My question is about Athlon XP CPUs which have FSB at 200 MHz frequency, and use DDR 400 on a motherboard with chipset which support dual channel (nVidia nForce2-400ultra or VIA KT880). My understanding is that bandwidth between the chipset and memory is "800" MHz because it's dual channel DDR400, but is the CPU <-->chipset FSB also 800 MHz ?
Is 800 MHz FSB required to fully utilize dual channel DDR400 ?
- Thread starter user1234
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but isn't the 800MHz FSB needed required to take advantage (=fully utilize) of dual channel DDR400 memory configuration ?
LTC8K6
Lifer
- Mar 10, 2004
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It's 200/400 for Amd and 200/800 for Intel.
They are both only 200mhz. Amd is dual pumped and Intel is quad pumped. The frequency is "400" or "800" because of the dual/quad pumping, not dual channel.
Dual channel doesn't really have to do with the frequency as far as I know.
They are both only 200mhz. Amd is dual pumped and Intel is quad pumped. The frequency is "400" or "800" because of the dual/quad pumping, not dual channel.
Dual channel doesn't really have to do with the frequency as far as I know.
DDR400 operates at 200 MHz but transfers data twice each cycle so it's like 400 MHz. Then if you have two DDR400 modules in dual channel config, both of them transfer the data at the sime time, so the memory controller needs an equivalent of 800 MHz bus (for example a quad pumped 200 MHz) bus to transfer the data to the CPU at the same speed it's being transferred from the RAM. So either it's a quad pumped 200 MHz bus, or a double pumped, double wide bus. For Intel it's well known to be a quad pumped 200 MHz, but what is it exactly for AMD (Athlon XP) ? I think that for AMD64 there is a hypertransport bus with even much higher bandwidth than the dual channel DDR400 bandwidth.
LTC8K6
Lifer
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So then you would be claiming that Intel's DC quad pumped 200mhz is really 1600mhz. It isn't. You don't double the frequency because of DC, you theoretically double the BW. Amd is 200/400 because it's double pumped, and Intel is 200/800 because it's quad pumped. Intel/Amd were 200/800 & 200/400 before they were dual channel. DDR ram already made them that way without dual channel.
If you put one stick of ram in an Intel 865 board for example, it is still running 200/800, but not dual channel.
Yes Amd's Hypertransport is faster.
If you put one stick of ram in an Intel 865 board for example, it is still running 200/800, but not dual channel.
Yes Amd's Hypertransport is faster.
Dude, DDR400 runs at 2x200 = 400 MHz, and that's the same for any platform. So the memory controller reads data from DDR400 module at 400 MHz, so stop talking about 200 (which is just the underlying FSB clock rate). As I said the memory controller reads data from DDR400 module 400 million times a second. So even AMD is 400. Moreover, that's the max speed for reading/writing to a single DDR400 module. But if you have two of them in dual channel config, you could read/write to them double the amount of data at 400 MHz, so you're still doubling the bandwidth. The question is how CPU can handle this unless it has a wider and/or faster access to the memory controller ?
LTC8K6
Lifer
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Well, you're no longer making sense to me then. I'll stop talking about 200mhz when I feel like it and not before.
Why wouldn't the CPU just be designed to accept 2, 4, 500, or whatever number of memory transfers per cycle?
Since it all works fine, I don't sweat it. Athlon XP's weren't BW hungry anyway and gained little from DC.
Since you can also run many different ratios between FSB and memory speeds such as 5:4, 4:5, 3:2, etc.,
I'm guessing that AMD and Intel have it all worked out fine between the CPU, the bus and the memory controller.
Why wouldn't the CPU just be designed to accept 2, 4, 500, or whatever number of memory transfers per cycle?
Since it all works fine, I don't sweat it. Athlon XP's weren't BW hungry anyway and gained little from DC.
Since you can also run many different ratios between FSB and memory speeds such as 5:4, 4:5, 3:2, etc.,
I'm guessing that AMD and Intel have it all worked out fine between the CPU, the bus and the memory controller.
I'm not sure I fully understand this stuff, so I'm reading about it and trying to explain it at the same time. So far, I'm thinking the AMD and Intel cases are slightly different, so lets start with Intel.
