So i want to learn more about RAM...

[SideWinder]

I don't know enough about it and i need some simple questions answered really.

What exactly does PC mean? Major difference between PC4200 and PC8500? Or does it only make a difference when it comes to overclocking?

Second question, overclocking. I am confused by 'timings'. Does it really make such a huge difference? Can anyone show me some benchmarks i guess of the differences of overclocking your RAM?

 

[Talrinys]

I have absolutely no idea what the two letters PC in the RAM actually means, suppose it's just to show it's for PCs or something, i don't know?
The number after however is the theoretical maximum bandwidth, and yep this is extremely important, it's basically how many GB per second that can move through it, with a comma after the first number, for example PC4200 is 4.2GB/s, and PC8500 is 8.5GB/s, which is course a lot faster.
There are some things to take into the equation, for instance AMD usually can muster a lot quicker RAM bandwidth due to the memory controller being integrated on the CPU itself(on Intel chips/chipsets the memory controller is located on the motherboard, and can be improved on too, the new P35 motherboards have slightly higher memory bandwidth than the P965 boards).
The reason i said theoretical is that it isn't always true, because of two things:
1: First of all, is there even being produced enough data to make it worth it? We're looking at the processor here, and on the newer Core 2 Duo chips you should get pretty quick RAM(DDR2-800, that is PC-6400) to match it, but on older processors the amount of data processed probably isn't enough to saturate the bandwidth.
2: Timings, this is essentially the amount of time it takes for various operations to be done inside the RAM, and this is to a certain degree nessecary. They do make a difference, and are a crucial part of tightening performance at quicker speeds, but i'd pick RAM speed over lower timings any day.

Hope i can help, if anyone has noticed any mistakes in this, hit me

 

[Admiral Huddy]

Just to follow what talrinys posted, it might be worth reading the memory FAQs to get your started.

In addition to selecting the correct memory, you’ll have to workout the frequencies of both the CPU and the memory you intend to use. So for example, a CPU which has an FSB of 333MHz (1333MHz effective) provides 10.6GB/sec of bandwidth so you’ll need memory to satisfy this requirement. In this example PC5300 is ample.

So why PC5300?

One major consideration when selecting the correct RAM, is the theoretical maximum bandwidth. PC5300 provides a maximum bandwidth of 5.3MHZ per second but a maximum peak of 1.6GB/Sec per channel in dual channel mode (twice the bandwidth), hence that PC5300 is enough to run a CPU with an FSB of 333MHz.

The reason why that people buy PC6400 and beyond is to increase overclocking potential without compromise to the bandwidth settings.

Here are a few calculations:

To calculate the memory bandwidth of your CPU, you need to know the FSB:

Effective CPU Speed (MHz) = FSB (MHz) x bits per clock cycle (4)
Required Frequency (GB/S) = Effective CPU Speed x bits per byte (8)

For example:

(266MHz x 4) x 8 = 8.5GB/s
(333MHz x 4) x 8 = 10.6GB/s

Now you need to find the correct memory:

PC2-3200 = 100MHz (400MHz effective)
PC2-4200 = 133MHz (533MHz effective)
PC2-5400 = 166MHz (667MHz effective)
PC2-6400 = 200MHz (800MHz effective)

PC5300 operates at a memory core speed of 166MHz. This equates to a bandwidth of 5.3Mhz :

So,

Bandwidth = Memory core speed x bits per clock cycle (4) x Bits per byte (8)

For Example,

(166MHz x 4) x 8 = 5333


In dual channel mode this equates to 1.6GB/s of peak bandwidth.

The above only applies to DDR2.

DDR is a bit simpler to understand:

Again you need to calculate the memory bandwidth of your CPU and again you need to know the FSB:

Effective CPU Speed (MHz) = FSB (MHz) x bits per clock cycle (2)
Required Frequency (GB/S) = Effective CPU speed x bits per byte (8)

For example:

(166MHz x 2) x 8 = 2.7GB/s
(200MHz x 2) x 8 = 3.2GB/s

It’s quite simple to see.. the 166MHz FSB requires PC2700 and the 200Mhz FSB chip requires PC3200.



The big difference between DDR and DDR2 is that the core speed of the later is half that of DDR allowing it to reach higher frequencies by transferring data at 4 bits per clock cycle rather than 2. The net effect however is the same.




I believe this is true but feel free to correct me where wrong.