motherboard to know that the RAM is present, as well as tell it what settings to use in order to access the RAM.

When you power on your computer, it executes a boot check, also called POST (power-on self-test). That’s a series of basic tests for the hardware, performed under the instructions of the UEFI/BIOS.

As described in your Wikipedia article, the UEFI/BIOS communicates through specified addresses using the computer's SMBus, and the SPD chip answers according to a known protocol.

The answer is then : The XMP profiles are stored on the SPD chip, as installed and burnt into this chip by the RAM manufacturer.

harrymc
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  • I'm not asking about XMP profiles, but the non-XMP timing data displayed by CPU-Z. – Sam Morris Jan 12 '23 at 17:44
  • Everything that CPU-Z is showing came from information that was read from the SPD. This mostly includes the XMP profiles. The Wikipedia article lists all this information. – harrymc Jan 12 '23 at 19:27
  • I'm not seeing which bytes from the DDR3 SPD the timings table is taken from. There's only a single set of timing data (14-29, 34-38). – Sam Morris Jan 13 '23 at 09:33
  • CPU-Z is probably showing the data from the XMP profile that was selected by the motherboard. I can't think of any other information that it should show instead. – harrymc Jan 13 '23 at 09:38
  • I think it would display XMP-nnnn above the timing data if it was read from the XMP data, rather than JEDEC #n. Can't be 100% sure though because none of this is documented... – Sam Morris Jan 13 '23 at 09:47
  • CPU-Z might not have that information, unless it compared the current settings with those of all the XMP profiles. The developer apparently left this for the user and didn't try to guess.. – harrymc Jan 13 '23 at 09:50