Computer Desktop Memory Speeds & Compatibility

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Rather than give memory modules catchy names, the industry refers to modules by their specifications. But if you don't know a lot about memory, these numbers can be confusing.
Here's a short guide to the most popular types of memory and what the numbers refer to and hopefully you will be slightly wiser when thinking of purchasing memory for your desktop PC.

Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory.
Confused?
Hopefully this guide will eliminate most of the confusing aspects of RAM modules and their various technologies and compatibilities.

What type of memory do you need?
It is always good policy to have matching memory types from the same manufacturer when adding multiple modules to a motherboard. If you don't then problems may occur such as random crashing, system freezing, blue screens of death, endless reboots, and worst of all a failure to boot up at all.

We will begin with the most popular of memory types on the market : DDR2

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LATEST POPULAR MEMORY:
DDR2
240 pins (Desktop PCs Only)



Above: Typical 1GB DDR2 240 pin memory module stick.

512MB DDR2 PC2-4200

1GB DDR2 800 Non-ECC Unbuffered SO-DIMM

2GB DDR2 PC2-5300

2GB DDR2 533 Non-ECC SDRAM

1GB DDR2 667

512MB PC2-6400 CL5

1GB DDR2 PC2-8000 Non-ECC Unbuffered

Above: Some example types of DDR2 memory advertised in shops around the market. Confused? Read on.

You would think that all the above memory examples wouldn't really be that much different from each other except for the memory size such as 512MB, 1GB, 2GB and rated speed and of course the price.
However one of the above listed isn't even for your desktop PC!

The "SO-DIMM" DDR2 (small outline dual in-line memory module) listed above is a special smaller designed memory module suitable for LAPTOPS (also notebooks, high end printers, networking hardware etc.) due to the miniature design and contains 200 pins instead of the desktop 240 pins and is physically about half the size of a DDR2 memory module.


Above: Typical 1GB DDR2 SO-DIMM (LAPTOPS)


Normally there are 8 types of memory attributes to consider when purchasing modules:

1. Memory manufacturer (e.g. OCZ, Corsair, Kingston, 3rd party generic)
2. Memory type (e.g. DDR, DDR2, DDR3)
3. Memory size (DIMM or SO-DIMM)
4. Memory module design (single sided (SS) or double sided (DS))
5. Memory speed (e.g. 400MHz, 667MHz, 800MHz, 1066MHz)
6. Memory bandwidth transfer rate (e.g.3200, 6400, 8500 MB/s)

Unfortunately some retailers do not provide such specific information using the above when detailing their RAM modules.

Other specifications you may also see advertised:
7. Memory latency CL5, CL3, 5-5-5-15
8. Other Memory terminology (unbuffered or buffered/registered memory)

These are less important information due to the way they are designed for high end servers that demand critical operational systems and as such cost vast more to produce or for the extreme desktop gamer or operational user that requires such technology.
Memory Latency: Most computer users don't need to worry about SDRAM  DDR latency, because the computer can handle the auto-adjustment to RAM timing and is system managed without the requirement for the typical end user to configure. Dedicated PC gamers have lots of information on the internet with advice about the best memory timings to ensure their games run as fast as possible. Typically the lower the latency values the faster the memory works.
Memory Terminology: The most common type used is "unbuffered" or "unregistered".
Buffered (or registered memory) technology is more likely required for high end servers and dedicated machines that demand the integrity to reduce failure in data.
Some systems do not support unbuffered memory, others require unbuffered memory, and many more give you the option to use unbuffered or registered memory.
The use of unbuffered memory is reasonable for gaming systems. It is not recommended for server-class systems.


Short Facts of Various Memory Technologies:
When you upgrade or build a PC one thing to keep in mind is that the memory DOES NEED TO BE THE SAME TYPE  — for example, the old PC133 SDRAM, for example, cannot be mixed with DDR, and DDR cannot be mixed with DDR2 (even though some motherboards might support both technologies, only 1 technology can be used or risk permanent damage to the motherboard and/or memory modules).
Confusingly DDR, DDR2 & DDR3 contains SDRAM technology and as such is advertised as so.
i.e.
DDR SDRAM
DDR2 SDRAM
DDR3 SDRAM

DDR3 is the latest newly introduced technology, DDR3 DIMMs have 240 pins, the same number as DDR2, and are the same size, but are electrically incompatible and have a different key notch location. (the small cut in the pins so it is aligned correctly and only 1 way into the motherboard's memory slot). DDR3 is very expensive technology and as such is only recommended for the elitist and performance seekers.
DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM.

TODAY'S MEMORY:

DDR2

SUMMARY:
DDR2 has 240 pins (connectors to the motherboard) and 1 notch.

A quick table summary of the exact same memory types but named differently by shops & suppliers. Probably one of the most confusing aspects when purchasing extra memory for your PC.

