Memory help

[Fabio Santoro]

I bought a ASUS motherboard for Pentium 4....and I'm planning to put a 2.GHz
HT 800 MHz FBS. Prescott CPU but I currently have a AMD system with a
ATLHON XP 2200+ CPU with a 512MB PC-2100 memory module....if I use that
memory module in that configuration how will be the performance?

 

[Fabio Santoro]

I wrote the CPU wrong....it's a 2.8GHz HT 800 MHz FBS. Prescott CPU

I bought a ASUS motherboard for Pentium 4....and I'm planning to put a but

I currently have a AMD system with a

 

[Jim]

If you mean use the PC2100 from the other system in the new P4 mobo, well,
assuming the mew mobo supports memory options of 266/333/400 DDR, you'll be
running your memory at the lowest option! And asynchronously to the CPU
FSB. That pretty much tells the story, and it's not a good one.

Your P4 800MHz is actually FSB 200MHz (clock). The PC2100 memory is 133MHz
clock. To use these in combination requires a CPU/DRAM ratio of 3:2
(probably supported by the chipset, assuming this is the 865/875 Intel),
calculated as (200 / 3 * 2 = 133).

In general, it's best to run the CPU FSB and memory clocks "in sync", you
just get better performance. Ideally that P4 800MHz would be much better
served w/ PC3200 (200MHz clock). And if you intend to overclock the CPU,
matters get only worse. Every increase in CPU FSB also increase the memory
clock, and since the PC2100 taps out so early, there's little if any
headroom in that memory for overclocking (not without increasing the
CPU/DRAM ratio even further). For people who intend to OC the CPU FSB, they
often look for *more* than PC3200 (e.g., PC3500, PC3700), just to prevent
this sort of problem. But for non OC'ers, PC3200 is the best match.

Anyway, as long as the chipset supports a 3:2 CPU/DRAM ratio with those
component, it all still works. But memory will be a drag, no doubt about
it. Most system suffer from memory bandwidth limitations anyway, even using
PC3200. By using PC2100, you're already losing 1.1Gb/sec (theoretically).
How this affects real world applications just depends on how you intend to
use the system. Obviously, memory intensive applications will suffer most.

And we didn't even talk about dual channel yet. While rather overblown as
to its affects on performance, even the worst cases show a 5-10%
improvement. But unless you have at least a "pair" of matched memory
modules, you can take advantage of it. So if that PC2100 512MB is only a
single stick, you're even worse off -- you can't even exploit the dual
channel features of your new mobo! (assuming it is dual channel capable).

HTH

Jim

 

[Fabio Santoro]

Thank you for responding.

So Jim this will be the same senario as in the past with PIV on 400Mhz. FBS
and using PC-133 SDARAM.

I bought a ASUS P4C800 Motherboard with INTEL 875P Chipset ( called
Canterwood, i think).

The manual says on 800 Mhz FBS configuration 266/333/400 Mhz DDR can be
used.

 

[Fabio Santoro]

Thank you for responding.

So Jim this will be the same senario as in the past with PIV on 400Mhz. FBS
and using PC-133 SDARAM.

I bought a ASUS P4C800 Motherboard with INTEL 875P Chipset ( called
Canterwood, i think).

The manual says on 800 Mhz FBS configuration 266/333/400 Mhz DDR can be
used.

 

[Jim]

To restate, PC2100 will work as long as the mobo indicates so in the spec
(133MHz clock, or 266MHz DDR) *and* there's a 3:2 CPU/DRAM ratio supported
by the chipset to insure both the CPU and memory can actually run at spec,
at the same time (as I recall, this is the case).

IOW, what the mobo specs show is ALL the options on CPU and memory, but what
they don't explicit say is which *combinations* are supported, that requires
an examination of the manual. You can't, for example, run a P4 800MHz CPU
(w/ 200MHz clock) and PC2700 (166MHz clock) AT THE SAME TIME (not at clock
anyway) because that would require a 6:5 CPU/DRAM ratio (200 / 6 * 5 = 166),
which is not supported in the Canterwood chipset. The closest available
CPU/DRAM ratio is 5:4 (200 / 5 * 4 = 160). If you installed an 800MHz CPU
*and* PC2700 together, for example, the Asus mobo would have no choice but
to underclock the memory slightly (160MHz) using 5:4 to make it run at all.
IOW, that PC2700 would actually be running 320MHz DDR (2 x 160Mhz clock),
which is PC2560 (320 * 8 bit wide).

To illustrate, examine pg. 2-11 of the P4C800-E Deluxe manual, notice the
asterisk next to the PC2700 entry on the 800MHz CPU line, in the matrix (
ftp://ftp.asus.com/pub/ASUS/mb/sock478/P4C800E-DX/e1347b_p4c800-e_deluxe.pdf
). That's why that's there!

But back to your PC2100, the only *caveat * is, it will run asynchonously to
the 800MHz CPU in terms of clock, thereby not ideal. Of course, I'm
assuming you would never undeclock the CPU from 200MHz down to 133MHz FSB
*just* to gain synchronization w/ the PC2100! If that's what you're
thinking, no. That would be letting the cart drive the horse, and only make
matters worse.

Jim