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Understanding PC2700 (DDR333) and PC3200 (DDR400) Memory

  • Dec 1, 2002
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Understanding PC2700 (DDR333) and PC3200 (DDR400) Memory

DDR (double data rate) SDRAM memory has replaced regular SDR (single data rate) SDRAM memory as the mainstream RAM for most current PC systems. DDR memory performs two operations per clock cycle, unlike regular SDRAM, which performs only one operation per cycle. As processors get faster and as their CPU bus (also called FSB for front-side bus) also speeds up, a faster memory bus is needed to keep pace. PC2700 (known as DDR333 in chip form) memory is the latest member of the DDR family, with PC3200 (known as DDR400 in chip form) and even faster speeds being proposed for the future.

Another form of memory used in the PC, called RDRAM (Rambus DRAM) also has several planned improvements over the next few years which will allow it to keep pace with faster processors as well. RDRAM is generally faster than DDR, however it is also more expensive and support is currently limited to specific Pentium 4 based chipsets (memory controllers).

In this article, I'll focus on the more popular DDR memory. Youll learn how the new faster DDR memory types differ from the older and slower versions of DDR memory, how they compare to RDRAM memory, and how to make sure youre buying the best memory modules for your needs.

Comparing Memory Speeds

DDR memory modules are named in reference to their throughput in MB/sec, while the raw chips are named in reference to their effective clock speed in MHz. This means that a module using DDR333 chips will run at 166.66 MHz, perform 2 transfers per cycle (for an effective clock speed of 333.33 MHz) and transfer 8 bytes at a time. Thus the throughput would be calculated as:

166.66 MHz * 2 transfers per cycle * 8 Bytes = 2666 MB/sec

The following table details and compares the speeds of past and future SDRAM, DDR SDRAM and RDRAM memory standards.

Module Standard

Module Format

Chip Type

Clock Speed (MHz)

Cycles per Clock

Bus Speed (MT/s)

Bus Width (Bytes)

Transfer Rate (MB/s)

PC66

SDR DIMM

10ns

67

1

66

8

533

PC100

SDR DIMM

8ns

100

1

100

8

800

PC133

SDR DIMM

7.5ns

133

1

133

8

1,066

PC1600

DDR DIMM

DDR200

100

2

200

8

1,600

PC2100

DDR DIMM

DDR266

133

2

266

8

2,133

PC2400

DDR DIMM

DDR300

150

2

300

8

2,400

PC2700

DDR DIMM

DDR333

167

2

333

8

2,666

PC3000

DDR DIMM

DDR366

183

2

366

8

2,933

PC3200

DDR DIMM

DDR400

200

2

400

8

3,200

PC3600

DDR DIMM

DDR444

222

2

444

8

3,555

PC4000

DDR DIMM

DDR500

250

2

500

8

4,000

PC4300

DDR DIMM

DDR533

267

2

533

8

4,266

RIMM1200

RIMM-16

PC600

300

2

600

2

1,200

RIMM1400

RIMM-16

PC700

350

2

700

2

1,400

RIMM1600

RIMM-16

PC800

400

2

800

2

1,600

RIMM2100

RIMM-16

PC1066

533

2

1,066

2

2,133

RIMM2400

RIMM-16

PC1200

600

2

1,200

2

2,400

RIMM1200

RIMM-16DC

PC600

300

2

600

2

2,400

RIMM1400

RIMM-16DC

PC700

350

2

700

2

2,800

RIMM1600

RIMM-16DC

PC800

400

2

800

2

3,200

RIMM2100

RIMM-16DC

PC1066

533

2

1,066

2

4,266

RIMM2400

RIMM-16DC

PC1200

600

2

1,200

2

4,800

RIMM3200

RIMM-32

PC800

400

2

800

4

3,200

RIMM4200

RIMM-32

PC1066

533

2

1,066

4

4,266

RIMM4800

RIMM-32

PC1200

600

2

1,200

4

4,800

RIMM6400

RIMM-64

PC800

400

2

800

8

6,400

RIMM8500

RIMM-64

PC1066

533

2

1,066

8

8,533

RIMM9600

RIMM-64

PC1200

600

2

1,200

8

9,600

Note: all information listed in italics refers to future proposed standards that are not yet validated, certified, or commercially available.

  • MT/s = Mega Transfers per second
  • MB/s = Megabytes per second
  • ns = Nanoseconds (billionths of a second)
  • SDR = Single Data Rate
  • DDR = Double Data Rate
  • SIMM = Single Inline Memory Module
  • DIMM = Dual Inline Memory Module
  • RIMM = Rambus Inline Memory Module
  • DDR = Double Data Rate

Note that there are essentially two ways to refer to a given type of memory, one way refers to the raw memory chips (for example DDR333), while the other refers to the assembled modules using the chips (for example PC2700). Note that even though modules using DDR333 chips have a bandwidth of 2666 MB/sec, the modules are called PC2700 instead.

