The Different Types of Random Access Memory (RAM)

In this article we are going to discuss Memory, in particular we're going to talk about Random Access Memory (RAM). Now RAM is the place in a computer where programs and data currently being used are stored. The term random means that the CPU can access any part in the memory storage, any address in the memory storage directly at any time, rather than having to start at the beginning and proceed it's way through to find a particular memory location.

In modern PC Systems RAM is implemented as a series of DRAM chips soldered onto an interface board, which we usually call a stick of RAM. Each DRAM chip is made up of a bunch of memory cells, each of these cells is assigned an address using hexadecimal notation. Now this addressing system is very similar to houses on a street, there's a chip implemented on your motherboard called a memory controller chip, or MMC. The job of the MMC is to keep track of the address, to keep track of all those cells, where they are and what's in them. RAM comes in various standards.

The first standard we are going to look at is SD RAM. One of the main characteristics of SD RAM is that the frequency of the memory is synchronized with the frequency of the system clock..If you think of ever tick of the clock as a pulse the memory is synchronized with that pulse to act in rhythm. SD RAM sends one instruction per cycle, each instruction can be 64 bits in length, now if you look at a memory board you'll normally see that it has eight memory modules, some may have less then eight but eight is the norm. The 64 bits of data is called a word, so with each cycle of the clock one word can be submitted the memory module, with typically 8 bits being stored on each module.

SD RAM uses 3.3 volts and operates at frequencies between 33 and 166 Mhz, the frequency must match the frequency of the system board.

The next type of memory is Double Data Rate Memory (DDR RAM). As it's name implies DDR can accept double the amount of data as SD RAM. It accepts one command per clock cycle plus two data sets. So for each cycle the DDR RAM can accept two 64 bit words of data and because of this DDR RAM has twice the bandwidth as SD RAM. DDR operates at 2.5 volts and at frequencies between 100 and 200 Mhz.

The next type is DDR2. DDR2 doubles the bandwidth of DDR1. It accepts four data sets per cycle, it operates at 1.8 volts and a frequency of 200 to 533 Mhz. Another improvement with DDR2 is that there is now a buffer between the data BUS and the memory. This buffer helps to achieve these four data sets and increased speed per clock cycle.

The latest type of RAM is DDR3 which doubles the data rate of DDR2, so it can accept eight data sets per cycle, it also has a buffer and it operates at 1.5 volts with frequencies between 400 and 1,000Mhz.

As you can see as we move from SD RAM through to DDR3 we lower the voltage in each case, while increasing the operating frequency We also take the amount of data that can be submitted in each cycle and double it.

One final type of RAM is RD RAM also called Rambus RAM. Rambus was an alternative to DDR. Rambus was actually the first specification to double the data over SD RAM. DDR was a competing standard that eventually became the accepted specification and evolved into DDR3

RD RAM instead of transferring data in 64 bits transfers data in 32bits or 16 bit at a time,but it does send two data sets per cycle. Another consideration with RD RAM is that in addition to having a memory module installed in your motherboard any spare memory sockets must be filled with continuity modules. This is a module that does not contain RAM chips.

Another way to increase memory bandwidth is by using multiple channels. A typical system configuration has a single memory controller that sends data to multiple memory modules. All data that is intended for you RAM must pass through this single controller. Dual channel memory adds a second or third memory controller to the motherboard. In this case your memory modules are assigned to one of the memory controllers. With dual and triple channel you can often assign multiple chips per channel. So for instance if I had a dual channel system I could have four memory modules with two modules assigned to each controller.

It's important to to note that when using dual or triple channel memory configuration that this is a configuration specific to the motherboard and not the memory itself. The memory you purchase would be the same.

For a video tutorial on how to install RAM sticks please visit the tutorial section or click this link.