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FB-DIMM 667 MHz in Mac Pro 2008 ?
- Thread starter Spacedust
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If you mix and matched 667 Mhz ram in with 800 Mhz ram it will work, but it will slow all the 800 Mhz ram down to 667. Depending on what your doing the difference between 800 Mhz and 667 Mhz ram could be negligible, and having more ram might cancel out the speed difference.
If you mix 667 and 800MHz memory, it will downclock all of it to 667MHz. That translates to an FSB of 1333MHz in practical use, as the memory's not running fast enough to use the remaining bandwidth.
So if you need/want the FSB to remain at 1600, you need to get the 800MHz RDIMM's for your system.
Experience. That's just how Intel's parts work together for that system (MP or PC variant with the same components on the board).
...would the speed difference in the modules be offset by the much larger capacity of RAM being used as the OP has 12 GB of the 667 Mhz RAM available vs the 6 GB on order?
Generally speaking, it's not a good idea to mix memory like that. Though it will downclock, the actuall position even matters, as the SPD (timings) are read on one of the DIMM's per channel, not all of them. So you can actually confuse the system (i.e. attempt to OC the memory in some cases). It gets down to the firmware.
So to prevent issues, stick with the same memory. Now if you have an '08, you could go ahead and use all 667MHz memory. The throughput is slower, but there are cases where the speed is less important than capacity. It's up to the user to determine that. But the cost difference between the two types isn't that bad last I checked, so might as well get what the system can actually utilize fully.
It's just easier to prevent issues by keeping it the same if possible. I've seen issues with some systems (in general), as the SPD was determined by one of the DIMM slots per channel (specific location). So if that location was filled with the faster sticks, problems came about (unstable).
I didn't have the differing clocked memory to test in an MP, but knew it's technically possible to mix it, as others such as yourself have (posted some time ago).
As per the FSB, it's not that it actually slows down, but the memory can't exceed 1333. Other aspects of the system (i.e.chipsets for example), will remain the same.
1066MHz DDR2 RDIMMs would probably work, but...
1. It would downclock to 800MHz on a 2008 MP.
2. I don't think anyone makes 1066MHz DDR2 RDIMMs with Apple-approved heat spreaders. Most of the ones I've seen don't have heat spreaders.
...so, it's kind of pointless.