Memory matters

Double data rate simply means that for every bus clock ‘tick’, two packets of data can be transferred, giving a theoretical memory bandwidth of twice the nominal clock speed. So 100MHz DDR SD-Ram in a PC with a
64-bit system bus can transfer data at 200MHz; that’s 64 bits of data 200 million times per second, or 1.6Gbits/sec.

In the interests of compatibility, memory manufacturers have agreed various design standards and also adopted a naming convention. For example, the 200MHz DDR memory just described is known as PC1600; the ‘1600’ refers to the 1,600Mbits/sec transfer rate possible on a PC’s 64-bit memory bus. See the attached PDF, Memory decoder, for an explanation of memory labelling.

To confuse matters, there are several variations, such as PC4000 (250/500MHz), which don’t correspond to normal system bus speeds and are generally produced for enthusiasts who want to overclock their memory. Some motherboards allow you to run the memory bus at a different speed to the system bus, but the benefits are dubious: synchronous memory is designed to work at the system bus speed.

DDR2
The original DDR memory was superseded by DDR2, also labelled as PC2-xxxx; for example, 400MHz DDR2 is known as PC2-6400. The number represents the theoretical data transfer rate: in this case, 6.4Gbits/sec.

Because DDR2 runs at a lower voltage (1.8V) than DDR (2.5V), it generates less heat and consumes less power. Standard DDR2 clock speeds are 200, 266 and 333MHz, although variants for overclockers exist, including 500MHz/1GHz PC2-8000 memory modules.

DDR2 Dimms are not compatible with DDR motherboards, as they use a 240-pin memory module rather than the 184-pin module of DDR memory.

DDR3
The latest type of memory module, introduced with the Intel Core i7 processor, is DDR3. It is even more power-efficient than DDR2, running at 1.5V or even 1.35V for some specialised low-power modules. It adds some technical features to boost data rates and improve reliability, such as the ability to clear the memory on reboot so the PC can start with a clean slate, as it were. It also increases the depth of the pre-fetch pipeline (see box on page 46) to eight bits versus DDR2’s four bits, enabling data transfer rates of up to 800MHz (12.8Gbits/sec).

DDR3 uses the same 240-pin modules as DDR2, but the notch is in a different place to physically prevent the modules being used in the wrong motherboards.

But there are some more significant changes with the Core i7. The memory controller is now integrated in the processor, dispensing with the FSB. More importantly as far as your pocket is concerned, Core i7 supports triple-channel DDR3 memory with up to two Dimms per channel, the implications of which we look at in the Refresh you memory section.