CanadaRAM - Clarification please on dual channel access with 3.3 GB configuration


macrumors newbie
Original poster
Oct 30, 2007
CanadaRAM - I have a question about dual channel access with the 3.3 GB configuration (i.e. 2 GB x 2 with Core2Duo (not Santa Rosa)).

I read the guide at These two items:

"All Intel Core Macs support dual channel memory access if matching modules are installed."


"Earlier Core 2 Duo machines [i.e. not Santa Rosa] can recognize a potential 3.3 GB (with 2 x 2 GB installed... the top 700 MB is wasted, so the practical maximum is 3 Gb with 1 x 1 Gb and 1 x 2 Gb). This is a hardware limitation. With 4 GB installed, these machines will report 4 GB in System Profiler, but the operating system will only use 3.3 GB."

The question is, what does dual channel memory access look like for the 3.3 GB configuration?

a) The 3.3 GB configuration does not support dual channel access despite the matching modules (due to the hardware limitation, the sticks aren't seen as matching?)

b) Up to 1.3 GB (the usable amount of the second 2 GB stick) from each stick gets used in dual channel access mode (i.e. 2.6 GB total used as dual-channel, 0.7 GB used as single-channel; seems unlikely)

c) None of the above (if so, please explain)



macrumors G5
You get dual channel across both modules - Dual Channel is all or nothing, you can't have dual channel on part of a module.

The 3.3 limitation is due to the other hardware on the motherboard hogging the address spaces that would be used to address the top 700 Mb of the memory space.
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