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What is DDR, SDR and RDRAM and how do they differ?

SDR RAM is what you will be using in your Pentium II, III or older Athlon boards. SDR stands for single data rate. DDR stands for Double Data Rate and works (in theory) twice as fast as SDR RAM. Because a lot of the time a PC relies on the CPU's internal cache rather than system memory, the difference in performance between DDR and SDR is not close to 2x. DDR is a small evolution of SDR technology, it differs in that it is capable of transferring data on both the rising and falling edges of each clock cycle.

RDRAM commonly known as Rambus, is an entirely different design in memory technology and relies on extremely high operating frequencies for performance. To attain such high clock speeds, however, RDRAM features a slimmer bus than current SDRAM. As opposed to SDRAM’s 64-bit bus, RDRAM operates on a 16-bit (and now 32bit) bus. The fastest RDRAM is faster than any DDR in most applications. It does however suffer from a higher latency, and this is due to the slimmer bus requiring a slight serialization of the commands. RDRAM runs very hot, and thus needs to power down when not in use. In effect this means that only one chip on a RIMM will be actively sending data at one time. While one chip transmits, the others enter a ‘standby’ mode in which they run much cooler. So that the active chip does not burn out, the heat must be "spread" along the length of the module, hence RDRAMs requirements for heatspreaders. A drawback to this design is that each module, upon leaving ‘standby’ mode, must go through a power-up cycle which imposes a further latency penalty.

RDRAM is available only for the Intel Pentium 4 (and the Pentium III for a short time), where as SDR and DDR SDRAM is available for all current platforms.

Q. How is DDR memory speed calculated?

(Operating Speed) x (xx-bit Bus) x (1/8 bytes per bit).

So for 333MHzz DDR, 333 x 64 x 1/8 = 2700MB/s (Hence PC2700)

Note that this is the theoretical maximum bandwidth if all subsystems worked with 100% efficiency.

Q. My PC 3500 / PC3200 / PC2700 is not getting close to this speed Sandra in my Athlon based system:

A: Unlike in P4 systems, an asynchronous bus does little to increase performance in an Athlon based system. There is a simple explanation for this phenomenon, which is that the Athlon XPs archaic 266Mhz CPU bus (or 333MHz in the 2600+ and up) simply cannot accept any more information flowing through it. VIA could pump up the asynchronous bus to 133/500 and it would still remain close to KT266a speeds, because the CPU bus is stuck panting along at 2100MB/s. Your system is working as it should.

Q. So, what use is this fast memory for my athlon?

A: By unlocking the Athlon processor (join all the L1 bridges in a palomino, or the right-most L5 bridge on a thoroughbred) and upping the front side bus, you can see a substantial increase in performance, as the CPU bus can now match the faster memory bus. At 200Mhz you will start to see the 3000+kbs speeds you were expecting with the PC3200 you just bought!

Q. What do all of these numbers people in the forums keep mentioning, and how do they affect performance?


1) The most famous of these is the CAS Latency, which stands for 'Column Address Strobe'. It is important in determining how fast memory modules will respond to a request for data. It is normally called the CAS delay, and has a value of 2 or 3 in SDR SDRAM. For DDR RAM however, the typical values are 2 or 2.5. Some 845 boards allow a CAS of 1.5 to be selected but the performance increase is next to zero, despite a considerable reduction in system stability. These numbers correspond to a number of clock cycles. Lower is better, and in SDR SDRAM the difference between CAS 2 and 3 can affect performance considerably. In DDR however the difference is not so pronounced and is less important than others.

2) An important setting in most Athlon boards is the Bank Interleaving setting. Because you have to wait for all these latencies to occur before you can actually do anything with the RAM, it didn’t take long before there was more waiting going on than actually reading or writing. To get around this, memory is often interlaced. Consider the simplified memory cycle of “CAS -> CMD -> RAS -> output”. Imagine if while half your RAM is charging the column (in the CAS step), the other half is already charging the row (in the RAS step). As you could imagine, this will mean the output step will occur twice as often. The default for most RAM is no interlacing. To improve performance, set it to 2-way, or even 4way, and setting your ram to 4-bank will considerably improve performance.

3) Command Rate determines whether a chip select can be executed in a single clock or whether it needs two or more clocks. In DDR the CMD rate offers a bigger boost in performance than CAS latency, with 1T command being the best currently available.

