Random Access   chris, kp & rob
In The Lab
Build Your Own PC Chapter 3: The Final Pieces
by chris & rob

Chris: After last month's product grab, it's about time to start putting the pieces together. None of the hardware boxes are open yet, but I can tell there are going to be some empty sockets inside this system, ones that are kind of important. Although we talked about two items in the first installment of this project, we did not go so far as to collect either the processor or memory for our new system.

Rob: What do we have to consider when purchasing a processor? Can't we just get any processor, heat sink and fan and slap it in?

Chris: Oh no Rob, that would be inviting disaster. The DFI LanParty system board is an Intel based motherboard, and we talked about getting a 3.2GHz P4 CPU with Hyper-Threading technology. By getting the Intel CPU kit we can be sure that we have a heat sink and fan that is designed to work with these high performance processors. On faster processors heat is a real issue, both from the performance and the operation of the system. If you choose to replace the Processor Heat Sink and Fan assembly with a showy LED-illuminated or speed-controlled one, make sure it is designed for the CPU in your system. For example, a Thermaltake brand heat sink and fan with speed control specifically indicates that the unit was designed for "Intel P4 478 FMB2, 3GHz and higher." Heat Sinks for AMD and Intel processors are not the same, and generally are not interchangeable between system boards. For improved air flow through the case, and specifically across the processor, system chassis design has started to add side fans or at least an additional air intake on the side of the case near the CPU position.

Rob: I didn't know there was that much involved in matching a system board to a processor. But I know what socket we'll need to fill next. Memory.

Chris: That's right. The board requires DDR (Double Data Rate) SDRAM modules capable of running at 400MHz. RAM timing is critical with almost all of the high-speed Intel and AMD compatible chipsets, and when you have intermittent start-up problems, reboots, or other strange behavior, memory should be one of the first things to check. Memory labeled as DDR-400 is only part of what to check before purchasing. One of the other specifications you should find for the memory is the Clock or Latency value. For example, DDR-400 memory "Clocks" might be specified as CL2, CL2.5 or CL3. Latency may also be identified as CAS, JEDEC, or as a series of numbers in the product description such as "3-4-4-8". Of this group of numbers, the first is the bus transaction speed of the memory in CPU clock cycles. (The next three numbers identify the clock cycles for burst cache fill.) This provides an indication of how fast the memory can respond to an access request by the CPU in terms of clock cycles. Basically, if the CPU requires information that has not been loaded into its cache memory, then a request must be made to the system memory where the data is stored. Once the request has been made, the CPU then must wait to receive the data before continuing processing. Latency is measured by how many clock cycles the CPU must wait before receiving the information.

One of the other features of the system board is the Dual Channel memory support. For the best performance in Dual Channel systems, memory must be installed in matched pairs across the channels. We carry both Centon and Corsair memory in matched expansion kits just for this reason. If you look at the motherboard image, you can see that there are four memory slots organized as two and two. With DDR memory and a P4 system board, each slot is a bank. Each pair of slots represents Bank0 and Bank1 for each of the two memory channels. For the maximum performance, the same type of RAM must be installed in Bank0 of Channel-A and Bank0 of Channel-B. (If you fill both slots next to each other, you are only installing memory in one of the two available channels.) Memory access speed will be effectively 800MHz if memory of the same size and speed is installed in Bank0 of Channel-A and Bank0 of Channel-B. It drops to at least 400MHz if installed in Bank0 and Bank1 of one of the Channels. In some systems, the memory access speed may be throttled to 333MHz if any of the Channels, Banks or memory module size or speeds are different. Check both the CPU and the system board specifications for any minimum Clock requirements or for compatible memory listings. With an Intel P4 CPU running at 3GHz or faster, we will probably need CL2 compatible memory.

Next month we start putting all the pieces together... but not before we take the correct precautions.

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