front side bus on G5's

[radeon_cali]

Would any one know the real fsb of the g5? what i mean by real is:
The P4 systems say they have a fsb of 800 and how they got 800 is my 200 x 4 bits of data going down the pipe line so the real fsb would be 200 ...

I was thinking that the g5's would be somthing like 250x2 = 500 for each CPU then that would mean a fsb of 1000 for both cpus ..But i could be wrong!

This is bin on my mind for some time.. thanx for any info

[Whisper]

I don't know if the FSB is double or quad pumped, but I do know that each CPU gets its own bus. So instead of 250MHz*2 for each CPU, it'd be 250MHz*4 (or 500MHz*2) for a total of 2000MHz.

[kupan787]

If I am not mistaken, the G5 bus runs at 1:4 ratio currently. So a 2.0GHz G5 would have a 500MHz FSB. This is of course double pumped to the 1GHz figure. I think the G5 supports 1:6, 1:4, 1:3, and 1:2 ratios for its FSB. Effectively, if you look at the "double pumped" figures, that is more like 1:3, 1:2, 1:1.5, and 1:1.

Question for someone else out there. What is more important, throughput values or the MHz/GHz of the FSB? I know there is some linkage between the two, but if I remember correctly the P4's 800MHz FSB had the same theoretical peak data rate (7.2GB/sec) as the 1.8 GHz G5's 900MHz FSB. So that would lead me to think that they aren't really that related. And then there is the Opteron which has no FSB (it has a 800MHz Hypertransport link, which is effectivly the same thing, right?)

Also, I believe that the G5 bus is different (bi-direcitonal vs unidirectional?) so I am sure that plays into the data rate figures as well...

[Londor]

http://www.apple.com/g5processor/

"Magic Bus
And to get at that data, the PowerPC G5 features an industry-leading 1.25GHz frontside bus for each processor, offering a staggering 20GBps throughput on dual 2.5GHz PowerPC G5 systems. That’s a huge leap over the Power Mac G4, with a bus speed of 167MHz. That means you won’t have a bottleneck getting information to the chip for processing. Each G5 model features a bus that runs at half the speed of the processor. So you’ll get dual 900MHz frontside busses on a dual 1.8GHz Power Mac G5, dual 1GHz frontside busses on a dual 2.0GHz Power Mac G5 and dual 1.25GHz frontside busses on a dual 2.5GHz Power Mac G5."

[wmf]

The Pentium 4 bus is 6.4 GB/s.
The G5 bus is 4.4+4.4 GB/s (since there are separate read and write channels).