Memory Bus Speed and Front Side Bus

[Razorfist]

Hi I am looking into putting together a computer and eventually overclocking it.

Looking at all the information about CPU speeds and memory speeds, I just have one question maybe someone can answer.

DDR2-800 has a memory clock of 200 MHz and an I/O Bus clock of 400 MHz.

Using this in combination with an e8400, multiplier 9x, FSB 333 MHz:

1. Would the default setup slow the memory down to 333 MHz?

Or...

2. Is the memory already too slow at 200 MHz?

In the event of #1, would the memory divider then be 1:1 with a FSB of 333 and memory at 333? Could I then overclock the FSB slowly, along with the memory trying to get close to 400 MHz FSB to match the rated DDR2-800?

If the memory wont go above 400 MHz, but the CPU will, would it be worth it to change the memory divider to something other than 1:1?

Say 450 MHz CPU and 400 MHz memory at 1:125? Is that even possible? What are the increments you can change the memory divider in?

If anyone could answer these questions I would appreciate it, as the answers will affect my decisions for purchases of parts.

 

[Mondoman]

Just worry about the datarate in each case. Remember that on Intel systems, all data to/from RAM has to go through the FSB, so it doesn't really help to run the memory bus at a faster datarate.
Take your FSB datarate (e.g. 1333MHz for e8400 default) and match that to the memory bus datarate. Since you will almost certainly be running the memory bus in dual-channel mode, divide the FSB datarate by 2 to get the datarate needed for a single RAM DIMM. Thus, for a 1333MHz FSB, to get a 1333MHz memory bus datarate, use RAM DIMMs running at 1333MHz/2 = 667MHz (DDR2-667) speed. Similarly, DIMMs rated at DDR2-800 will be able to keep up with up to a 1600MHz FSB without overclocking the RAM.
If you want to run the FSB faster than 1600MHz datarate, you will almost certainly have to overclock the RAM, as essentially all of the RAM available at specs faster than DDR2-800 is just overclocked RAM sold according to an overclock spec.

 

[BTRY B 529th FA BN]

Originally posted by: Mondoman
Just worry about the datarate in each case. Remember that on Intel systems, all data to/from RAM has to go through the FSB, so it doesn't really help to run the memory bus at a faster datarate.
Take your FSB datarate (e.g. 1333MHz for e8400 default) and match that to the memory bus datarate. Since you will almost certainly be running the memory bus in dual-channel mode, divide the FSB datarate by 2 to get the datarate needed for a single RAM DIMM. Thus, for a 1333MHz FSB, to get a 1333MHz memory bus datarate, use RAM DIMMs running at 1333MHz/2 = 667MHz (DDR2-667) speed. Similarly, DIMMs rated at DDR2-800 will be able to keep up with up to a 1600MHz FSB without overclocking the RAM.
If you want to run the FSB faster than 1600MHz datarate, you will almost certainly have to overclock the RAM, as essentially all of the RAM available at specs faster than DDR2-800 is just overclocked RAM sold according to an overclock spec.

Well put

 

[myocardia]

Razorfist, you're right about PC6400 only running @ 200 Mhz, however since DDR2 transfers data 4x per clock cycle, instead of two times, like "old" DDR, you get 800 Mhz worth of performance out of it. What does that mean to you? It means that an E8400 will run @ 3.6 Ghz before you even start overclocking the RAM, and all DDR2 overclocks some. Most decent, name brand PC6400 DDR2 will do ~450 FSB/900 Mhz DDR2, which gets an E8400 up to 4.05 Ghz.

If you're feeling really lucky, you could buy some PC8000, which will take an E8400 up to 4.5 Ghz, before you start overclocking the RAM. Of course, you aren't going to be able to run anywhere near 4.5 Ghz with air cooling, but cooling isn't what we're talking about, is it?

 

[Razorfist]

Thanks for the replies, this makes everything extremely clear.

Now... onto my next topic... the best air cooling solution for under $100 that doesn't deafen me. Gonna go find the forum for that and do some research on the net. I wanna hit that 4 GHz.

 

[Razorfist]

EDIT: I made a double post by accident, so...

I found the best cooling solution for me, the Xigmatek HDT-S1283

WOW. This cooler is amazing for an air cooling solution. Gonna attempt 4 with it. Will make a post of my system when I get it all up and running.