CPU vs RAM

[BaldEagl]

I've been looking at computer components and have a question.

Back in the old days of the FSB you could calculate RAM speeds to synch with your CPU clock.  For instance I knew I'd run my CPU at 3.2 Ghz so bought DDR2 800 RAM to synch (3200/2 cores/2 (double pumped cores)/2 (dual channel) = 400 Mhz CPU core clock speed x 2 (DDR2) = 800 DDR2 RAM to remain synched).

Do these calculations still hold true with todays newer processors?  If they do is this then correct for a CPU that can run up to 4.0 Ghz?  4000/4 cores/2 (still double pumped?)/2 (still dual channel? ... thinking quad core I5) = 250 Mhz CPU core clock x 4 (DDR4) = 1000 DDR4 RAM. 

Something seems wrong in scenario 2 as everything I've read indicates that RAM speeds have to increase due to increased CAS latency.  I'm guessing I'm off assuming dual channel which would increase the CPU core clock to 500 and the RAM speed to 2000 which would seem about right.

Anyone know?

[Bino]

To see which RAM to use for Intel chips, find the CPU you want on the Ark site and then click on that CPU's "memory specifications" link.

From what I read, chips now use a variety of different ratios of the main CPU clock for memory access.

And they just keep getting faster. On my "4th gen" 4770K the max memory bandwidth is 25.6 GB/s on DDR3-1333. The latest "6th gen" 6700K goes up to 34.1 GB/s on DDR4-1866

[38ruk]

Intel ditched the Front side bus back in the core 2 days for a DMI , which is a direct link from the cpu to the southbridge. Amd has used a  hyper transport link for quite awhile as well , at least as far back as my last Phenom II 550 be.  The 550 and my I5 both had/have unlocked multipliers so I haven't had to mess around with a fsb in years.

[BaldEagl]

Intel ditched the Front side bus back in the core 2 days

Yes, I knew that which is why I'm trying to determine if there's still a meaningful relationship between CPU and RAM clock speeds.

[38ruk]

I guess it comes down to what you primarily use your PC for . Memory speed scales nicely with some programs like photoshop, but for games it hit or miss. In some games the higher speed memory will increase as much as 10fps.  Basically it comes down to what your motherboard will run up to and the minimum spec ram of what the Cpu, but they are not tied together. Intel uses a Base clock rate (bclk) that is set at 100 mhz for my I5. That times the multiplier is what determines cpu speed . The bclk is kinda like an old FSB but has little room for movement as it is tied to the PCI pci-e slots  sata and usb ports , so it doesnt like to be messed with a whole lot.