
It's a fact that modern systems need just scads of memory - 512 MB at least, although 1 GB isn't exactly rare anymore, either. Things get a bit more complicated once you stroll down to your local computer store, where they have a huge selection of DDR400 RAM modules from innumerable vendors in all imaginable variations. So what should you look out for? Should you really listen to what the friendly salesperson has to say?
In any discussion of RAM, somebody is bound to drop the term "CAS latency", or CL for short. But there are a slew of other factors that also affect how fast your RAM is. In this article, we'll take a closer look at these factors and explain the concepts behind the cryptic numbers given to the different modules.
Then we'll move along to the real purpose of this article - determining how a given system will perform on best-case, average and worst-case memory timings. We ran 19 individual benchmarks on all the available platforms (Athlon XP, Athlon 64, Athlon 64 FX, Pentium 4, Pentium 4 EE) in order to get you the dirt on the timings.
State-of-the-art RAM modules generally transfer data in 64 bit chunks. They contain DRAM chips that send data synchronously with the clock pulse signal and generally use the double-data-rate method (DDR). The difference between DDR and SDR-SDRAMs is that the DDR modules transfer data during both the rising and the falling edges of the clock pulse. That means that DDR400 RAM really only sends data at 200 MHz using the DDR method.
A better measure of memory speed is the module's cycle time, which is the amount of time needed to complete one clock cycle. A cycle time of 10 ns means that 100 million cycles are possible per second, and the chips run at up to 100 MHz. To reach 133 MHz, you need 7.5 ns; for 166 MHz, 6.0 ns.
| Cycle Time T | Max. Frequency f | Bandwidth SDR* | Bandwidth DDR* |
|---|---|---|---|
| 10 ns | 100 MHz | 800 MB/s (PC100) | 1,600 MB/s (DDR200) |
| 7.5 ns | 133 MHz | 1,064 MB/s (PC133) | 2,100 MB/s (DDR266) |
| 6 ns | 166 MHz | - | 2,700 MB/s (DDR333) |
| 5 ns | 200 MHz | - | 3,200 MB/s (DDR400) |
* Here's how to calculate bandwidth: frequency x interface width (64 bits are 8 Bytes). DDR RAM offers twice the transfer rate of SDR RAM.
Nomenclature: RAM Names
The name game was a lot easier with conventional SDR-SDRAM, which was simply named for the clock speed (PC100, PC133 SDRAM). The rules changed with the advent of DDR RAM. The modules are now titled using the maximum bandwidth (in MB/s). So PC2100 is DDR266, PC2700 is DDR 333, etc. This sea change was based on the nomenclature used for Rambus DRAM (RDRAM), whose names - PC800 or PC1066 - were also derived from their frequency. The following table provides more information.
| Name | TypeName: | Effective Clock Speed | Data Bus | Bandwidth |
|---|---|---|---|---|
| PC66 | SDRAM | 66 MHz | 64 Bit | 0.5 GB/s |
| PC100 | SDRAM | 100 MHz | 64 Bit | 0.8 GB/s |
| PC133 | SDRAM | 133 MHz | 64 Bit | 1.06 GB/s |
| PC1600 | DDR200 | 100 MHz | 64 Bit | 1.6 GB/s |
| PC1600 | Dual-DDR200 | 100 MHz | 2 x 64 Bit | 3.2 GB/s |
| PC2100 | DDR266 | 133 MHz | 64 Bit | 2.1 GB/s |
| PC2100 | Dual-DDR266 | 133 MHz | 2 x 64 Bit | 4.2 GB/s |
| PC2700 | DDR333 | 166 MHz | 64 Bit | 2.7 GB/s |
| PC2700 | Dual-DDR333 | 166 MHz | 2 x 64 Bit | 5.4 GB/s |
| PC3200 | DDR400 | 200 MHz | 64 Bit | 3.2 GB/s |
| PC3200 | Dual-DDR400 | 200 MHz | 2x 64 Bit | 6.4 GB/s |
| PC4200 | DDR533 | 266 MHz | 64 Bit | 4.2 GB/s |
| PC4200 | Dual-DDR533 | 266 MHz | 2 x 64 Bit | 8.4 GB/s |
| PC800 | RDRAM Dual | 400 MHz | 2 x 16 Bit | 3.2 GB/s |
| PC1066 | RDRAM Dual | 533 MHz | 2 x 16 Bit | 4.2 GB/s |
| PC1200 | RDRAM Dual | 600 MHz | 2 x 16 Bit | 4.8 GB/s |
| PC800 | RDRAM Dual | 400 MHz | 2 x 32 Bit | 6.4 GB/s |
| PC1066 | RDRAM Dual | 533 MHz | 2 x 32 Bit | 8.4 GB/s |
Information is stored by first separating the memory area into rows and columns. The capacity of the individual chips determines the number of rows and columns per module. When several arrays are combined, they create memory banks.
The chips are actually accessed by means of control signals such as row address strobe (RAS), column address strobe (CAS), write enable (WE), chip select (CS) and several additional commands (DQ). You also need to know something about which row is active in the memory matrix at any given moment.
In today's computers, a command rate is defined in BIOS - generally 1-2 cycles. This describes the amount of time it takes for the RAS to be executed after the memory chip has been selected.
The memory controller selects the active row. But before the row will actually become active so that the columns can be accessed, the controller has to wait for 2-3 cycles - tRCD (RAS-to-CAS delay). Then it sends the actual read command, which is also followed by a delay - the CAS latency. For DDR RAM, CAS latency is 2, 2.5 or 3 cycles. Once this time has lapsed, the data will be sent to the DQ pins. After the data has been retrieved, the controller has to deactivate the row again, which is done within tRP (RAS precharge time).
There is one more technical restriction - tRAS (active-to-precharge delay). This is the fewest number of cycles that a row has to be active before it can be deactivated again. 5-8 cycles are about average for tRAS.
Memory timings are generally cited in order of importance:

