DDR SDRAM MEMORY

Which DDR RAM to buy?

You want to purchase an ECS K76SA motherboard, and 256MB of PC2100 or PC2700 DDR RAM. You want to run an AMD Duron 1.3GHz processor on the motherboard's front-side bus (FSB) that runs at 200MHz with this processor installed. You know that both PC2100 and PC2700 DDR RAM run faster than 200MHz, so you want to know if the RAM you purchase will be able to run at the slower bus speed used by the processor.

Even though the processor has a maximum speed (frequency) of 1.3GHz (1300MHz), the effective data transfer rate of the processor is limited to the speed (frequency) of the motherboard's FSB, which in this case, doubled by DDR technology, is only 200MHz. The FSB speed is the speed with which the processor is able to communicate with the rest of the system. It can do its own calculations at 1.3GHz, but, in this case, can only communicate with the rest of the system at 200MHz. This is going to limit the effective speed of the DDR RAM, because the RAM can but won't tranfer data faster than the processor.

The chipsets on AMD Athlon/Duron motherboards allow the RAM bus to run at a different speed from the processor bus.

For example, the base FSB on the above ECS motherboard running a Duron processor is 100MHz, because that is the FSB used by that processor. The processor is able to operate at the DDR (double-data rate), which is 200MHz. (Later Athlon processors use a 133MHz FSB.) If PC133 SDRAM, which runs at 133MHz, is used, the Duron processor has an effective data transfer rate that is 67MHz faster than the RAM, so in this case the slower RAM is creating the bottleneck that is limiting the data transfer speed. But with DDR RAM installed, the processor's effective data transfer speed would be responsible for creating the bottleneck.

Below is a table providing information on the different types of DDR RAM.

If you purchase PC2700 RAM, the Duron 1.3GHz processor runs on a base FSB of 100MHz, doubled to give an effective data transfer speed of 200MHz, while the RAM runs on a base FSB of 166MHz, which is effectively 333MHz using the DDR technology. So, in this case, the processor is creating the bottleneck by having an effective data transfer rate that is 133MHz slower than the RAM.

The motherboard's manual provides the information on the types of RAM that the motherboard can run. You must set the RAM clock speed (in the BIOS, or by setting jumpers on the motherboard) to the speed of the RAM that your purchase. This is 133MHz for PC2100 and 166MHz for PC2700 DDR RAM.

Therefore, if you wanted the RAM speed to match the processor speed, you would install PC1600 DDR RAM. However, you could purchase the highest type of DDR RAM supported by the motherboard. If you decided to upgrade the processor to the highest AMD Athlon processor that it supports, that processor will run on a base FSB of 133MHz, providing an effective data transfer speed of 266MHz. The ECS motherboard supports PC2700 DDR RAM, so if that is installed with the new processor, the effective data transfer rate would be increased from 200MHz to 266MHz, and the bottleneck between the RAM and processor would be reduced from 133Mhz to 66MHz.

The latest AMD XP processors run on a base FSB of 166MHz (DDR 333MHz), so they will transfer data across the system bus at the same speed as PC2700 (DDR 333) RAM. But PC3200 (DDR 400) RAM runs at 400MHz, so the base motherboard FSB would have to run at 200MHz for one of these processors to transfer data across the system bus at the same speed as PC3200 RAM.

Websites that sell DDR SDRAM Memory

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