First, I think there needs to be a differentiation between FSB (bridge between CPU and Memory controller) and the Memory Channels (bridge between memory controller and RAM)
So, in a single channel memory config you are transferring 64 bits of data between the memory and memory controller, while in a dual channel config you are transferring 128 bits of data. This bus is run @ the memory speed (typically 200 MHZ x 2 for DDR - so 400 MHz effective speed). This equates to a 6.4 GB/sec bandwidth.
Now, on the FSB, you have only one 64 bit data path, so you do need an 800 MHz effective speed to realize that same 6.4 GB/sec data rate. Therefore, on the Intel solution, you would want to match a chip with an 800 FSB capability to a dual channel DDR400 RAM MoBo.
The AMD side is a bit different. Instead of an FSB, they have the HyperTransport link. I'm not sure how fast it operates on each chip, but I know the chipsets on the MoBo operate anywhere from 600 MHz to 1000 Mhz (coincidentally, 800 seems to be what most chips use).
So, on the AMD system the "FSB" is the HTT link, and is already operating @ the appropriate speed (and can be adjusted in the BIOS using various dividers that I am not familiar with)
So, I guess technically you would say "YES, you do need at least an 800 FSB on an Intel system to take full advantage of dual channel DDR 400 memory, and on the AMD system you'd need at least an 800 MHz HyperTransport link speed."
So, that's my best answer so far.
-D'oh!
First, I think there needs to be a differentiation between FSB (bridge between CPU and Memory controller) and the Memory Channels (bridge between memory controller and RAM)
So, in a single channel memory config you are transferring 64 bits of data between the memory and memory controller, while in a dual channel config you are transferring 128 bits of data. This bus is run @ the memory speed (typically 200 MHZ x 2 for DDR - so 400 MHz effective speed). This equates to a 6.4 GB/sec bandwidth.
Now, on the FSB, you have only one 64 bit data path, so you do need an 800 MHz effective speed to realize that same 6.4 GB/sec data rate. Therefore, on the Intel solution, you would want to match a chip with an 800 FSB capability to a dual channel DDR400 RAM MoBo.
The AMD side is a bit different. Instead of an FSB, they have the HyperTransport link. I'm not sure how fast it operates on each chip, but I know the chipsets on the MoBo operate anywhere from 600 MHz to 1000 Mhz (coincidentally, 800 seems to be what most chips use).
So, on the AMD system the "FSB" is the HTT link, and is already operating @ the appropriate speed (and can be adjusted in the BIOS using various dividers that I am not familiar with)
So, I guess technically you would say "YES, you do need at least an 800 FSB on an Intel system to take full advantage of dual channel DDR 400 memory, and on the AMD system you'd need at least an 800 MHz HyperTransport link speed."
So, that's my best answer so far.
-D'oh!
Thanks, that makes sense, and is more or less what I thought. My question is specifically about the Athlon XP 3200+ which operates at core speed of 200 x 11 = 2.2GHz. What is the FSB bandwidth (width and rate) for this CPU ? (I'm asking because I never heard the number 800 MHz mentioned in regard to this CPU's FSB).
And LTC8K6, read the previous post carefully, I think it can clarify your confusion about the difference betweem the FSB and the memory channels. Only the FSB is quad pumped. The memory channels (between the RAM modules and the chipset) can only operate at fast as the memory is capable of. That is, DDR400 module transfers twice in each cycle and operates at 200MHz clock rate. It is not quad pumped, quad pumping applies only to the FSB (link between CPU and chipset).
And LTC8K6, read the previous post carefully, I think it can clarify your confusion about the difference betweem the FSB and the memory channels. Only the FSB is quad pumped. The memory channels (between the RAM modules and the chipset) can only operate at fast as the memory is capable of. That is, DDR400 module transfers twice in each cycle and operates at 200MHz clock rate. It is not quad pumped, quad pumping applies only to the FSB (link between CPU and chipset).
The Front Side Bus on the Athlon XP is 400 mhz while the Athlon 64 is 800 Mhz (equal to). I can't see what the gain would be using dual channel memory on the Athlon XP since the FSB is maxed out but keep in mind that when it was introduced the fastest DDR was PC2100 and not PC3200. Two channels of PC2100 got the job done.
A front side bus of 400 mhz would be 3.2 GB of memory bandwidth so truthfully a single channel of DDR would get the job done today. Even the dual channel Athlon 64 doesn't show that much improvement over the single channel socket 754.
