The following guide describes what DDR2 you should be buying and how to get the best from it.
Before we begin to work out which DDR2 we need, it’s important to understand the difference between DDR and DDR2.
Firstly, it’s important to remember that DDR2, like DDR, continues to transfer data on both the rising and the falling edge of each clock cycle, so it effectively carries out twice as many operations per clock cycle (compared to SDRAM which only carries out one operation per cycle). Therefore, although the memory continues to run at the designated clock speed, the I/O bus effectively operates at twice the speed. For example, while the memory speed runs at is 200MHz the effective speed is doubled at 400MHz. This method is known as Double Pumping and DDR (Double Data Rate) gets its name from this method.
So where is DDR2 different from DDR? Well the main difference is that DDR2 modules have half the core memory speed of standard DDR modules. However, the data is transferred across the bus at twice the speed whereas standard DDR core memory and transfer bus speeds operate at the same speed.
This is done by increasing the width of the memory’s prefetch buffer to 4 bits wide as opposed to 2 bits wide of DDR. The net result produces the same bandwidth but since DDR2 operates with a lower clock speed and therefore less power consumption, there is plenty of headroom for greater bandwidths. The offset for this is increased latency timings. The forthcoming DDR3 will have a memory prefetch of 8 bits wide, allowing further increased throughput.
Identifying DDR2 RAM
RAM is normally identified in two ways and either is correct:
DDR2-xxxx is the standard name where xxxx denoted the quoted transfer speed (effective)
PC2-xxxx is the module name where xxxx denoted the theoretical bandwidth which is rounded up or down.
So, for example PC2-3200 is sometimes referred to as DDR2-400 and visa versa.
The following table shows the available DDR2 models with their names, speeds and their bandwidths:
Understanding the Bandwidth
When you hear RAM being referred to as PC5300 for example, the numbers actually refer to its theoretical maximum bandwidth which it uses to communicate with the north bridge and the rest of the system.
To calculate the bandwidth: Bandwidth (GB/s) = FSB frequency x DDR x bpcs
FSB frequency = bus clock speed (not the effective speed) in MHz
DDR = this value is always 2 as it refers to double pumping (as described above).
Bpcs = Bytes Per Clock Cycle. The data is transferred on a data that’s 64 bits wide and since there’s 8 bits for each byte so the bytes per clock cycle equates to 8 bytes per transfer.
For example: A bus with a speed of 266MHz requires memory to run at 533MHz because it doubles the data rate and therefore requires DDR2-533 (266MHz x 2). DDR2-533 runs at a frequency of 4200MHz (533MHZ x 8 bits). Thus DDR2-533 is also referred to its bandwidth. In this case PC2-4200.
233 x 2 x 8 = bandwidth
In dual channel mode, the bandwidth is theoretical doubled. So, our PC4200 for example, would have an operating peak bandwidth of 8.52GB/s.
Here are the following dual channel peak bandwidths:
PC2-3200 = 6.40 GB/s
PC2-4200 = 8.52 GB/s
PC2-5300 = 10.67 GB/s
PC2-6400 = 12.80 GB/s
PC2-8500 = 17.00 GB/s
Selecting the right DDR2 RAM
The basic concept of choosing DDR2 remains the same as DDR in that the minimum speed of the RAM you choose must match that of the CPU FSB otherwise known as synchronous operation. So you’ll have to know what speed your CPU operates at in MHz before you think about purchasing memory.
Beware though; you may see some CPUs quoted in effective FSB speeds. Do not confuse the effective FSB of the CPU with the effective FSB of the system. They are not the same thing. For example, an Intel Core 2 Dual processor (C2D) may operate with an FSB of 266MHz, but since these CPUs are Quad pumped they have an effective FSB speed of 1066Mhz (266 x 4 rounded up). The effective speed of the CPU has nothing to do with the effective RAM or system speed. However, the speed of the CPU in GHz is still the product of the FSB x CPU multiplier. So a 2.4GHz E6600 CPU is based on 267MHz x 9.