| Intel Processors (Socket 478) | |
|---|---|
| 200 MHz FSB (Dual DDR400) | Pentium 4 3.20 GHz Extreme Edition
(3200 MHz 12-8/512/2048 kB) |
| 200 MHz FSB (Dual DDR400) | Pentium 4 3.20 GHz (3200 MHz 12-8/512 kB) |
| AMD Processors (Socket A) | |
| 200 MHz FSB (DUAL DDR400) | Athlon XP 3200+ (2200 MHz 128/512 kB) |
| AMD Processors (Socket 940) | |
| 200 MHz FSB (DUAL DDR400) | Athlon FX-51 (2200 MHz 128/1024 kB) |
| AMD Processors (Socket 754) | |
| 200 MHz FSB (SINGLE DDR400) | Athlon 64 3200+ (2000 128/1024 kB) |
| Memory | |
| DDR400 (200 MHz) | Corsair TWINX PC3500
2x 512 MB CL 2.0-3-2-6 (Socket 754 and Socket A) 4x 256 MB PC3500, CL 2.5-2-2-5 (Socket 478) |
| DDR400 (200 MHz) | Legacy Electronics Registered ECC PC3200
2 x 512 MB CL 2.0-3-2-6 (88S6HDAR-1TDG 5733) Infineon Chips |
| Motherboard | |
| Intel 875
(Socket 478) |
Asus P4C800-E Deluxe Rev : 1.02
Bios : 1011 BETA 006 4 x 256 MB Corsair TWINX PC3500 |
| NVIDIA nForce 2 Ultra
(Socket A) |
Asus A7N8X Rev : 2.00
Bios : 1006 2x 512 MB Corsair TWINX PC3200 |
| VIA K8T800
(Socket 940) |
MSI K8T Master 1-FAR (MS-9130)
Bios : 1.0Bd Rev : 1 2 x 512 MB Legacy Electronics (Reg. DDR) |
| VIA K8T800
(Socket 754) |
MSI 8KT Neo (MS-6702)
Bios : 1.0 Rev : 1.0 2 x 512 MB Corsair TWINX PC3200 |
| Common Hardware | |
| Sound Card | Terratec Aureon 7.1 Space
96.00 kHz sample rate |
| Graphics Card | MSI FX5900U-VTD256
GPU : NVIDIA GeForce FX 5900 Ultra Memory : 128 MB DDR-SDRAM Memory Clock : 400 MHz DDR Chip Clock : 450 MHz |
| Hard Drive (AMD System) | FastTrak S150 TX2plus (Bios : 1.00.0.30)
2 x SATA Maxtor 6Y080M0 (Raid 0) 80 GB / 8 MB Cache / 7200 rpm |
| Hard Drive (Intel System) | Intel FW82801ER ICH5R
2 x SATA Maxtor 6Y080M0 (Raid 0) 80 GB / 8 MB Cache / 7200 rpm |
| DVD/CD-ROM | MSI MS-8216 16x DVD |
| Network (Intel - i875) | Intel 82547ET (CSA) |
| Network (AMD - nForce 2) | NVIDIA nForce MCP Networking Controller |
| Network (AMD - VIA 8KT800) | Broadcom BCM5705KFB |
| Software | |
| Intel Chipset | V 5.00.1012
Intel Application Accelerator RAID Ed. V 3.51 |
| NVIDIA Graphics Driver | Detonator V 53.03 WHQL |
| VIA K8T800 | Hyperion 4in1 Ver. 4.51 |
| DirectX | 9.0b |
| OS | Windows XP, Build 2600 SP1 |
While it’s next to impossible to find one single RAM module that will work on all four platforms, that won’t really affect our test results. We weren’t aiming to compare different platforms, but to see what kind of an impact best-case and worst-case timings would have on performance.
We used the fastest-possible memory configuration for each platform. For the Pentium 4 and Pentium 4 Extreme Edition on the Asus P4C800-E (Intel 875P), we used four 256 MB DIMMs from Corsair because they allowed us to set the fastest-possible timings of CL2.0-2-2-5.
This was not possible with two 512 MB DIMMs because the fastest ones we could find - also from Corsair - supported "a mere" CL2.0-3-2-6.
That means that we had to test all the other platforms because the Athlon 64 FX and the Athlon 64 would only run stably with four DIMMs and much slower timings, while the nForce2 board from Asus only ships with three DIMM slots. For the Athlon 64 FX, we swapped out the Corsair RAM for a matched pair of Legacy Electronics modules.
| System | Platform | RAM Used | Reason Why |
|---|---|---|---|
| AMD Athlon XP | nVIDIA nForce2 Ultra 400 | 2x 512 MB Corsair TWINX, CL2.0-3-2-6 | No faster 512 MB DIMMs available |
| AMD Athlon 64 | VIA K8T800 | 2x 512 MB Corsair TWINX, CL2.0-3-2-6 | No faster 512 MB DIMMS available, 4x 256 MB only possible with slow timings |
| AMD Athlon 64 FX | VIA K8T800 | 2x 512 MB Legacy Electronics, CL2.0-3-2-6 | The fastest-possible RAM since no faster 512 MB DIMMs were available |
| Intel Pentium 4 | Intel 875P | 4x 256 MB Corsair TWINX CL2.0-2-2-5 | Fastest-possible RAM |
| Intel Pentium 4 EE | Intel 875P | 4x 256 MB Corsair TWINX CL2.0-2-2-5 | Fastest-possible RAM |
| Best Case | Average | Worst Case | |
| AMD Athlon XP | 2.0-3-2-6 | 2.5-3-3-6 | 3.0-4-4-8 |
| AMD Athlon 64 | 2.0-3-2-6 | 2.5-3-3-6 | 3.0-4-4-8 |
| AMD Athlon 64 FX | 2.0-3-2-6 | 2.5-3-2-6 | 3.0-3-2-6 |
| Intel Pentium 4 | 2.0-2-2-5 | 2.5-3-3-6 | 3.0-4-4-8 |
| Intel Pentium 4 EE | 2.0-2-2-5 | 2.5-3-3-6 | 3.0-4-4-8 |
OpenGL Games: Quake 3


