Difference in RAM speed (533MHz vs. 667) for an older iMac

Question

I have an iMac (late 2006 Core 2 Duo model) that originally came with two 512MB DDR2 533MHz for a total of 1 Gig. About 2 years ago, I replaced one of them with a 2GB DDR2 800MHz. Now, I have recently replaced the remaining module with a 1GB DDR2 667MHz.

Now, according to the specs of my machine, it is capable of memory speeds up to 667 MHz. Yet, the system info is reporting that both are running at 533MHz.

Why is that? I would have expected that they both would run at 667MHz.

For the record, the first one was made by PNY and the second by Kingston.

Answer 1

There are two likely explanations for this. One is that one of the modules requires an elevated voltage to operate at 667MHz.

The other relates to the modules being mismatched. When you have two mismatched modules, the system has to find a configuration that both modules support.

It may be that there's no suitable set of timings for 667MHz that works well with both modules. It can actually be faster to drop the memory clock to 533MHz and keep the latency down.

This is especially true if your Core 2 Duo has a 1066Mhz FSB. The FSB is quad-pumped and DDR2 memory is double-pumped. 1066/4 = 533/2, and there's a benefit to matching them on Core 2 CPUs.