Could somebody explain the difference in speeds between RAM (bus) and
CPU (fsb). Do they have to run at the same speed to take advantage of
maximum performance? I have a Atlhon 2.2mhz xp. Right now it is only
running at 1.8mhz because I have regular ddr 266 ram. Since the price
of higher RAM speed is more expensive, I want to know if it is worth
(the money) to upgrade to a higher speed or to just buy more regular
ddr266? Remember that I am not using my CPU at full speed?
Thanks
Rod
http://www.qdi.nl/support/CPUQDISocketA.htm
The PR or Performance Rating is 2200, while the actual clock rate
is 1800 at stock speeds.
2200+ 1800MHz/FSB266 FSB266 = 2*133MHz clock.
64bits 64bits
Processor------------Northbridge-------------DDR_DRAM
| x266Mt/s x266Mt/s |
| |
133MHz 133MHz
Clock Clock
Some Northbridge chips have rate adaptation between DRAM
and the processor. That means the transfer rate on one
side can be different than the other side. As to whether
this helps or not, and to what degree, varies from chipset
to chipset.
An efficient chip design changes how it passes data between
sides, when it is known that both sides of the chip are using
the same clock. This is termed synchronous operation.
The Nforce2 Northbridge, for example, demonstrates better
performance when it is run in sync. It even has a bug,
where data is corrupted (in dual channel mode?) when the
sides are not synchonized.
It is possible, if the DRAM was lifted quite high (DDR400,
while the FSB is 266 million transfers per second), that
performance again could be increased. This is trading off
clock speed, versus the losses caused by having to use
resync circuitry to clean up the clock rate difference.
However, the Northbridge might not support such a ratio
between the two sides of the chip.
In the manual, I see this table:
FSB DDR266 DDR333 DDR400
200/266MT/sec Yes Yes Yes
333MT/sec No Yes No
which means the designers felt only synchronous operation
made sense at 166MHz (FSB333). The first line of the
table means you can experiment with DDR400 if you want,
and it probably does make a slight difference.
I'm afraid you'll have to visit some of the review sites
and see if any of them have useful benchmarks for the
conditions you are interested in. If you look here for
example, DDR400 was slower than DDR333:
http://www.anandtech.com/mb/showdoc.html?i=1696&p=7
If you do buy DDR400 memory, it is backward compatible
with DDR333 and DDR266, so you can experiment with whatever
clock rate combinations your motherboard supports.
I think it is fair to point out, that the improvement in
memory performance does not strongly influence application
performance. At the best of times, a 30% improvement in
memory bandwidth, might give a 10% faster application
performance. So, you'll be spending cash, for little gain.
If anything, you could get a 3000+ and buy some PC3200 CAS3
and run it at DDR333. That is probably the best you can do
with your board (short of experimenting with overclocking
a Mobile processor, and I don't know how your BIOS would
react to one of those). That might give you a 20%-30%
improvement, but would you even notice ?
http://www.asus.com.tw/support/cpusupport/cpusupport.aspx
Athlon XP 3000+(333 MHZ FSB)(Model 10)(Barton) ALL ALL
$146USD retail boxed Newegg
http://www.crucial.com/store/listpa...ards&mfr=ASUS&cat=RAM&model=A7V8X-X&submit=Go
The CT6464Z40B is CAS3 DDR400, and the CAS timing is the same,
at 200MHz=5ns, 5nsx3=15ns CAS, as the slower clocked CT6464Z335
PC2700 CL=2.5, which at 166MHz=6ns, 6nsx2.5=15ns CAS also. The
PC3200 is only $3 or $4 more, and the resale value of PC3200
will be higher than other memory speeds, when you want to sell it.
So, one stick of Z40B would be $97US. (You can only use two
double sided sticks at DDR333 speeds, and only one at DDR400,
so keep those limits in mind when buying the memory. I.e. You
should plan on running DDR333 most of the time, and when you want
to experiment with DDR400, only do it while running one 512MB
double sided stick.)
Paul