A front side bus of 400 mhz would be 3.2 GB of memory bandwidth so truthfully a single channel of DDR would get the job done today. Even the dual channel Athlon 64 doesn't show that much improvement over the single channel socket 754.
Dual channel has nothing to do with the speed of the ram. All dual channel is, is two sticks of the same amount and speed of ram. You can run pc2100 in dual channel, no matter the fsb.
Not sure if your brain is working in reverse, but this is all about bandwidth. If you transfer data in dual channel that means it's double the bandwidth, because the memory controller is transferring data simultanously to two ram modules. The FSB (link between the memory controller and CPU) need to have enough bandwidth to be able to transfer the data to the CPU at the same bandwidth it is being read from memory (assuming synchronous operation, which is the optimal case), but the FSB width cannot be doubled, therefore it's speed needs to be doubled. If you have no clue about what you're talking about, don't bother posting. When I ask a question here, I expect people that know the answers to help shed light on the subject, and I'm not interested in the opinion of some shmoe that doesn't even understand the difference between quad pumped,dual channel, and double data rate, let alone FSB and memory channel.
Who are you referring to?
If you are talking about me, then let me say this:
I gave a simple answer to your question. This does not mean that I'm some schmoe that doesn't understand what he's talking about.
Edit: I actually just read your OP, and after looking at the question again....that answer is yes. A front-side bus of 800mhz is required to give the full effect of dual channel DDR400 in a 1:1 configuration.
Thank you ! now we're in agreement. My follow-up question is "does Athlon XP 3200+ have 800 MHz FSB, and if not isn't dual channel wasted on the Athlon XP ?" Anyone (who has a clue) ?
No, the XP3200+ does not have a 800mhz fsb.
As for the second part.....my best guess is that (meaning info is 2nd-hand) dual channel won't be wasted on a athlon xp 3200 because the slower fsb....I'm not sure that "wasted" is the right word. It may have less of an effect on the athlon compared to the P4, but doubling the bandwidth is always good. I mean, is there such a thing as too much bandwidth?
After thought: The fsb determines the speed of the cpu. The memory speed determines the performance of the ram. So, although the fsb my be a little slower, the ram will still be as fast.
....Well...look at any benchmarks of a 3200 running on a nforce board (with dual channel), and compare it to a via chipset board (single channel). The nforce board will usually win most, if not all, of the benchmarks. That should prove that dual channel isn't wasted on the athlon
Yes, I know Athlon XP works faster in dual channel mode (and btw, this is not limited to nForce2 Ultra chipset, VIA has the KT880 chipset which supports dual channel as well). So even though the FSB's bandwidth is smaller than needed to transfer data from memory continously at full speed in dual channel mode, I guess that the increased memory speed allows for increased utilization of the FSB up to its "400 MHz"(3.2 GB/S) limit.
Lol, I forgot about the KT880. I was thinking about an old magazine article......dang, can't remember which one.
"I guess that the increased memory speed allows for increased utilization of the FSB up to its "400 MHz"(3.2 GB/S) limit."
Maybe dual channel just decreases the inefficiency -like how you can't fill a vacuum's bag completely without losing performance. (lol - vacuum performance ?)
Here's an excerpt from an article I ran across, about the nForce2 dual channel chipset and its effect on system performance (bear in mind this is from 2002 and relates to Athlon XPs running on 166MHz FSB) :
"The key feature of the North Bridge in nForce2 is its memory controller. It's dual-channel, like in the previous chipset, and thus can provide 128bit access to the memory. Thanks to that, the nForce2 based systems can maintain record-breaking memory bandwidth. For example, it will be 5.4GB/sec when working with DDR333 SDRAM memory modules in 128bit access mode. Of course, there are no Socket A processors available yet that can use up so much memory bandwidth to the full extent. The maximum bandwidth in the new Athlon XP processors supporting 333MHz bus is 2.7GB/sec. But there are other consumers of the memory bandwidth in a system. For example, it's an AGP graphics card, various I/O devices and DASP that will be discussed later on. The dual-channel 128bit memory bus can also help to get relatively high performance of the integrated graphics core as in nForce2 IGP where system memory is used as graphics memory as well."
Enjoy !
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