FSB and Mem. Clock speed.

[kioria]

Examplary components:

Pentium IV 2.4 Ghz Northwood B 533 FSB
Asus P4P800 AI
Samsung DDR 400 Mhz.

Technical Details:

1. The motherboard made by ASUS allows the CPU to run at 533 Mhz FSB
2. Samsung DDR 400 Mhz is then connected along side with the CPU, however some limitations were found.

Technical Problems:

1. P4B 533 Mhz FSB is quad-pumped architecture, 4 x 133 Mhz. And ASUS motherboard handles this in a perfect manner.
2. The problem lies within the link between CPU's FSB and Memory's clock speed.
- ASUS P4P800 AI automatically adjust or under-clocks memory's clock speed down to DDR 333 Mhz, 2 x 166 Mhz.
- BIOS excludes an option of having a clock speed of 400 Mhz for a ram module.

The part I don't get is, why does the motherboard underclock my DDR 400 Mhz down to DDR 333 Mhz? Where is the link hidden behind this?
For all the intel fans out there, can you please guide me the details of how intel chipsets/fsbs/memory clock speeds work?

Thanks

 

[johnjkr1]

For the most part, its better to run FSB and RAM synchronous anyway..so why worry?

 

[boshuter]

If I read you right, the motherboard is using SPD to set ram timings, speed, etc., this has nothing to do with the mb. I'ts programed into the memory by the manufacturer (Samsung). It does the same thing with my Corsair, just set your memory speed and timings manually and don't worry about the "automatic" stuff.

For the most part, its better to run FSB and RAM synchronous anyway..so why worry?

This is not quite correct IMO, I get better performance and benches running my memory at 5:4 with 2-2-2-3 timings than I do at 1:1 with slower timings. (this has been argued over and over and over...)

 

[kioria]

Originally posted by: johnjkr1
For the most part, its better to run FSB and RAM synchronous anyway..so why worry?

What difference does it make between having my FSB and DDR in synchronous or non-synchronous mode?

 

[johnjkr1]

From Toms Hardware:

"Anyone who regularly uses different chipsets will soon realize that the models from the house of Intel do not generally allow asynchronous memory operation. The reason lies in the only slight advantages offered by this mode: The overhead or wait times are simply so large that the theoretically higher data rates cannot be translated into performance gains. "

http://www20.tomshardware.com/motherboard/20030812/ddr500-01.html



"The reason for the non-existent power increase is the fact that the memory can no longer be run synchronously with the processor speed or, more precisely, the FSB speed. An Athlon XP FSB with a speed of 266/ 133 MHz (double-pumped), for example, works excellently and without delays with a memory run at the same speed; so does the Pentium 4 at 533/ 133 MHz (quad-pumped), including DDR266 at 133 MHz (or PC1066 RDRAM at 533 MHz double-pumped).

However, if the main memory is operated asynchronously, wait times need to be figured in now and then. The data transfer is like jumping onto a moving merry-go-round: more often than not, you have to wait for your "chance." With synchronous or pseudo-synchronous transfer, there are no unnecessary wait times, which is why Intel prefers this model in the E7205.

The next generation of Pentium 4 with 800 MHz FSB (200 MHz quad-pumped), too, is only being operated synchronously for now, and it will actually work with DDR400.
"