A. DDR2 PC2-4200 = DDR2-533 = 533MHz = FSB 266 Mhz

B. DDR2 PC2-5300 = DDR2-667 = 677MHz = FSB 333 Mhz

C. DDR2 PC2-6400 = DDR2-800 = 800MHz = FSB 400 Mhz

D. DDR2 PC2-8000 = DDR2-1000 = 1000MHz = FSB 500 Mhz

In DDR2 modules, the numbers that come after the "PC2" refer to the total bandwidth of the module. For this type of memory, a higher number represents faster memory, or more bandwidth. Occasionally DDR2 is referred to as "DDR2-533" or "DDR2-667," for example. When written this way, the numbers after "DDR2" refer to the data transfer rate per second (/s) of the components. DDR2 is not backward-compatible with DDR.

DDR2 PC2-4200 (commonly referred to as DDR2-533) memory is DDR2 designed for use in systems with a 266MHz front-side bus (providing a 533MT/s data transfer rate). The "4200" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 4200MB/s, or 4.2GB/s.

DDR2 PC2-5300 (commonly referred to as DDR2-667) memory is DDR2 designed for use in systems with a 333MHz front-side bus (providing a 667MT/s data transfer rate). The "5300" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 5300MB/s, or 5.3GB/s. PC2-5300 is backward-compatible for PC2-4200.

DDR2 PC2-6400 (commonly referred to as DDR2-800) memory is DDR2 designed for use in systems with a 400MHz front-side bus (providing an 800MT/s data transfer rate). The "6400" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 6400MB/s, or 6.4GB/s. PC2-6400 is backward-compatible for PC2-4200 and PC2-5300.

DDR2 PC2-8000 (commonly referred to as DDR2-1000) memory is DDR2 providing a 1,000MT/s data transfer rate). The "8000" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 8000MB/s, or 8GB/s. PC2-8000 is backward-compatible for PC2-4200, PC2-5300, and PC2-6400.

My motherboard states that it can support DDR2 533/667 MHz memory what does this mean and can I use faster frequency memory?
This means that your motherboard can support the following memory module speeds:
DDR2 PC2-4200 533
DDR2 PC2-5300 667
This will also mean it can also support FASTER modules such as:
DDR2 PC2-6400 800
DDR2 PC2-8000 1000
BUT the memory will run at the reduce rate of the highest motherboard supported speed, as in this case 667 MHz.

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OLDER MEMORY:

DDR


SUMMARY:
DDR has 184 pins (connectors to the motherboard) and 1 notch.
DDR PC1600, PC2100, PC2700, and PC3200
Like DDR2 modules, in DDR modules the numbers that come after the "PC" refer to the total bandwidth of the module. For this type of memory, a higher number represents faster memory, or more bandwidth. Occasionally DDR is referred to as "DDR400" or "DDR333," for example. When written this way, the numbers after "DDR" refer to the data transfer rate per second (/s) of the components.

DDR PC1600 = DDR 200 MHz = FSB 100 MHz

DDR PC2100 = DDR 266 MHz = FSB 133 MHz

DDR PC2700 = DDR 333 MHz = FSB 166 MHz

DDR PC3200 = DDR 400 MHz = FSB 200 MHz

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EVEN OLDER MEMORY:


SUMMARY:
SDRAM has 168pins (connectors to the motherboard) and 2 notches.
SDRAM PC100, 125MHz, and PC133
In SDRAM modules, the numbers that come after the "PC" refer to the speed of the system's front-side bus.

PC100 memory is SDRAM designed for use in systems with a 100MHz front-side bus. It is used in many Pentium II, Pentium III, AMD K6-III, AMD Athlon, AMD Duron, and Power Mac G4 systems. PC100 has been replaced by PC133, which is backward-compatible.

125MHz memory is SDRAM designed for use in systems with a 125MHz front-side bus. 125MHz has been replaced by PC133, which is backward-compatible.

PC133 memory is SDRAM designed for use in systems with a 133MHz front-side bus. It is used in many Pentium III B, AMD Athlon, and Power Mac G4 systems. PC133 is backward-compatible for PC100 and 125MHz.
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GLOSSARY

DDR SDRAM = double data rate synchronous dynamic random access memory
DDR2 SDRAM = double-data-rate two synchronous dynamic random access memory
DDR3 SDRAM = double-data-rate three synchronous dynamic random access memory

latency = SDRAM latency refers to the delays incurred when a computer tries to access data in SDRAM. SDRAM latency is often measured in memory bus clock cycles. Because a modern CPU is much faster than SDRAM, the CPU has to wait for a relatively long time for a memory access to complete before it can process the data. SDRAM latency contributes to total memory latency, which causes a significant bottleneck for system performance in modern computers.

notch = the small "cut" in the memory module's pins so the component can only fit one way (polarised) into the motherboards memory socket.

SDRAM = synchronous dynamic random access memory which is a type of solid state computer memory.

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