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From what I gather, transfer rate is how many bits you transfer via the bus at once with every clock. So the formula would be: frequency (in MHz) * 2 (because of DDR) * bus width (because I think it's a hard limit on how many bits can pass through at once?).

But the thing is, I was looking at some of my teacher's slides on RAM and it claimed a DIMM with code PC3-12800U and 1600 MHz had a transfer rate of 12800 MB/s; and doing the math it doesn't make sense, as 1600 * 2 * 64 (which, looking up, is the bus width for DDR3) equals 204800, (which I assume is something like megabits per second? Really lost on what the actual unit of measures would actually be here, as I have no idea why megabits would be being used here, since converting it directly to bytes give it a value which could only be megabytes per second accourding to the transfer rate) or 25600 in bytes. So why is the transfer rate half of what it is supposed to be? Is it not taking into account DDR for some reason (i'd imagine it wouldn't be the actual rate data is being transfered then)?

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PC3-12800U is DDR3-1600, which is 1600 Mega-Transfers per second, not 1600 MHz. The factor of 2 caused by DDR signaling is already accounted for in that "1600 MT/s" figure.

DDR3-1600 operates at 800 MHz. If you run the numbers with that frequency again, you'll arrive at the correct result.

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  • \$\begingroup\$ I always thought MHz and MT/s were used interchangebly. By the way, does that mean that the information is plain false then? I see so many marketplace ads for DIMMs with PC3-12800U claiming they have 1600 MHz. Or are all DIMMs marketed like that? Is it because they operate it at X MHz but after you factor in Double Digital Rate it essentially doubles their speed so they can claim it's the same as X * 2 MHz? \$\endgroup\$
    – WaveCave
    Commented Apr 28 at 4:37
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    \$\begingroup\$ @WaveCave The MHz is typically listed as twice its actual speed is really just marketing, truthfully. Saying "MHz" is more relatable than MT/s and it's nomenclature that closely aligns with CPU vocabulary. Marketing likes simplicity and it's not good to confuse the consumer. \$\endgroup\$
    – Colin
    Commented Apr 28 at 5:53
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    \$\begingroup\$ @WaveCave With DDR interfaces it is common to refer to the doubled data rate with MT/s and the base clock rate with MHz. In reality it's an artificial distinction since if you probe the data line you will not be able to distinguish a 1600 MHz SDR line from a 1600 MT/s DDR line. You'd see a 1600 MHz fundamental and it's harmonics either way. In that sense they're the same thing, but it can be useful to distinguish them for clarity. \$\endgroup\$
    – user1850479
    Commented Apr 28 at 12:13

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