What is RAM?

RAM, short for Random Access Memory is the short storage area for your PC. Often just called memory or system memory RAM is just an electromagnetic storage that loses all its data once the power has been removed. RAM is used by your operating system and other programs and games in order to store data that is required at speed. Computers with large amount of memory often perform faster simply because the CPU is not idle waiting for data to be retrieved from slower components such as the hard drive which can be stored in memory, if your system has enough.

Why is RAM considered Random Access

RAM can be accessed by the computer for any piece of data in any cell of the memory bank. There is no need to go through the entire memory bank to get to the data you require. The opposite to Random access is serial access. SAM or Serial Access Memory needs to be read from the start to get to the data you require. An example of SAM storage would be a cassette tape. Of course there are things which fit in-between these two examples such as a CD or DVD. Because these have tracks they can be random access to a point but then need Serial access to get to the exact spot required.

How does RAM store data

RAM consists of many capacitors and transistors. A capacitor and a transistor are paired together to make a memory cell. The capacitor represents one "bit" of data, the transistor is able to change the state of the capacitor to either a 0 or a 1. the Zero's and ones when read in a sequence represent the code which the computer understands. This is called binary data because there is only two states that the capacitor can be in.

In order for a capacitor to achieve a value of 1 it needs to be filled with electrons. To achieve a value of 0 the capacitor needs to be emptied of electrons. You may of noticed when purchasing RAM that many types of RAM you buy are called DRAM or SDRAM. DRAM or Dynamic Random Access Memory has a small problem with the capacitors holding a value of 1. It is called dynamic RAM because its state of 1 or 0 needs to be constantly refreshed on order to stay in the correct state. In order to demonstrate this and for a much more detailed look into the workings of RAM a good site site is How stuff works. This article will show you a much more technical look at the workings of RAM and has a great Diagram to illustrate the nature of Dynamic Random Access Memory

There is a type of RAM that doesn't have to be refreshed constantly. This is called SRAM or Static RAM. Static RAM uses a type of Flip Flop to hold the data in the cell. This can take around 6 transistors on a chip per cell rather than just the one., The consequences are that Static RAM needs more chips per Mb than DRAM and therefore is much more expensive.

Types of RAM

Down the years the face of RAM has changed dramatically in the early days of computing we had the SIMM. SIMM was a Single Inline Memory Module. Moving on the next logical step was the DIMM, Double Inline Memory Module. The introduction of DIMM's brought with it  new speeds and sizes to give computers more power than ever before. The big advances in recent years has been the introduction of DDR RAM (Double Data Rate). DDR effectively doubled the speed that RAM could transfer data without actually increasing the MHz. E.g. a stick of PC133 (133MHz) RAM with DDR would effectively transmit data at 266MHz but still have a bus speed of 133Mhz.

After the introduction of DDR, a new standard of DDR2 was released. DDR2 improves on the previous DDR standard by allowing for higher frequencies to be used and lower power is required. The power savings are made by better manufacturing process using smaller dies which require less voltage to operate. DDR2 modules are mostly labled as DDR2-xxxx where xxxx is the data rate of the module, however some manufactures has decided not to stick to the standard labeling and started to use PC2-xxxx where xxxx is the theorectical bandwith of the module. DDR2 is not forwards or backwards compatible with other DDR memory modules, to make sure than users dont use incorrect memory types or try to mix memory types, the notches on the modules that fit into motherboards are moved into different positions.

Now DDR3 modules are also available. DDR further enhances what DDR2 had already improved on by giving higher frequencies and lower power usage by once again having smaller dies and using lower voltages to operate. Like the DDR2 memory modules they are not backwards compatible and again the modules notches are moved to a different location. Also like the the previous standard, some manufacturers have chosen to use the labeling of PC3-xxxx istead of the standard DDR3-xxxx labeling.

The other big memory type of the recent past is RAMBUS memory. Rambus memory uses a memory module called a RIMM (RAMBUS Inline Memory Module) RIMM's are specials high speed memory chips. They require special motherboard support and are priced higher than that of standard memory modules.

Measuring the Speed of RAM

Like CPU's RAM is measured in MHz. The higher the MHz the greater the speed of the RAM. To keep this calculation simple when dealing with DDR RAM, retails often simply put the effective MHz rating. For example a 133Mhz DDR module would be advertised as 266Mhz. That is basic speed test of the memory module. However there is another important factor in high performance memory and that is CAS latency.

CAS meaning Column Access Strobe, the basics of CAS latency are that CAS latency is the amount of clock cycles it takes for the response from the memory for a query. Shorter is obviously better. CAS-2 is a 2 cycle delay and CAS-3 a 3 cycle delay. To learn more about CAS latency Corsair have a good article here

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