[Introduction]
Often users are confused about memory timings, frequencies, bandwidth or simply what memory to buy. This article will cover all these topics and more, starting with what memory is and why we need it. Memory is also often referred to as RAM (Random Access Memory) and acts as a “memory” buffer for the computer, allowing it to think quickly. The hard disk drive (HDD) should not be confused with memory as they are two completely different things.

When processing large amounts of information the CPU (Central Processing Unit) needs some where it can quickly store or access the data. This is of course where memory (RAM) comes into play. Memory operates best when synchronized with the CPU as it allows for a constant and uninterrupted flow of data. The term memory “stick” is often used to describe a single memory module. Memory modules found in desktop computers all look much the same, yet they can all be quite different. For example, depending on the computers age and design it can either make use of EDO RAM, SD RAM, DDR RAM, DDR2 RAM or RAMBUS memory.

[Types of Memory]
Most users are unsure about the differences between the original DDR memory and the new DDR2 memory type. Most of the questions were “Which memory type is faster?”, “Can both DDR and DDR2 be used at the same time?” and of course “Which memory should I buy?”. Most will be surprised to find that there is not a great deal of difference between DDR and DDR2. First here is some history regarding the original DDR memory standard.

The name DDR stands for “Double Data Rate”. DDR memory works by carrying information on the rising and falling point of a clock cycle, where as the much older SD-RAM carried data at only the rising point of each cycle. Theoretically, this means that DDR memory has double the memory bandwidth. The first DDR memory was rated PC1600 which ran on a 100MHz memory clock, this enabled it to produce a maximum data bandwidth of 1600MB/s.

The next step was PC2100 which ran at 133MHz which resulted in a maximum transfer of 2100MB/s, and then PC2700 which operated at 333MHz giving a peak bandwidth of 2700MB/s. Many more standards emerged, while the official standard for DDR memory ended at PC3200 (DDR400). There were a number of memory manufacturers that offered memory as fast as PC4400 (DDR550). This kind of memory was strictly aimed at overclockers.

The new DDR2 memory picks up where the original DDR memory left off, starting at DDR2 400 it is now available clocked at 1000MHz. While DDR2 is almost identical to DDR memory, there are a number of enhancements. The biggest feature of DDR2 is its 4-bit prefetch, allowing it to read/write four times the amount of data per clock cycle. This feature effectively doubled the data bus speed while keeping the same 64-bit interface that the original DDR memory used.

This new DDR2 memory has been available for quite some time now, but it will not overtake its predecessor for quite some time. The demand for DDR2 has only slowly grown over the last year and there have been a number of problems with availability. DDR2 memory cannot simply replace original DDR memory due to the physical difference in the pin count, as DDR has 184-pins and DDR2 has 240-pins. The power requirements are also different, as DDR2 requires less power to operate at higher frequencies.

[Memory Frequencies]
Although I did just briefly talk about memory speeds I will now show you the available DDR & DDR2 frequencies. The below list includes all the official frequencies along with some of the non-official frequencies.

DDR Speed---Front Side Bus---Bandwidth---Relase
DDR200---100MHz FSB---1.6GB/s---Official (known as PC1600)
DDR266---133MHz FSB---2.1GB/s---Official (known as PC2100)
DDR333---167MHz FSB---2.7GB/s---Official (known as PC2700)
DDR400---200MHz FSB---3.2GB/s---Official (known as PC3200)
DDR433---216MHz FSB---3.4GB/s---Non Offical (known as PC3400)
DDR500---250MHz FSB---4.0GB/s---Non Official (known as PC4000)
DDR550---275MHz FSB---4.4GB/s---Non Official (known as PC4400)
DDR600---300MHz FSB---4.8GB/s---Non Official (known as PC4800)

These are single channel bandwidth figures meaning when used in dual channel the bandwidth will be twice as large. This memory market official means it has been announced by JEDEC as an official memory specification. The non-official memory still exists and you can purchase it today. However, non-official memory has no real specification and is just overclocked or improved memory built to the official specification. This memory is popular as it will allow for much greater performance when overclocking. Motherboard manufacturers often provide “official” memory support for “non-official” memory, if that makes sense.

