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Topic Title: Guide to Memory Bandwith
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Created On: 04/26/2004 02:42 PM
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 04/26/2004 02:42 PM
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Aubrey
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Posts: 3663
Joined: 12/03/2003

DDR Memory Bandwidth

The calculations below illustrate how the peak bandwidth for DDR memory modules is calculated.


Peak Bandwidth = (Memory Bus Width) x (Data Rate)

where Data Rate = (Memory Bus Speed x Operations/Clock Cycle)



Each DIMM module is 64 bits wide, or 8 Bytes wide (each byte = 8 bits).

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Peak bandwidth for PC1600 DIMMs

(8 Bytes) x (200 MHz Data Rate) = 1,600 MB/second

or 1.6 GB/second.

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Peak bandwidth for PC2100 DIMMs


(8 Bytes) x (266 MHz Data Rate) = 2,128 MB/second

which is rounded to 2,100 MB/second or 2.1 GB/second.

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Peak bandwidth for PC2700 DIMMs


(8 Bytes) x (333 MHz Data Rate) = 2,664 MB/second

or 2.7 GB/second.

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Peak bandwidth for PC3200 DIMMs


(8 Bytes) x (400 MHz Data Rate) = 3,200 MB/second

or 3.2 GB/second.
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Memory Speed
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Technically, the DDR memory bus runs at memory-bus clock rate of 100 MHz for PC1600, 133 MHz for PC2100, 166 MHz for PC2700 and 200MHz for PC3200. However, each DDR memory module and memory chip run at an effective (data) rate of 200 MHz, 266 MHz, 333 MHz and 400MHz respectively. The computer industry has adopted a practical convention of just referring to the data rate as the DDR DIMM speed. So, PC1600 DIMMs are said to run at 200 MHz, PC2100 DIMMs at 266 MHz, PC2700 DIMMs at 333MHz and PC3200 DIMMs at 400MHz.

PC1600 PC2100 PC2700 PC3200
200MHz 266MHz 333MHz 400MHz
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Facts:
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With all things equal, a stick of DDR memory capable of running 2-2-2-5 will make the computer operating experience seem faster than a DIMM which may only run at 3-4-4-8. This is because the delay from when the memory receives an instruction, retrieves the data, and sends it back out is less
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Most DIMM's that run tight timings, such as certain PC3200 & PC3500 modules, have to run the memory at lower MHz than the FSB. However, when overclocking to extreme speeds these DIMM's are bandwidth limiting the processor. What I mean by this, is that when the processor requires a great deal of bandwidth, the CPU will have to wait for another clock cycle before being filled, as the memory is just not fast enough to keep up at the same pace. Having a large pool of bandwidth is great when you're working with applications that process a lot of raw data, such as Photoshop or databases for example.
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The other point of view is that CAS2-rated PC3200 & 3500 memory can make up for the lack of bandwidth because the memory has a lower latency that in effect moves data between the CPU and memory faster. Programs that do not require a large amount of bandwidth tend to benefit more from quicker data transfers between the memory and the rest of the computer; such as games or 3D applications.
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Double the Bandwidth
Since high-speed DDR brings with it so many potential concerns, it's only natural
to look for ways of extending the bandwidth of lower-speed DDR. The most popular angle is to adopt a dual-channel bus to system memory, thereby doubling the potential bandwidth without having to jack memory clock speeds into the stratosphere. This is a great solution to the bandwidth impasse, since vendors can
use JEDEC-authorized specifications yet still achieve higher-than-DDR400 data transfers.

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A working brain is no substitute for possession of a powerful computer.
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