There is several types of memory with for each type several standards. Here are the 4 main types :
- SDR-SDRAM : This is an obsolete type of RAM, but you can still find it in very old computers. A chip had 168 pins and 2 keyways.
- DDR-SDRAM : This is an evolution of the SDR, with better frequencies. It has 184 pins and one keyway. It’s not selled today and have been replaced by DDR2.
- DDR2-SDRAM : This is the standard type of mmemory. All the new motherboards support this type. It has better frequencies. It has 240 pins.
- DDR3-SDRAM : Really recent technology and still almost few used. The motherboards support more and more that technology.
Now, the main used chips are DDR2 and the new motherboards are all compatible with this norm. This is why, i’ll advise to directly buy DDR2. If you want better performances use DDR3, but it’s only necessary with big configurations.
For each technology there is several formats. This formats indicate the frequency of the chip. The DDR have a internal frequency and an external one. Here are the different formats :
- DDR2 400 – PC2-3200 : External Frequency (EF) : 200Mhz, Internal Frequency (IF) : 100Mhz
- DDR2 533 – PC2-4200 : EF : 266, IF : 133
- DDR2 667 – PC2-5300 : EF : 333, IF : 166
- DDR2 800 – PC2-6400 : EF : 400, IF : 200
- DDR2 1000 – PC2-8000 : EF : 500, IF : 250
- DDR2 1066 – PC2-8500 : EF : 533, IF : 266
- DDR2 1200 – PC2-10000 : EF : 600, IF : 300
More the frequency is high, more the access to the data is fast. Nevertheless, like you will see in the next chapter, it’s not enough to have a high frequency. In fact, a very high frequency has no utility with high latencies.
The motherboard have also a frequency, so have to keep in mind to not decrease the performances of the memory with the motherboard and vice-versa. The motherboards doesn’t support any formats, so you have to control the compatibility between the memory and the motherboard. More the frequency is high, more it will be costly. So have to choose the right middle between performances and price.
Note : Some mainboards have limitations on the supported number of chips of a certain type of chips. For example a mainboard XXX can support only three chips of type YYY.
For the persons that want to make overclocking, i will advise to take chips with very high frequencies (DDR2 1066 and superior). But for all that want a simple configuration, lower frequencies will be enough.
For the persons interested of the very high frequencies, here are the caracteristics of the DDR3 norm :
- DDR3-800 : 100 MHz- 400 MHz
- DDR3-1066 : 133 MHz – 533 MHz
- DDR3-1333 : 166 MHz -à 667 MHz
- DDR3-1600 : 200 MHz – 800 MHz
- DDR3-1800 : 225 MHz – 900 MHz
- DDR3-1866 : 233 MHz – 933 MHz
- DDR3-2000 : 250 MHz – 1000 MHz
- DDR3-2133 : 266 MHz – 1066 MHz
- DDR3-2200 : 275 MHz – 1100 MHz
- DDR3-2300 : 287 MHz – 1150 MHz
To choose the DDR3 memory, the best criteria is the price. Take the format the most according to your budget.
An other caracteristic of memory is the number of clock cycles necessary to access a data. This caracteristic is represented by 4 numbers (x-x-x-x) :
- CAS (Column Address Strobe) Delay or CAS Latency : Time to access a column. Namemly the number of cycles between the read request and the answer.
- RAS (Row Address Strobe) Precharge Time : Number of cycles between 2 RAS operations.
- RAS To CAS Delay : Access time to swith from a row to a column.
- RAS Active Time : Access time for a row.
More the timings are low more the chips is fast. But you have to balance the 4 numbers.
It’s possible to change timings in The BIOS, but these operations can be dangerous for the stability of the chips and also for their life time.
This parameter is not the most important to keep in minde, but you cannot ignore it. Timings of 3-3-3-8 or less will be fine. If you have the budget, you can choose 2-2-2-5, but it’s very expensive. Keep in mind that timings are lower for DDR3 than for DDR3.