What Type of RAM (memory) Should I Buy For My Computer?

What Type of RAM (memory) Should I Buy For My Computer?

Posted on January 21, 2010. Filed under: What Type of RAM (memory) Should I Buy For My Computer? |

Memory is one of the most popular, easy and inexpensive ways to upgrade a computer. Within limits, it affords the greatest performance increase.

First you need to know how much memory you have already by clicking on Start>right clicking on My Computer or Computer> clicking Properties> From here you should be able to see how much RAM you have installed.

The next step is to open your computer and look at the RAM (memory module(s)  that are already installed. It is easy to identify- look for a long circuit board card(s) installed in expansion slot(s).  Count how many free slots you have that are identical in size and orientation to the one with the installed memory. Write down the exact kind of memory that is installed already.

When you are purchasing RAM for a memory upgrade, you need to pick the right kind.  You must match the RAM to the motherboard’s memory expansion slots.

If you know what motherboard you have you may not have to open your computer just yet- simply read the motherboard’s documentation in print or online to find out what kind and how much memory your computer will support.  If you don’t know what it is; however, it will require you to open your computer up and see what you have already got in there. You need to simply purchase the exact same kind- preferably, the same manufacturer. If you don’t have any installed yet, see if you can identify your motherboard by looking for the name of the motherboard stamped on it somewhere. Then google that name or code to get more information. You can also figure out what kind of RAM to buy by counting the number of pins in the expansion slot. See the descriptions below. However, you need to be careful with this because DDR2 and DDR3 DIMMS both have 240 pins, but they have a little notch that is in different positions and they are not cross compatible.

You can also download CPUZ: http://www.cpuid.com/cpuz.php . CPUZ is a small utility which gives information about your CPU (Processor), Cache, Mainboard, Memory, SPD etc.

You need to understand that at least one of your motherboard’s  memory banks (composed of one or more slots) have to be completely filled for your memory to work.

You should not install disparate memory (cards) modules (speeds, capacities and sides) in your memory slots on your motherboard- even two different manufacturers can cause problems. Get all the same kind.

Usually each RAM slot on the motherboard is a complete memory bank which is filled up when you put the correct card in, but sometimes a motherboard may combine one or more RAM slots into one bank. Remember that the bank has to be filled to work, so this may require you to fill one or all of the slots.

Usually RAM slots on your motherboard are black and close together, but if they are color-coded it means that you need to fill the matching colors together to complete a bank or it could be because you have dual-channel memory.

Dual-channel memory is when the memory controllers (on the motherboard) uses  two memory banks at the same time that work together synchronously, freeing up the bottleneck that can occur between the CPU and the memory making your computer faster. This requires pairs of memory to be installed at the same time.

Single-channel memory- which is more typical- only requires one card to fill the slot/bank and is slower.

Your motherboard documentation will explain what your motherboard’s needs are. It will list which type(s) of memory it supports as well as its maximum speeds and required pairings. If  your motherboard’s FSB is rated at a maximum speed of 533MHz, and you install memory that is rated at 300MHz, the memory will operate at only 300 MHz, if it works at all.

You also need to understand that RAM can be single-sided (SS) or double-sided. Double-sided RAM cards are equivalent to 2 single-sided cards. This has nothing to do with dual or single channel memory as just explained. It just means that double sided cards are useful because they free up space inside your computer. They don’t create more memory capacity. Your computer has a limit and you can’t push it by putting in a lot of double-sided RAM.

For example, lets say that your motherboard’s memory controller supports a total of four memory modules. If you are using double-sided memory cards, you can only use 2 cards (modules) even though you have 4 slots. Also, you can’t fill the 3 slots with single-sided cards (modules) and then try to add a double-sided card in the last slot! It will only take 4 single sided or 2 double-sided.

Motherboard documentation will tell you the quantity and type of memory cards (modules) that it can support.  If you try to get the latest and greatest RAM out there- be careful- your motherboard may not recognize them! It has to be the right kind.

You need to know that SDRAM is what people mean when they say RAM. SDRAM  means synchronous dynamic random access memory. All of the different types of RAM correspond with your motherboard’s expansion slots (which corresponds with its clock signal).  None of the memory module cards are cross- compatible. For example- you can’t use DDR3 memory in a motherboard that only accepts DDR- it won’t work physically or electronically!)

SDRAM includes the following (I am calculating this assuming a 64-bit processor (versus a 32-bit) because that is the most common):

SDR (single-data rate SDRAM) If the the motherboard’s clock signal is 100MHz than you would have an FSB of 100 and need an SDR SDRAM PC100 memory module (card) which produces 800 MBps (megabytes) output. If your FSB is 133MHz than you will need a PC 133 memory module card which produces 1067 MBps output.

DDR doubles the data rate, so if the clock signal is 100MHz then the FSB is advertised as 200MHz and you will need the DDR-200 SDRAM PC1600 memory modules which produce 1600MBps output. If the clock signal was 200MHz, then the FSB would be 400MHz and you will need the DDR-200 SDRAM PC 3200 memory module. You might have to round up. For example, a clock signal of 166MHz, with an FSB speed of 333MHz might lead you to look for a hard to find PC2667 memory module- instead of a more common PC2700 which will do just fine.

DDR2 quadruples the data rate, so if the clock signal is 100 MHz then the FSB is advertised as 400MHz and you will need the DDR2-400 SDRAM PC2-3200MBps memory modules. If the FSB advertised for your motherboard is higher than 1333MHz you can assume that you need to use DDR3 memory modules.

DDR3 doubles the quadruple data rate (x8), theoretically  if the clock signal is 100 MHz then the FSB is advertised as 800MHz and you will need the DDR3-PC3-6400, but actual clock speeds for DDR3 tend to be from 133MHz at their lowest to 250MHz.  So if you have a clock speed of 200, then the FSB is 1600 and you will need the DDR3-1600 SDRAM PC3-12800.

DRDRAM doubles the data rate (like DDR) and is proprietary memory. This means it is specific memory for specific motherboards. If you have a clock speed of 400 you will have an FSB of 800 and purchase PC800 DRDRAM. The name reflects the FSB speed instead of the output. Newer modules do reflect the output, rather the FSB in the name, such as the 32-bit RIMM 6400.

Your CPU (processor) is going to support one kind of memory packaging and it will be one of the following:

DIMM (memory cards (modules) that are used as a package for the SDRAM family: SDR, DDR (184 pins), DDR2 (240 pins), DDR3 (240 pins) None of them are cross-compatible.

RIMM carries DRDRAM and varies whether it is a 16-bit (184 pins) or 32-bit module (232 pins) . Dual-channel memory is required using either two 16-bit RIMMS or one 32-bit RIMM. RIMM modules must have every memory slot occupied or the computer won’t work. So if you have four slots and 2 are filled with 32-bit RIMMS and that is your max, you need to fill the other two with inexpensive blank cards.

SODIMM’s are for notebook or small computers. They include their own version of DDR, DDR2, etc. They are not interchangeable with the DIMM version.

MicroDIMM (64-bit data rate memory cards)  are 50% smaller than SODIMMs and are often used in laptop computers.

You should now be well equipped to understand what RAM to purchase for your computer.

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