As you can see from the table, individual PC2700 (DDR333) modules are faster (greater throughput) than individual RIMM1600 (PC800) modules, however RIMM-16 modules are normally used in dual-channel (RIMM-16DC) setups where two modules are accessed simultaneously, thus doubling the effective bandwidth. Therefore RIMM1600 modules run in a dual-channel memory bus would have 3,200 MB/sec bandwidth. This means that the effective bandwidth of dual-channel RIMM1600 (PC800) memory is 1600*2 or 3,200 MB/sec, which is somewhat faster than PC2700 memory at 2,666 MB/sec.

Future proposed PC3200 (DDR400) memory on the other hand will have exactly the same memory bandwidth as dual-channel RIMM1600 (PC800) RAMBUS memory. This speed will make PC3200 a possible alternative to RIMM1600 dual-channel memory for Intel-based systems, and will enable future Athlon-based systems to enjoy faster memory performance as well. All this is provided of course that the PC3200 standard is certified and chipsets materialize which will allow their use.

The fastest PC memory coming are the new RIMM4200 (PC1066) modules. As of yet there are no chipsets released which support this memory.

PC2100 (DDR266) memory is the most common memory type used by Athlon and Athlon XP processors (all of which have up to 266MHz CPU bus speeds), as well as by Intel Pentium 4 processors running on DDR-compatible motherboards. However, users who want to overclock their systems or those with the most recent Pentium 4 DDR chipsets may want to consider PC2700 memory. This is because the current Athlon CPU bus is limited to 2,133 MB/sec, while the Pentium 4 bus runs at either 3,200 MB/sec or 4,266 MB/sec. This means that technically only the Pentium 4 can properly make use of current PC2700 or forthcoming PC3200 modules. If your computer has one of the latest VIA Technology chipsets, the KT333, KT400, and P4X400, PC2700 (DDR333) memory may be important for your consideration if you want to get the best performance from your system.

Factors Accounting for Faster Memory Speed

There are three factors that enable faster memory performance:

  • CPU bus speed
  • Memory bus speed
  • CAS (Column Address Strobe) Latency (CL) ratings

CPU bus speed (also called FSB or front-side bus) indicates how fast the CPU communicates with the motherboard, including especially the memory controller (part of the motherboard chipset) on the board. Memory bus speed indicates how fast the memory modules can communicate with the memory controller. All data moving from the CPU to memory and vice versa must go through the memory controller, which means it must flow over both the CPU bus and the memory bus. Therefore we have a situation where the lowest common denominator rules, meaning the slower of the two buses dictates maximum performance.

The most ideal situation is therefore when the memory bus runs at the same bandwidth (throughput speed) as the CPU bus. This means there are no bottlenecks in the system that can hurt performance. For example, if you are using an Athlon processor with a 266 MHz (2,133 MB/sec bandwidth) CPU bus, then the best memory for that system would be PC2100 DDR, since it runs at exactly the same speed. Using faster PC2700 memory would not improve the transfer of data between the memory and the CPU, since all data going to the CPU can only move at 2,133 MB/sec maximum anyway. In fact in many cases using the faster PC2700 memory may even slow things down since it will not be running in step with the CPU bus. Of course, running memory with a bandwidth slower than the CPU bus is not good either, as that will definitely hurt performance.

The CPU bus speeds of the Pentium 4 and Athlon processors are as follows:

Bus Type

Bus Width (bytes)

Bus Speed (MHz)

Bandwidth (MB/sec)

200MHz Athlon/Duron FSB

8

200

1,600

266MHz Athlon/Duron FSB

8

266

2,133

400MHz Pentium 4/Celeron FSB

8

400

3,200

533MHz Pentium 4 FSB

8

533

4,266

If you compare these CPU bus speeds to the memory bus speeds, you'll see that in general, PC1600 or PC2100 DDR SDRAM matches up best with the slower Athlon/Duron CPU bus, while dual-channel RIMM1600 or RIMM2100 or RIMM4200 RDRAM matches up best with the faster Pentium 4/Celeron CPU bus. There are some other matches, for example the proposed PC3200 (DDR400) memory will work with the slower 400 MHz Pentium 4 bus as well. In general, you are best when the speed (meaning throughput or bandwidth and not MHz) of the CPU bus matches the memory bus.

Once you have matched the speed of memory to that of your CPU bus (or it is as close as possible), then you can consider the third parameter, CAS latency. This specification measures the delay in clock cycles between when a Read command is given and when the data is ready to be read, and determines how quickly the processor can perform repetitive memory read operations.

The standard CAS Latency value for PC2700 modules is 2.5, meaning that 2.5 clock cycles are required between the Read command and the availability of data.

If you plan to run your system at its standard speed (no overclocking), any of the top-quality memory brands on the market will work in a satisfactory manner: I prefer Micron memory sold by Crucial.com. However, if you plan to overclock your system by increasing the FSB speed or want to use more aggressive memory timing settings in the system BIOS, you need to look at DDR modules which go beyond the basics.