Anyway, when matching the correct DDR2 to your CPU, you need to choose RAM that operates to at least the CPU bus speed. In the example above, you’ll need RAM that operates to a minimum bus speed of 266MHz. Therefore, to run a CPU that has an FSB of 266MHz (1066MHz effective), then DDR2-533 (PC2-4200) would be the minimum RAM requirement. This is because DDR2-533 operates at 266MHz. (see above table) remembering that DDR (or DDR2) carries out two operation per clock cycle, therefore the effective speed is doubled from 266MHz to 533MHz.
You may have notice above I said minimum; this is because most systems today will allow memory to run faster than bus speed which means you can install faster memory beyond minimum requirements for a decent performance boost. This is called Asynchronous operation. The speed of the RAM is still determined by the bus speed but uses a memory multiplier or ratio to increase the memory speed without changing the bus speed (although there are valid reason for doing this too – we’ll come to that later). Therefore, choosing faster memory for your system isn’t a bad idea because you’ll want to run your system as fast as possible. It also provides some headroom to change your CPU bus speed.
However, for default “out of the box” CPU/RAM combinations; you will need the minimum RAM requirements as described for successful operation.
You can use the table earlier to select your minimum DDR2 RAM requirements. Just by knowing your CPU FSB speed and match it against the Bus Clock column but bear in mind you may want that extra performance from your system.
Overclocking DDR2 RAM
If you read the above, you will have noticed that I mentioned minimum RAM requirements. This is because your system will allow asynchronous RAM/FSB operating speeds. This is to say that the RAM does not have to run at the same speed as the FSB. We can do this using a FSB/RAM divider set in the BIOS to alter the relationship between the FSN and RAM but this needs some careful planning and foresight, so be careful.
So when looking at what we've mentioned so far, for the RAM/FSB to be running at the same speed (i.e. synchronous) then the RAM divider would be set to a ratio of 1:1. For example, if the FSB is running at 266MHz (533MHz effective) then by setting the RAM divider to 1:1 the RAM would also run at 266MHz (533MHz effective).
Let’s say however, you have a system that has an FSB of 266MHz running with PC2-5400 (DDR2-677). By changing the divider to 5:6, would force the RAM to run at 320MHz as per the calculation below:
266MHz / 5 * 6 = 320MHz (640MHz effective)
This is within the capabilities of DDR2-677
By the same example, I could then attempt to overclock the RAM by changing this divider further as below:
266MHz / 4 * 6 = 400 MHz (800MHz effective)
In succeeding (although unlikely) in overclocking the RAM to 400MHz (DDR2-800 speeds) then this would have been achieved without actually altering the FSB and therefore the CPU speed at all.
Of course, it’s far easier to buy faster RAM which exceeds the minimal FSB requirements in the first place as this allows you to increase the FSB (thereby increasing the CPU speed) whilst keeping the RAM at the same speed (synchronous) This is why some RAM is known as “overclockers” RAM simply because it’s used by enthusiasts who intend to overclock the FSB and CPU speeds without changing the divider.
The basic concept of choosing DDR2 RAM remains the same as DDR in that the speed of the RAM must operate that of the CPU FSB which is otherwise known as synchronous operation. So you’ll have to know what speed your CPU operates at in MHz first.
Beware though; you may see some CPUs quoted in effective FSB speeds. Do not confuse the effective FSB of the CPU with the effective FSB of the system. They are not the same thing. For example, an Intel Core 2 Dual processor (C2D) may operate with an FSB of 266MHz, but since these CPUs are Quad pumped they have an effective FSB speed of 1066Mhz (266 x 4 rounded up). The effective speed of the CPU has nothing to do with the effective RAM or system speed. However, the speed of the CPU in GHz is still the product of the FSB x multiplier. So a 2.4GHz E6600 CPU is based on 267MHz x 9.
Anyway, when matching the correct DDR2 to your CPU, you need to choose RAM that operates with a bus speed that matches that of the CPU bus speed. In the example above, you will need RAM that operates at an actual bus speed of 266MHz. Therefore, to run a CPU that has an FSB of 266MHz (1066MHz effective), then DDR2-533 (PC2-4200) would be the minimum RAM requirement. This is because DDR2-533 operates at 266MHz. (see above table) remembering that DDR (or DDR2) carries out two operation per clock cycle, therefore the effective speed is doubled from 266MHz to 533MHz.
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