Question about DDR3 MAX Speed vs system's current FSB Speed.

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arlenreyb

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Aug 3, 2016
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Sorry if this has been asked in some way before. I tried searching, but I'm still having trouble finding the exact explanation I'm looking for.

I have two 8GB DDR3 1600 (PC12800) DIMMs in my system for a total of 16GB of RAM.
So, correct me if I'm wrong, but this means each cycle, it grabs twice (since it's DDR), and each grab it grabs 4x because it's DDR3. So, 8x the clock speed, right? Meaning the base clock speed is 200 MHz, right?
Going back up; a 200 MHz base cycle, twice per cycle, so 400 MHz, DDR3, so a total of 1600 MHz per cycle. That's where the PC12800 comes from, it's that 1600 MHz x 8.

When I check CPU-Z, it tells me my FSB speed is 100 MHz. Not 200 MHz.
Under the Memory tab, it tells me that my DRAM frequency is 800 MHz. Not 1600 MHz.
Under SPD, it tells me the max bandwidth is PC3-12800J, but then it says 800 MHz. I assume this is because it's DDR, and CPU-Z doesn't double this rate, correct?

So ... are FSB clock and memory clock NOT the same? Can they be set separately? I was under the impression that they were the same.

If they are the same, and since my FSB clock is 100 MHz, that would mean twice per cycle, so 200 MHz, DDR3, so 800 MHz, meaning ... am I only using 1/2 of my RAMs potential, since it's DDR3-1600?

Or does it just mean that I could use the same 2 sticks of RAM in a system with an FSB speed of 200 MHz?

What am I missing, here?

Thanks for your help.
 
Solution
M
Hello,

I'm not really an expert but AFAIK that is not how you count the speed.
Most modern mb only have 100MHz FSB (you can overclock this on good mb). As for RAM stick windows and or detecting software shows half of it's speed because of the "DDR" as in Dual Data Rate and thus the effective speed is actually double.

The equation is like this:
DDR3 SDRAM gives a transfer rate of (memory clock rate) × 4 (for bus clock multiplier) × 2 (for data rate) × 64 (number of bits transferred) / 8 (number of bits/byte)

and the "200MHz" you are looking for isn't the FSB rate you are looking for but it's the RAM Stick clock rate (it becomes 800Mhz because of clock multiplier). https://en.wikipedia.org/wiki/DDR3_SDRAM#JEDEC_standard_modules
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myitinos

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May 19, 2015
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Hello,

I'm not really an expert but AFAIK that is not how you count the speed.
Most modern mb only have 100MHz FSB (you can overclock this on good mb). As for RAM stick windows and or detecting software shows half of it's speed because of the "DDR" as in Dual Data Rate and thus the effective speed is actually double.

The equation is like this:
DDR3 SDRAM gives a transfer rate of (memory clock rate) × 4 (for bus clock multiplier) × 2 (for data rate) × 64 (number of bits transferred) / 8 (number of bits/byte)

and the "200MHz" you are looking for isn't the FSB rate you are looking for but it's the RAM Stick clock rate (it becomes 800Mhz because of clock multiplier). https://en.wikipedia.org/wiki/DDR3_SDRAM#JEDEC_standard_modules
 
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Solution
A

arlenreyb

Commendable
Aug 3, 2016
2
0
1,510

That's exactly what I said, though. Just in a different order.

"Going back up; a 200 MHz base cycle, twice per cycle, so 400 MHz, DDR3, so a total of 1600 MHz per cycle. That's where the PC12800 comes from, it's that 1600 MHz x 8."

Rephrased:
200 MHz base cycle (memory clock rate), twice per cycle (x2 for data rate) so 400 MHz, DDR3 (x4 for bus clock multiplier), so a total of 1600 MHz per cycle. That's where the PC12800 comes from, it's that 1600 MHz x 8 (64 bits transferred / 8 bits/byte = 8).



But I think this answers my question. FSB rate is not the same as RAM stick clock rate.

Thanks for your help.
 
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