4) RAS-to-CAS Delay (tRCD) is becoming an increasingly important limiting factor in determining overall memory performance. It can usually be set to 4, 3 or 2, with 2 being the fastest. Because RAS Precharge (tRP) is also set as 4, 3, or 2, the two settings are often grouped together or confused. However, the former setting has a significant impact on system performance where as the latter does, in general, not.

5) Bank cycle time (tRAS) is the number of cycles necessary to develop the full charge differential between bit and reference lines to restore the data in the memory cells. Lower is faster, with most motherboards having settings for 7, 6 and 5.

Q. Woah, woah, thats too technical. Just tell me what the common timings do and what 6-2-2 etc refers to!

CAS (2.5 or 2) = Lower gives significant performance increase
tRAS (7,6 or 5) = Gives very small performance increase
tRCD (2, 3 or 4) = Lower gives significant performance increase
tRP (2, 3 or 4) = Gives very small performance increase
CMD (1T, 2T or 3T) = Lower gives very significant performance increase
Bank Interleave (None, 2B or 4B) = Higher gives very significant performance increase.

Q. Is it safe to run my memory at over 2.8V, and will it increase overclockability?

A: Not really, 2.8V (2.9 if you are using VERY high quality brands) is the absolute maximum you should run your memory at if you want it to last a decent amount of time. Some makes such as GeIL advertise 3.1V or higher, but this is component suicide. Higher voltages give better overclockability up to a thermal limit.

Q. What is chip speed and how is it relevant

Chip speed (in ns) is the speed at which the wafer manufacturers (eg Samsung, winbond, nanya etc) rate their product to work at. You can work out the MHz by dividing 1000 by the speed in ns. EG 5ns RAM, as featured in XMS3500 is actually rated to 200MHz. 4.5ns RAM as featured in OCZ PC3500 EL-DDR is rated to 222MHz.

Q. Do heatspreaders make a difference, should I add them?

A: In RDRAM heatspreaders are absolute requirements, in DDR all chips do equivalent work when processing a command. Heatspreaders are little more than a cosmetic improvement.

Q. The fast 4:3 setting does not appear to be available on my Pentium 4 motherboard

A: Select HW strap to "low" and it should appear. Not all boards support this speed, especially if based on the 845PE chipset.

Q. My OCZ PC 3200 / Corsair XMS 3200 / Twinmos 3200 is not working well with my nforce board in dual channel mode.

A: No, this is a known issue and it is very difficult to optimise SPD timings for both single and dual channel operation. Use Corsair Twin X or OCZ Dual-channel optimised matching pair kits.

Q. How much memory do I need?

A: This depends entirely on how you use your PC. For smoother computing you should not run a windows XP based system with less than 256MB DDR. If you play a lot of high-end games, especially flight sims, 512MB is preferable. 1GB is required only for users who need to run a LOT of windows simultaneously including high-memory applications such as Photoshop, rendering packages and multiple explorer windows.

Q. Which memory brand should I buy?

A: This depends on your application. If you want the absolute best possible performance out of your system, either OCZ's EL-DDR or Corsairs LL XMS series is for you. If you are interested in a bit of overclocking but nothing to serious, or are on a limited budget, twinmos is excellent. If you don’t plan to overclock, Crucial is unbeatable.

Q. should I go RDRAM or DDR in my new P4 system?

A: DDR - definitely. There is very little benefit of choosing an RDRAM chipset over the latest granite bay. Intel has confirmed there will be no forthcoming RDRAM chipsets. It is a dieing technology (perhaps unfortunately).

Q12. Which chips are the best?

A: At time of writing, winbond have the best chips available, with Samsung’s newer smaller die chips being inferior to their previous "overclockers favourites"

Q. What speeds to the memory chips run at?

PC100 = 100MHz
PC133 = 133MHz

PC800 = 800MHz
PC1066 = 1066MHz
PC1200 = 1200MHz
PC4800 = 1200MHz (same as above)

PC1600 = 100MHz (200MHz effective)
PC2100 = 133MHz (266MHz effective)
PC2700 = 166MHz (333MHz effective)
PC3200 = 200MHz (400MHz effective)
PC3500 = 217MHz (433MHz effective)
PC3700 = 233MHz (466MHz effective)

Q. Isn't DDR SDRAM a contradiction?

No, SDRAM (Synchronous Dynamic Random Access Memory) should not be confused with SDR (single data rate). Both SDR and DDR are SDRAM.

Any more questions you think need adding to our memory FAQ? PM me!