Guide to Memory

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Guide:

Memory (RAM)
Most desktop computers now use a type of memory called DDR2. Some high performance ones might use DDR3, but this is quite expensive. Avoid systems using the older DDR memory standard. In terms of amount, it is best to have at least 1GB of memory in the system and preferably 2GB. Memory speeds can impact performance as well. The faster the memory, the better the performance should be. When buying memory, try to buy as few DIMMs as possible to allow for future memory upgrades if needed.

How Much is Enough?
The rule of thumb that I use for all computer systems for determining if it has enough memory is to look at the requirements of the software you intend to run. Pick up the boxes for each of the applications and the OS that you intend to run and look at both the "minimum" and "recommended" requirements. Typically you want to have more RAM than the highest minimum and ideally at least as much as the highest listed recommended requirement. The following chart provides a general idea of how a system will run with various amounts of memory:

Minimum Functionality: 1GB
Optimal: 2GB
Smooth Sailing: 4GB or more
The ranges provided are a generalization based upon most common computing tasks. It is best to check the requirements of the intended software to make the final decisions. This is not accurate for all computer tasks because some operating systems use more memory than others.

Note: If you inted to use more than 4GB of memory, you must have a 64-bit operating system to get past the 4GB barrier.

Does Type Really Matter?
The type of memory does matter to the performance of a system. There are two basic types of memory currently used: DDR2 and DDR3. There are older RDRAM and SDRAM modules, but these forms are both outdated and should be avoided in a computer purchase. DDR is still available, but unlike to be installed in any currently sold PCs. Beyond the type of memory, each memory type also has a related speed to that ram as well. Below is a chart detailing the order of the type and speed in order of slowest to fastest:

DDR2 400 MHz or PC2-3200
DDR2 533 MHz or PC2-4200
DDR2 667 MHz or PC2-5300/PC2-5400
DDR3 800 MHz or PC3-6400
DDR2 800 MHz or PC2-6400
DDR3 1066 MHz or PC3-8500
DDR3 1333 MHz or PC3-10600/PC3-10666
DDR3 1600 MHz or PC3-12800
These speeds are all relative to the theoretical bandwidths of each type of memory at its given clock speed when compared to another. A computer system will only be able to use one type (DDR2 or DDR3) of memory and this should only be used as a comparison when the CPU is identical between the two systems. These are also the JDEC memory standards. Other memory speeds are available above these stardard ratings.

Memory Expansion
One other thing that you might want to consider is how much memory the system can support. Most desktop systems tend to have a total of four memory slots on the boards with modules installed in pairs. Smaller form factor systems typically will only have a two RAM slots. The way these slots are used can play a key role in how you can upgrade memory in the future.

For example, a system may come with 2GB of memory. With four memory slots, this memory amount can be installed with either two 1GB memory modules or four 512MB modules. If you are looking at future memory upgrades, it is better to purchase a system using two 1GB modules as there is available slots for upgrades without having to remove modules and RAM to increase the overall amount.

Memory Types
While there are many different types of memory on the market, the most common types that are used in PCs right now are:

Double Data Rate DRAM (DDR)
Double Data Rate 2 DRAM (DDR2)
Double Data Rate 3 DRAM (DDR3)
Double data rate or DDR memory is designed to function at two memory operations per clock cycle. This effectively doubles the speed of the memory over older synchronous memory modules. This memory type has been phased out in favor of the high clocked DDR2 standard.

DDR2 is an updated version of DDR that provides even faster clock speeds than the previous DDR memory standard. The two types of memory are not interchangable as they interface with the chipset and memory in a different method. To differentiate the two, each type has a different pin count and layout for the memory modules.

DDR3 is the latest version of memory to be released and once again provides speeds faster than those of DDR2. It is still fairly uncommon but will likely gain more acceptence over time as processor speeds increase requiring the faster memory.

Memory Speed and Bandwidth
The main purpose for the DDR3 memory standard is to increase the memory speed and bandwidth over the older DDR2 specifications. The original DDR memory modules ran between an effective clock speed of 200 and 400MHz. The DDR2 standards doubled this to between 400 and 800MHz. DDR3 is set to double the speed of DDR2 with clock speeds between 800MHz and 1.6GHz.

Even though the DDR2 memory standard officially only goes up to the 800MHz speed, there are modules that are sold that can run at the 1066MHz speeds. Technically these modules are overclocked standard modules as the JEDEC specs don't officially support speeds over 800MHz. This is often dealt with by requiring the memory modules to run with higher voltages or at different latencies for the increased speeds.

Lower Power Consumption
The other primary benefit of the new memory standard is another reduction in the voltage used to power the memory modules. The original DDR standard modules ran at a 2.5V level. A significant power reduction to 1.8V was created with the DDR2 standard. With the lower voltage levels, personal computers required less actual power to run the memory subsystem. DDR3 looks to further reduce the power usage by dropping the voltage levels to 1.5V. This is not as large of a jump as DDR2 from the DDR standard, but it does reduce the overall power consumption.

Increased Latencies
In order to increase the speeds of the memory modules with DDR3, it was also necessary to increase the latency of the modules. Latency is the amount of time that it takes for a memory module to process commands in a number of clock or command cycles. The higher the latency, the slower the memory will be at processing a command.

The typical latency for a DDR2 JEDEC standard was 5-5-5-15. The JEDEC standard latencies for the newer DDR3 memory are 7-7-7-15. Even with the increase in the latency for the DDR3 memory, the higher clock speeds allow the memory to still have a greater bandwidth than the older standards. This results in DDR3 memory running at 800MHz to be slightly slower than DDR2 memory that is also running at 800MHz. The real advantage comes when the clock speeds get higher than this mark.

Another thing with the latencies to be aware of is that these are the standards. As manufacturing improves with the memory modules, the modules will be able to run at lower latencies than the JEDEC specifications. It is possible to find DDR2 memory that is faster than the 5-5-5-15 speeds right now. It will take some time for DDR3 to get below the JEDEC latencies.

Conclusions
DDR3 is the next step for personal computers in expanding their memory bandwidth. It will take some time for it to really be widely implemented. AMD has only recently switched over their processors to use the DDR2 standard and will likely not move to the DDR3 standards for some time. In addition, the higher performance DDR3 modules will likely take some time compared to high performance DDR2 that is already readily available.

If you are already invested in a current memory standard with your existing computer, it is not worth switching to the new technology. After all, switching to DDR3 will require a completely new motherboard and possibly processor in addition to the memory cost. If you are looking at investing in a new computer system though and DDR3 is available, it may be worth it to go with the newer memory standard for longer term upgrade potential.