800MHz DDR2 single channel vs 667MHz DDR2 dual channel...

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KILOWATT

Hi everyones...thanks to read.

Two basic questions i could not get an answer after 1 hour of checking old
posts:

1- Since most Pentiums 4 CPUs sold are with a 800MHZ FSB, it is correct to
assume that the ram will always be a bottleneck unless i use the newer
800MHz DDR module?

2- If i use two 667MHz matched pair modules in dual channel, will the ram
fsb will still be a bottleneck compared to the 800MHz cpu fsb?

TIA for your reply.


--
Alain(alias:Kilowatt)
Montréal Québec
PS: 1000 excuses for grammatical errors or
omissions, i'm a "pure" french canadian! :-)
(If replying also by e-mail, remove
"no spam" from the adress.)
 
Bottlenecks aside, the 800 MHz is actually a quad pumped 200 MHz. So
for synchronicity using an equally fast RAM is recommended. The 667 is
a quadpumped 166 MHz. There would be some inefficiencies.

Another issue that you should look into is the bus width. Compare the
RAM and FSB bus widths. What really needs comparison is the total data
that can be sent by the RAM, and the total data than can be passed
through the Bus. Choose a configuration that optimises this. Getting
RAMs that exceed the FSBs capability would be a waste as would be the
converse.
 
KILOWATT said:
Hi everyones...thanks to read.

Two basic questions i could not get an answer after 1 hour of checking old
posts:

1- Since most Pentiums 4 CPUs sold are with a 800MHZ FSB, it is correct to
assume that the ram will always be a bottleneck unless i use the newer
800MHz DDR module?
Click to expand...

I don't think so. p4s use QDR (Quad data rate) to come up with the true
FSB rating. Their real FSB rating on a "800" MHZ chip would be 200MHZ.
if it took DDR it would be DDR 400 (pc3200). There is no such thing as
QDR SDRAM, at the moment. Not on the Commercial PC market anyhow. There
IS QDR SRAM, however.
2- If i use two 667MHz matched pair modules in dual channel, will the ram
fsb will still be a bottleneck compared to the 800MHz cpu fsb?
Click to expand...

See above. There is often a performance boost for Dual channel, BUT....
 
"KILOWATT" said:
Hi everyones...thanks to read.

Two basic questions i could not get an answer after 1 hour of checking old
posts:

1- Since most Pentiums 4 CPUs sold are with a 800MHZ FSB, it is correct to
assume that the ram will always be a bottleneck unless i use the newer
800MHz DDR module?

2- If i use two 667MHz matched pair modules in dual channel, will the ram
fsb will still be a bottleneck compared to the 800MHz cpu fsb?

TIA for your reply.


--
Alain(alias:Kilowatt)
Montréal Québec
PS: 1000 excuses for grammatical errors or
omissions, i'm a "pure" french canadian! :-)
(If replying also by e-mail, remove
"no spam" from the adress.)
Click to expand...

Don't forget that dual channel is 128 bits wide, and the
CPU bus is 64 bits wide. On a DDR based system, a FSB800
processor was perfectly balanced with two DDR400 DIMMs in
dual channel mode. (I.e. 6400MB/sec FSB is equal to two
3200MB/sec DIMMs).

If you look at the results here with a Netburst processor,
there is hardly any incentive to spend as much on memory
as you spent on the processor. Anything at or over DDR2-533
is enough.

If you used DDR2-800, the memory subsystem to FSB800 bandwidth
ratio would be 2:1, meaning the burst of read data is being
buffered in the Northbridge.

http://www.anandtech.com/memory/showdoc.aspx?i=2732&p=4

Paul
 
Thanks a lot for all your replies. Sorry for my very late reply.

From what i've read during the last few days... it seems that the memory
bandwidth must be close as possible as the CPU bandwidth. 6400 megabits/sec
is the theorical bandwidth of a 800MHz fsb cpu as is the one from a 800MHz
ddr2 module. I wish to buy the p5ld2 motherboard from ASUS, wich have a 800
MHz fsb. It only supports 667MHz ddr2 modules, nonetheless. But with dual
channel modules wich is supposed to give a 10% increase in bandwidth... i
suppose that the memory bandwidth will closely meet the one from the cpu.
 
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