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manitoubalck
Jul 21, 2003, 04:02 AM
Apple claim and I don't doubt that the new G5 supports up to 8GB of DDR 400. My misunderstanding however is that Apple claim that G5 has a memory bandwidth of 6.4GB/s which I donít doubt, but is however twice what DDR 400 can offer, being 3.2 GB/s hence its other name DDR 3200.

This leads me to the assumption that the new G5 runs Dual Channel DDR (developed by Nvidia.) Hence 4GB per channel if the memory is shared between both the processors, or 2GB per channel per processor to make up the 6.4GB/s bandwidth.

These configurations are the only way without using the now defunct RD RAM (which had a frequencies of up to 1066MHz and a bandwidth of 8GB/s) to come even close to making use of the amazingly fast FSB of 1000MHz.

If the second assumption is true about each processor accessing its own bank of ram Apple hasn't broken the 4GB barrier per processor.

Either way these insights show that the 8GB of DDR 400 may not be all itís cracked up to be.

blogo
Jul 21, 2003, 06:52 AM
Looks like they run dual channel
Because even the low-end config on the apple store comes with 2x128mb ram modules.

britboy
Jul 21, 2003, 07:04 AM
It's definitely dual-channel DDR. The RAM slots on the mobo are split into two sections, with 8 slots in total (4 on each side). The only way to upgrade the RAM is by adding it in pairs. For the proof, just try going to the apple store and pricing up a G5. When you try and upgrade the RAM, you'll notice that it's all multiples of 2.

AngryAngel
Jul 21, 2003, 10:36 AM
...and the 8GB limit is only because 2GB DIMMs aren't made in volume yet.

cc bcc
Jul 21, 2003, 11:27 AM
It is dual channel, Apple just calls it 128 bit wide.

It would make no sense at all for each cpu to have it's own ram.
The OS distributes the threads to the cpu's, so if one cpu needs to access some information that happens to be stored on the other cpu's ram, it would first have to be copied. This happen countless times per second, inefficiency to the max. A chipset designer would be instantly fired for proposing a memory setup like this.
A solution would be to keep double copies of all the data..

Anyway, it is true dual channel ram. That means we went from effectively 167 MHz RAM to 800 MHz!

(Man I'm so happy, today I ordered a 1.8 GHz G5 with Ati 9800 pro! Yeehaw!)

Catfish_Man
Jul 21, 2003, 11:58 AM
Originally posted by manitoubalck
Apple claim and I don't doubt that the new G5 supports up to 8GB of DDR 400. My misunderstanding however is that Apple claim that G5 has a memory bandwidth of 6.4GB/s which I donít doubt, but is however twice what DDR 400 can offer, being 3.2 GB/s hence its other name DDR 3200.

This leads me to the assumption that the new G5 runs Dual Channel DDR (developed by Nvidia.) Hence 4GB per channel if the memory is shared between both the processors, or 2GB per channel per processor to make up the 6.4GB/s bandwidth.

These configurations are the only way without using the now defunct RD RAM (which had a frequencies of up to 1066MHz and a bandwidth of 8GB/s) to come even close to making use of the amazingly fast FSB of 1000MHz.

If the second assumption is true about each processor accessing its own bank of ram Apple hasn't broken the 4GB barrier per processor.

Either way these insights show that the 8GB of DDR 400 may not be all itís cracked up to be.

It's full 36 bit addressing (42 bit from the processor), so it can address up to 16GB of ram. They do use interleaved memory (and have been using it off and on since the early 90s). New Intel chipsets and nVidia nForce do the same thing.