Removing the first bottleneck: DDR SDRAM

When talking about memory bandwidth, one of the most commonly discussed areas where there is a “lack of enough” has been on the current crop of 3D graphics cards.  Ever since the introduction of NVIDIA’s GeForce we have seen an increased focus on memory bandwidth and the bottlenecks its limitation provides us with when it comes to gaming performance. 

Discussion about memory bandwidth is present in other areas of the PC outside of the 3D graphics card arena, however it isn’t the first thing that comes to mind when you’re talking about CPUs.  Normally, after architecture, clock speed and cache size are the main factors that are taken into account when talking about CPUs, however memory bandwidth is critical to the performance of high speed CPUs. 

Take the 1.2GHz Athlon that was released earlier this month.  At 1200MHz, the CPU is running at 9 times the frequency of the memory bus (assuming PC133 SDRAM).  As the Athlon gets higher and higher in clock speed, the ratio of CPU clock to memory clock will increase to the point where the CPU has to wait around on the memory before proceeding with any calculations.

The AMD 760 chipset helps to alleviate this problem by introducing the first ever support for Double Data Rate (DDR) SDRAM on an Athlon platform.  By transferring twice as much data per clock (once on the rising edge and once on the falling edge), DDR SDRAM effectively offers twice as much memory bandwidth as an “equivalently clocked” Single Data Rate SDRAM solution.

As we mentioned in our initial preview of DDR technology on the Athlon, there are two flavors of DDR SDRAM that will be made available for use with the AMD 760 chipset: PC1600 and PC2100 DDR SDRAM.

The PC1600 and PC2100 names simply define the amount of available bandwidth each solution provides, so PC1600 essentially means a 1600MB/s or 1.6GB/s transfer rate and PC2100 means a 2100MB/s or 2.1GB/s transfer rate, both figures are theoretical maximums of course.

How are those two figures derived?  Let’s look at the below table to explain that:

DDR SDRAM Comparison
Memory Bus Width
Operating Frequency
Data Transferred per Clock
Memory Bandwidth
64-bits (8-bytes)
8 x 100 x 2 = 1600MB/s
64-bits (8-bytes)
8 x 133 x 2 = 2100MB/s

So as you can see, it’s quite simple.  PC1600 DDR SDRAM operates at 100MHz and PC2100 DDR SDRAM runs at 133MHz.  In terms of cost, PC1600 DDR SDRAM should be priced almost on par with PC133 SDRAM while PC2100 DDR SDRAM should carry a 15 – 25% price premium over PC133 SDRAM. 

The AMD 760, much like the ALi MAGiK 1, does not allow for asynchronous operation of the FSB and the memory bus.  For example, if you’re using PC1600 DDR SDRAM (100MHz operating frequency) your FSB must also be set to 100MHz.  And if you’re using PC2100 DDR SDRAM (133MHz) your FSB must also be set to 133MHz.  This means that both your FSB and your memory bus are running at 100 or 133MHz DDR.  According to motherboard manufacturers it is very difficult to produce a design that will run the two buses asynchronously reliably, for example running PC2100 SDRAM using a 100MHz DDR FSB. 

This obviously poses a problem as there have been no Athlon CPUs that use the 133MHz FSB.  In order to get around this, the AMD 760 chipset will allow you to use PC2100 DDR SDRAM in a system that’s running at a 100MHz DDR FSB setting, however in doing so it will underclock your memory bus to 100MHz DDR as well, effectively making your PC2100 memory, PC1600 SDRAM. 

So what’s the point of PC2100 SDRAM if you can never use it?  Let’s find out…

Index New Chipset, New FSB, New CPUs
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