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DRAM

 

These are the most common, least expensive, and pretty much describe all RAM types with the exception of SRAM (you'll learn about that in a little bit). DRAM uses capacitors (generally 30-50 femtofarads) in the chip to store an electrical charge (if it has a charge, that's a 1, if there is no charge, it's a 0). This type of RAM is called "dynamic", because it needs to be refreshed (updated) constantly (it generally holds a charge for 1ms (one thousandth of a second)). Why?? Well, capacitors normally lose their charge over a period of time, and this is accelerated each time the CPU scans (polls) the memory for it's data. This poll checks for the storage of voltage across the capacitor and this poll depletes quite a bit of the energy stored there. Therefore, it needs to replace this energy so that your one (1) doesn't become a zero (0). This type of RAM is also referred to as "volatile" RAM. Volatile means that the RAM loses all the information stored in it when power is removed.

Data is stored in cells, much like the way a spreadsheet is laid out. You have data stored in a cell at a certain row and column. Memory is accessed like this too. There are two signals, RAS (Row Access Strobe) and CAS (Column Access Strobe). These two signals together (RAS is first, then CAS) determine which row and column to find a cell containing the data you want. There is actually quite a bit more going on, but for the purposes of this article, I'm not going to get too deep into that. I would like to go ahead and dispel a common misconception right up front. Many people believe that if you install
faster RAM (a lower number is faster. i.e. 50ns is faster than 70ns) that you will see a performance increase. This is not true. If the speed of the RAM is faster than the CPU can access during it's polling cycle (bus speed), then the RAM sits idle until the next polling cycle. So the best option is to find out what your motherboard manufacturer recommends and use that speed of RAM. You can see the correlation of CPU speed (MHz) to speed in ns below:

 

Clock in MHz Cycle in ns
25 40
33 30
50 20
66 15
100 10
200 5


A ns or nanosecond, is a billionth of a second. This is the amount of time that it takes electricity to travel about one foot. Another point of interest, is that you can mix chips with different speeds, but the faster chips will only run as fast as the slowest chip installed.

Some RAM pins are gold and others are tin/lead. Many people say you should buy RAM whose pins are made of the same type metal as your sockets, while others say that it isn't important. I haven't tested this to prove one way or another, but I do know that dissimilar metals will react and begin corroding. The people who say it isn't important,
believe that amount of time in which this will begin to happen will generally be longer than the life of the system. Personally, I collect legacy systems, so to me it will make a difference. Those of you who buy a new computer every couple of years will probably not have anything to worry about.



Physical Packaging Types of DRAM
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