DDR-2 in Detail Continued
DDR-2 while having a similar command set, it requires a totally different memory socket. This is why DDR and DDR-2 modules can't be interchanged in the same sockets. DDR runs on a 184 pin socket, where DDR-2 uses a 240 pin socket. The extra pins are used for the second core on the chip as well as extra signal voltage pins. DDR-2 however, does give some advantages as it can be in a term retarded back to allow DDR memory modules to exist on the same motherboard as DDR-2. Some motherboards have appeared with both DDR and DDR-2 memory module sockets which just goes to show the similarities between the two technologies. Intel's own I915 series chipsets support both DDR and DDR-2 memory technologies. You can have Dual Channel DDR or Dual Channel DDR-2 and while you may find both sockets on the one motherboard, you can't use them both at once, as they share the same data address paths, and would simply conflict. However, it does allow users to go to I915 chipsets and keep their DDR until DDR-2 matures into a more mainstream product.
DDR-2 does have its advantages over DDR, but it falls behind in one aspect - latencies or the time it takes to perform certain tasks. DDR runs at usually CL3, CL2.5 and some high performance modules run at CL2, though CL2.5 is more common for higher speed memories over 400MHz. DDR-2, due to its nature of dual cores increases latencies to CL5 and CL4 and for the higher performance modules CL3 although CL3 isn't available yet which makes CL4 the lowest available at the moment.
This won't affect the Intel Pentium 4 series as much as it would the AMD series CPU's which seem to crave lower latencies rather than raw bandwidth which is one advantage for Intel as DDR-2 533 can offer 8.5GB/s and with 667 modules now starting at 10.5GB/. With DDR-2 speeds set to push bandwidth to the ultimate, you will soon see a lot of programs written for high bandwidth utilisation.
DDR-2 possesses a slight complication in adoption for AMD64 users, as it simply cannot be done on the current 754 and 939/940 packaging. As most already know, AMD places its DDR memory controller and primary Northbridge onto the CPU die. While this does eliminate the need for a memory bus controller and CPU to system controller as AMD wanted, when new memory standards hit, it's slow to change over.
With DDR-2 requiring a new socket and new pinouts, AMD will have to re-design its K8 CPU with a DDR-2 memory controller as well as a new socket, as extra pins will be required for the DDR-2 memory controller. When AMD swapped over from Single Channel memory in its 754 pin to Dual Channel Memory controller in the 939, the pin requirements jumped by 185 pins, the exact amount needed to add a second channel.
So if AMD wanted to have a Single Channel DDR-2 and Dual Channel DDR-2 solution in the K8, we would have to see a socket 808 to replace the 754 and a Socket 1048 to replace 939 which is simply too much cost to the consumer, especially with the already super exorbitant prices of AMD K8 on the 939 package.
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