DDR2 Memory Confusion..

[Admiral Huddy]

I've not had much dealing with DDR2 and i'm really getting in a mess understanding the CPU/Memory compatability, so I need some help..

For example, I'm failing to understand why a CPU which has a FSB of 1066 MHz is compatable with memory that runs at 800 MHz (PC2-6400)

I thought I had it cracked bu I'm still a little confused.

Can some one help please...

[Admiral Huddy]

this is what i've come up with so far, nut it's of no help:


The following table shows an example of this for each DDR2 module:



Module / Memory clock / Bus Clock / Transfer Speed (effective) / Peak Transfer rate
PC2-3200 / 100 MHz / 200 MHz / 400 MHz / 3.200 GB/s
PC2-4200 / 133 MHz / 266 MHz / 533 MHz /4.264 GB/s
PC2-5300 / 166 MHz / 333 MHz / 677 MHz / 5.336 GB/s
PC2-6400 / 200 MHz / 400 MHz / 800 MHz / 6.400 GB/s
PC2-8500 / 266 MHz / 533 MHz / 1066 MHz/ 8.500 GB/s




In dual channel mode, the bandwidth is theoretical doubled. So, PC5300 for example would have a operating peak bandwidth of 1.06GB/s making the

PC2-3200 = 6.40 GB/s
PC2-4200 = 8.52 GB/s
PC2-5300 = 10.67 GB/s
PC2-6400 = 12.80 GB/s
PC2-8500 = 17.00 GB/s


So, when choosing memory for a CPU which has a FSB of 333MHz (1333 MHZ effective), PC6400 is ample because the 1333Mhz CPU requires a bandwidth of 1.06GB/s. Since PC6400 produces 12.8GB/s of bandwidth in dual channel mode, the requirement is fore filled with plenty of room to spare.

[Admiral Huddy]

Ok, I think I’ve got this sus but correct me if I’m wrong. PLEASE!!

In DDR days, to work out the memory frequency requirements it was simple:

The I/O bus (FSB) = CPU clock speed x 2

The required memory frequency = FSB x 8

Therefore a CPU with a clock speed of 200 MHz required PC3200

(200 x 2 x 8) = 3200MHz

Simple.

With DDR, it’s really no different. However, the FSB runs twice the speed of the I/O bus because the prefetch buffer is now 4.

So when calculating the required frequencies for your CPU, it’s still the same but based on the I/O bus not the FSB..

For example,

A C2D with a clock speed of 266MHz has a I/O bus speed of 533 MHz (1066MHz effective FSB).

Therefore the minimum memory for that CPU would be PC4200!!


Am I right? Am I right..

Has the FSB confused me?

[semi-pro waster]

OK, my simpletons explanation (how I explain it to myself since I'm not particularly au fait with Ram timings etc) is that the FSB is in effect quad-pumped e.g. for 266.5mhz (PC4200) you get an FSB speed of 1066mhz which is correct so PC4200 is the minimum you need for any Core2Duo with that FSB speed.

If you up the Ram speed e.g. going for PC6400 (800mhz nominal - 400mhz real) then you will overclock the PC by that amount times the CPU multiplier - E6320 has a multiplier of 7x so the 7x400mhz = a real speed of 2.8ghz.

I'm not entirely sure if that is what you are asking or even if it helps but if not could you clarify a bit further and I will try again. :)

[Admiral Huddy]

ok, the bit i'm confused about is working out which RAM you need.. Reading your example, you state you need PC4200 as that runs at a peak frequency of 4200MHz. Your effective FSB runs at 1066MHz. So how does one come to PC4200.

1066Mhz x 8 = 8528Mhz???? Way over,

So this is why I'm confused which has lead me to believe the the CPUs aren't really quad pumped at all. They still work as DDR (ie. process on the up and down).. so the RAM frequency requirement is still calculated as per DDR.. i.e.

266 x 2 x 8 = frequency of 4256Mhz (PC 4200)

therefore, the ram actaully runs at 533MHZ.

The only difference is that DDR2 has a buffer size of 4 bits rather than 2 (DDR), so it pushes more data through on each cycle, hence making it appear as quad pumped.

Am I making sense..? :D

[killerkebab]

Huddy, PC3200 does not mean frequency :)

PC3200 is also commonly referred to as DDR400, because of it's speed of 400MHz. Hence why CPUs with a bus speed of 200MHz (Athlon XP anyone? :)) run with DDR400 since 200x2=400 (think DDRs double data rate).

The difference lies in the numbering. PCx refers to the module, wheras DDRx refers to the memory chips themselves. So a PC3200 memory module will have DDR400 chips on it. PC-x refers to the theoretical peak bandwidth whilst DDR-x is the effective clockspeed. The reason you found one number was typically eight times larger than the former is simply because the way to work out bandwidth is to multiply the effective speed by eight, since DDR works on a 64bit bus, and that 64 bits make up eight bytes of data. 8 bytes of data per second multiplied by a frequency of 800 million comes out with 6.4 billion bytes per second - 6.4Gb/s, hence PC2-6400 :)

As for DDR2, a PC2-6400 module will have DDR2-800 chips on it, for 800FSB. Now your question dealt with 1066FSB which is, I believe, the standard for the high end C2D chips.

My belief is that the high end C2D chip is nothing more than an 800FSB overclocked to 1066FSB, and the RAM is simply overclocked to keep up, since my understanding is RAM and FSB can never be at different speeds (if they are, the faster one will slow to the speed of the slower one to maintain sync, so if your RAM can't keep up with 1066FSB, your processor will slow to 800 for it). I know back in the days of single data rate on my old machine that is how it worked, anyway :p

Since there aren't any 'true' DDR2-1066/PC2-8500 chips, people tend to buy their slower DDR2-800 counterparts (I know I have for my processor, although mine has a true speed of 800FSB). Bear in mind I could have written a number of falsehoods here, but I think it is pretty sound...

[Admiral Huddy]

Ok so basically the minimum requirement for your CPU will be based on the bus clock speed which matched the type of DDRx.

So in the table above, if you have a 266Mhz CPU running an FSB of 1066MHz effective, then the minimum RAM would be PC8500. This is because both the bus and the RAM runs at 533MHz.

If this is the case, what is the minimum requirement for the 1333MHz CPU when they arrive?

This is what promoted these questions in the first place :)

[semi-pro waster]

Ok I'll try again, DDR2 means that the actual speed of the Ram is doubled so PC4200 runs at an effective rate of 533mhz (2x266.5mhz) but the CPU requires 1066mhz i.e. double the effective rate and four times the actual speed of the Ram so that is why I call it quad-pumped.

If it helps to take a specific example then lets use the E6320 - clock speed of 1.86ghz and a multiplier of 7x - so you multiply the actual speed (minimum of 266.5mhz) of the Ram by the multiplier to get 1.866ghz.

This isn't getting into the finer technicalities of what Ram chips are capable of and ignoring the fact that I would always try to pair a Core2Duo with PC5300 or above at the very least it does work as a basic guide. :)

For 1333mhz CPUs you should need a minimum of PC5300 as 1333/4=333.5 or PC5300 speeds (667mhz effective).

[Admiral Huddy]

Sorted.. thanks.. what it was I read an article that C2D chips are reliant on dual channel because it effectively doubles the bandwidth ie PC8500 has a theoretical peak bandwidth of 17GB/s.. It's this that threw the spanner in the works a bit and I got confused. Although this is true, it's theoretical and memory requirements for DDR2 is still calculated by the FSB x DDR(2) = RAM speed (which is what you said).

Thanks for clearing that up.. My head hurts now :D