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Old Mar 5, 2005, 11:49 AM   #1
andrewfee
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Do Powerbooks support dual-channel memory?

With my iMac G5 I made sure that I had 2x512mb that were a matched pair, as it significantly improves memory performance. Do the latest Powerbooks support this, or will it not matter if I just add 1gig to the 512mb already installed?

If they do support dual-channel, then I'll have to buy 2x1gig which will be very costly. (480 from crucial)

If not, I'm probably just going to go with 1.5gig, and possibly add the 1gig at a later date if I need it.
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Old Mar 5, 2005, 12:39 PM   #2
Erendiox
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I think you're a little confused as to what dual-channel memory is (or, I believe the proper term is dual datarate). Anyway, DDR memory has a very high bandwidth. I think its something like 4.2 GHZ or 4.7. The powerbooks, being G4 processors, cannot take full advantage of this bandwidth, as their front side bus can only do around 2GHZ. So, to put it simply, the powerbooks cannot take full advantage of DDR memory.

However, what you're saying about pairing two identical sticks of memory is true. It will improve performance if they are identical.
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Old Mar 5, 2005, 12:42 PM   #3
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I don't *think* they do, hell, they don't even support DDR fully IIRC. On a side note, iMacs don't either. Having identical DIMMS increases the BUS from 64-bit to 128-bit or something...but it's still not dual-channel.
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Old Mar 5, 2005, 01:04 PM   #4
andrewfee
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Quote:
Originally Posted by edesignuk
I don't *think* they do, hell, they don't even support DDR fully IIRC. On a side note, iMacs don't either. Having identical DIMMS increases the BUS from 64-bit to 128-bit or something...but it's still not dual-channel.
Oh, I thought that's what dual-channel was. Although Erendiox is saying that it will improve performance; is that right? I can't find anything about the Powerbook saying if it will or not.
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Old Mar 5, 2005, 01:09 PM   #5
yippy
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Matched pairs will not give you any speed advantage in the PowerBooks. Only the iMac G5 gain any advantage from matched pairs and that is only a slight change, not noticible for most anyone.
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Old Mar 5, 2005, 01:12 PM   #6
andrewfee
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Originally Posted by yippy
Matched pairs will not give you any speed advantage in the PowerBooks. Only the iMac G5 gain any advantage from matched pairs and that is only a slight change, not noticible for most anyone.
Thanks; I didn't think they supported it, but wanted to make sure. It'll save me a fair bit of money. (1.5gig is probably enough, and is much cheaper)


Seems to make a fair bit of difference in the iMacs though. (roughly 25% faster in most memory benchmarks)
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Old Mar 5, 2005, 02:51 PM   #7
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Originally Posted by andrewfee
[IMG]http://barefeats.com/image05/img5d-str.gif[IMG]
Seems to make a fair bit of difference in the iMacs though. (roughly 25% faster in most memory benchmarks)
Thats because the computer can now transfer data over a 128-bit bus instead of a 64-bit bus, essentially using both RAM cards at once.

It harkens back to why SIMMs had to be paired. Because the PowerPCs were 64-bit processors in actuality and therefore needed to use both sides of a DIMM or two SIMMs to be able to access RAM. (Had to do with the way you call memory) Now, it is the same idea but you don't NEED two DIMMs with the G5. You can use one, or both, depending on the RAM for more speed.
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Old Mar 5, 2005, 02:54 PM   #8
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Quote:
Originally Posted by andrewfee
Seems to make a fair bit of difference in the iMacs though. (roughly 25% faster in most memory benchmarks)
Interesting thing. On the x86 side, dual channel yields about 3 % advantage over single channel, e.g. on nVidia nForce 2 boards.

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Old Mar 5, 2005, 07:33 PM   #9
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Quote:
Originally Posted by Erendiox
I think you're a little confused as to what dual-channel memory is (or, I believe the proper term is dual datarate). Anyway, DDR memory has a very high bandwidth. I think its something like 4.2 GHZ or 4.7. The powerbooks, being G4 processors, cannot take full advantage of this bandwidth, as their front side bus can only do around 2GHZ. So, to put it simply, the powerbooks cannot take full advantage of DDR memory.

However, what you're saying about pairing two identical sticks of memory is true. It will improve performance if they are identical.
Actually, dual-channel and DDR (double data rate) are separate things. The PowerBook G4 takes advantage of neither, despite its "PC2700 DDR" RAM. The PowerBook front side bus is 167MHz (single data rate) and severely limits the performance and bandwidth of the RAM. DDR RAM is a c.2001 invention that allows for data to be carried on both the peaks and troughs of the oscillation waves (so data is carried twice per clock, hence the name double data rate), theoretically improving memory throughput considerably. In practice early on, the gains were minimal, but improvements have made DDR RAM far superior (not so much on the PowerBooks, but elsewhere in computing).

Dual-channel RAM (as covered in other posts here) requires a matched pair of DIMMs that can improve throughput by increasing the amount of data that the memory can handle at once. It partly has to do with the bus being wider (but that doesn't make it dual-channel, as edesignuk noted), but more technically, it involves staggering RAM modules so that the memory bus can be saturated continuously by running data through module #2 while #1 is busy. On the PC side, as another poster mentioned, it is a marginal improvement, but on the Mac side, it seems to make a big difference. Unfortunately, it is a recent development of modern DDR chipsets (chipset [system controller] support is required for dual-channel to work), and the PowerBook won't be able to benefit from it until it receives a fundamentally new system controller and bus design (i.e. a true DDR bus).
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Old Mar 5, 2005, 07:43 PM   #10
daveL
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Quote:
Originally Posted by yippy
Matched pairs will not give you any speed advantage in the PowerBooks. Only the iMac G5 gain any advantage from matched pairs and that is only a slight change, not noticible for most anyone.
Well it sure was noticeable on my wife's iMac G5. It came with a single 512 MB stick. When I added a second matched stick, it really became snappy (that's a technical term).
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Old Mar 5, 2005, 07:58 PM   #11
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Macs haven't supported interleaved memory in a long time, the dual channel is supported only in the G5 chipsets at the moment (while it is "required" in the XServe G5 and the PowerMac G5.)

So no benefits... no interleaved memory or dual channel support for that machine.

Would sort of be pointless since the G4 FSB saturates at slightly over 1GB/s when the memory is running at a max of 2.1 or 2.7GB/s.
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