How CAS Latency 2-Rated Memory Can Benefit You

Generally, its assumed that the faster the memory bandwidth, the faster the memory performance will be. In truth, the speed of the memory module can be misleading. This is especially true if you compare PC2100 modules using the faster CAS Latency rating of 2 with PC2700 modules using the standard CAS Latency rating of 2.5. Because CAS 2 modules require fewer clock cycles to provide data than CAS2.5-rated modules, in some cases a PC2100 system with CAS2-rated memory can outperform a PC2700 system with standard CAS2.5 memory according to Toms Hardware Guides comparison of PC2700 memory modules:

http://www17.tomshardware.com/mainboard/02q1/020220/kt333-08.html

Some CAS2-rated memory modules also allow aggressive users to tweak the memory timings in the system for even faster access than with standard settings. The ability to make memory timing adjustments depends primarily on the motherboard and BIOS Setup implementation, as well as the chipset. Intel brand motherboards (used by Dell, Gateway, and others) for example, have no provision for memory timing adjustments in their BIOS Setup, but third-party boards frequently allow such adjustments.

New Designs to Fight Heat Buildup

Memory, like all other electronic components, can get hot during operation, and heat can cause system lockups and unreliable operation. While most PC2700 modules still use the same surface-mount design that goes back to the 72-pin SIMM modules of the mid-1990s, some vendors have adopted new designs to minimize heat buildup to enable better overclocking and more stability.

One method is the addition of a heat spreader to the outside of the module. This is the approach taken by Corsair Memory in its overclock-friendly Extreme Memory Speed (XMS) series of memory modules for DDR333 and DDR400 speeds. Note that the PC3200 (DDR400) modules they offer are programmed to report they are PC2700 since DDR400 is not yet fully standardized. Most of these modules are also designed to handle CAS 2 timings. Another method is the use of smaller memory chips mounted using ball grid array (BGA) solder balls instead of normal surface-mount wire traces. This is the approach taken by KingMax for its PC2700 modules. Both of these approaches seem to work well in reducing heat buildup.

Whats the Optimum Number of Memory Modules to Install?

While many systems using DDR memory modules have three or even four memory sockets, recent tests performed by Anandtech, at http://www.anandtech.com/mb/showdoc.html?i=1636&p=20 indicate that many systems have problems running DDR333 memory reliably when more than two memory sockets are in use. Since you can buy PC2700 memory in sizes up to 512MB, using only two sockets isnt a big limitation for most users.

Even if you need only 512MB of RAM, it might be better to buy two 256MB modules (or move up to 1GB of RAM by buying two 512MB modules). Having two memory modules installed may allow some chipsets to interleave the memory banks, alternating between them for faster access.

Conclusion

PC2700 (DDR333) and PC3200 (DDR400) are faster versions of previous DDR standards such as PC1600 (DDR200) and PC2100 (DDR266). They differ in the speed of the memory chips they use and some also differ in the CAS Latency settings they provide. Although not yet fully supported by most motherboards, particularly if you want to overclock your system, you may be interested in these faster rated modules.

For Further Research

Tracking Down the Fastest PC2700 Memory Modules

If youre interested in overclocking memory or fine-tuning BIOS memory access settings, youll find a wide variety of online reviews that can help you choose the right memory and configure your system for maximum performance:

The VR Zone tested PC2700 modules made by Micron, Kingston, TwinMOS, Nanya, WinBond, and KingMax for both high FSB speeds and aggressive memory timings. See the results at: http://www.vr-zone.com/reviews/Memory/DDR333/

VIAarena matched Corsair XMS and KingMax memory against each other and also provides useful tips on BIOS memory setting configuration: http://www.viaarena.com/?PageID=99

KingMax DDR333 Memory

Read ClubOverclockers review and see close-ups of the unique BGA memory chip installation method at http://www.cluboverclocker.com/reviews/memory/kingmax/DDR333/

KingMax memory modules are available from:

CrazyPC Memory
http://www.crazypc.com

Memory Suppliers.com
http://www.memorysuppliers.com

Crucial DDR333 Memory

PC Stats tests Crucials standard PC2700 module at http://www.pcstats.com/articleview.cfm?articleID=1082. Crucial PC2700 memory modules are available direct from: http://www.crucial.com

Corsair DDR333 Memory

Learn more about Corsairs XMS memory series including third-party benchmark test results at: http://www.corsairmemory.com/xms/index.html

Corsair XMS and standard DIMMs are available from:

Aberdeen LLC
http://www.aberdeeninc.com

Axion Technologies
http://www.axiontech.com

The Importance of CAS 2 Instead of CAS 2.5

For benchmark tests indicating the benefits of CAS 2 memory, see Toms Hardwares Perfect Timing: DDR Performance Analysis article at:
http://www17.tomshardware.com/mainboard/02q2/020507/index.html

Learning More About Memory

You can learn much more about memory in my new book "Upgrading and Repairing PCs, 14th edition". You will also find a great deal of useful information on the subject presented visually in my upcoming Upgrading and Repairing PCs Video Training Course.

See more about my latest books and videos at: http://www.upgradingandrepairingpcs.com/books_by_scott/

For a more technical discussion of how memory works from the inside out, I recommend the following article from Ars Technica:
http://www.arstechnica.com/paedia/r/ram_guide/ram_guide.part1-2.html

Copyright©2002 Pearson Education. All rights reserved.

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