For example, while DDR2 800 is the fastest official memory specification, some memory manufacturers have released memory capable of operating at 1000MHz. In an effort to gain popularity among the enthusiast crowd, motherboard manufacturers are offering official support for this memory frequency. ASUS were among the first to offer official DDR2 888 support, while claiming their boards would also support DDR2 1000 which was an unbelievable claim at the time. Memory speeds (frequencies & bandwidth) differ from memory timings which I will now discuss.

[Memory Timings]
Now I believe the biggest cause for confusion involves memory timings and how they influence performance. Although DDR2 features a 4-bit prefetch, we are still finding it unable to out perform the original DDR memory at the same or similar frequencies. For example, DDR400 is capable of operating with timings of CAS2-2-2-5 where as standard DDR2 400 at the moment runs at CAS4-4-4-12. This means that despite the fact DDR2 may be working harder delivering twice as much data per clock, it is theoretically doing it twice as slow due to the poor latency timings.

This means it was not until DDR2 667 became a standard that we truly saw DDR2 take charge. At this frequency the poor latency timings of DDR2 are no longer a problem, as the frequency will pick up the slack. However this does leave me with the same questions I have been asked time and time again. What should “you” do now? DDR and DDR2 memory modules can not be used simultaneously. Therefore it makes little sense to replace current DDR memory modules with DDR2 modules. Clearly if you are building an AMD system then stick with the original DDR interface as AMD has no plans to adopt DDR2 just yet.

This decision only affects Intel users looking at upgrading to the LGA775 platform. At this point in time I would suggest purchasing DDR2 memory. The new DDR2 memory is now just as affordable and does offer slightly better system performance. Furthermore, manufacturers have been working hard to reduce the memory timings of DDR2 and have had quite a degree of success. Before we move on I will quickly explain what the term “CAS Latency” means.

Often the terms CAS and RAS will come into conversation when discussing memory in general. CAS means “Column Address Strobe”. Dynamic memory consists of vast arrays of rows and columns of bits, each row or column containing thousands. The term RAS means “Row Address Strobe”. The CAS Latency measures the time delay in cycles, for example CAS2 means the system must wait 2 cycles before the data has been buffered. Therefore, it is important to keep the CAS value as low as possible to avoid lag.

[Memory Capacity & Configs]
While it is important to have the fastest possible memory in order to achieve maximum system performance, memory capacity is just as important. Without enough memory capacity you could just be burning a lot of speed with little effect. Today the recommended memory capacity is 1024MB (1GB) as this will allow the computer to comfortably load WindowsXP and all its applications. Those looking at gaming will certainly need at least 1GB of memory to maintain a quick and safe operating system.

Thankfully the pricing of both DDR and DDR2 memory modules has decreased rapidly through 2005, making the purchase of 1GB very affordable. It is true that just 512MB of memory will be enough to run WindowsXP with basic applications, though the system may become sluggish from time to time. With the arrival of 64-bit computing, applications have become extremely hungry for memory. For this reason most users are now turning to 2GB configurations for all their gaming needs.

A few pages back in the “Memory Frequencies” section I displayed a list of official and non-official memory types. For example “DDR400 – 200MHz FSB – 3.2GB/s – Official (known as PC3200)”. I also mentioned that there is a way of further increasing this memory bandwidth. Using what is called “Dual-Channel” memory technology, supportive processors and chipsets can turn a pair of 3.2GB/s DDR400 memory modules into 6.4GB/s of bandwidth. This is done by placing the memory modules in pairs and then accessing them simultaneously.

Dual-Channel memory has become a standard amongst Intel and AMD platforms over the past few years. There are few processors that only utilize single channel configurations. The following processors all support dual channel memory configurations; AMD Athlon64 (939-pin), AMD AthlonFX, AMD Opteron, Intel Pentium Extreme Edition, Intel Pentium D, Intel Pentium 4 Extreme Edition, Intel Pentium 4.

The latest VIA, Intel, NVIDIA, SiS and ULi chipsets all support dual channel memory configurations at a range of frequencies.