Wolfenstein Enemy Territory


Warcraft III - The Frozen Thorne



AquaMark3


Xmpeg & Divx 5.1


Winrar 3.2

3D Studio Max 5.1






In most of the disciplines, you can see that it no longer matters as much what memory timings you have as it did only a few years ago, when SDRAM or the first DDR generation were still hot. Or, to put it another way, having faster or slower RAM will not tip the balance in favor of or against the latest AMD and Intel processors.
We observed one interesting result in many of the gaming benchmarks: while the Pentium 4 3.2 GHz is normally just a touch faster than the Athlon 64 3200+, it quickly falls behind the Athlon if you only use slow memory modules.
Things start getting untidy when you combine compute-intensive tasks with large quantities of data such as file compression. In such categories, the memory timings make or break performance - the Pentium 4 processors either take the lead or bring up the rear, depending on whether the memory timings are fast or slow. We were duly impressed by the Athlon 64 FX-51's scores, which maintained its ranking no matter what kind of memory it was given. This steadfastness is largely due to the integrated memory controller.
The moral of the story is clear: while we still recommend buying brand-name products to ensure compatibility (especially for dual-channel systems), but you don't necessarily need the fastest timings. In today's market, you only need fast modules if your computer will be computing a lot or encoding video. For any other application, slower RAM will definitely cut the mustard.