ACTUAL G5 front side bus speeds?

[simie]

http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/31366.pdf

This is the only white paper regarding the 64FX that I could find.

 

[psycho bob]

If you don't understand latencies, then you won't understand why AMD has an inherent memory bandwidth advantage (at least for now). If G5's could run their memory at cas 2 and Trcd (ras to cas) 2, they'd gain a substantial amt of bandwidth, but w/ the delay of communicating to a northbridge on the motherboard, the benefits of these low latencies are considerably reduced.

Considering I haven't really looked into the G5 firmware, I wonder if it is even possible to change those latencies in the OS. With Windows, it is done rather easily in real-time, so I'm sure if someone develops to software, Macs can at least get cas2 and trd2.

While looking through an Apple developer doc for something else I came across the CAS compatibility chart for the G5.
The officially supported lantencies are; 2, 2.5, 3, 4 and 5. No need to go fiddling with firmware just put the RAM in.

 

[Mav451]

Just b/c its supported doesn't mean it will run at that speed. How do you know if the G5 will default to JEDEC 3200 spec cas3 for stability's sake? On alot of PC motherboards, even if they are capable of 2-2-2-5 latencies, they won't initially run at that until you go into BIOS to set it.

If someone has a friend with Corsair XL or G.Skill RAM in their computer, ask them if you can borrow it and put in in your G5. If it does run at 2-2-2-5 w/o user intervention, I'll be pretty impressed, but I won't be shocked if it defaults to 2-3-3-6, or the standard 3-3-3-8.

 

[solvs]

So basically this thread crumbled into a "My FSB is better than your FSB."

What is the fastest FSB? Is it the 1.25 Ghz on the 2.5 Ghz G5 or something from AMD? Am I right or is my prof right?

There is no easy answer. The G5 is 1.25. Not Apple's marketing, that's what it is. The AMD is 2.6, but it's also 800. I'd just let it go.

 

[psycho bob]

Just b/c its supported doesn't mean it will run at that speed. How do you know if the G5 will default to JEDEC 3200 spec cas3 for stability's sake? On alot of PC motherboards, even if they are capable of 2-2-2-5 latencies, they won't initially run at that until you go into BIOS to set it.

If someone has a friend with Corsair XL or G.Skill RAM in their computer, ask them if you can borrow it and put in in your G5. If it does run at 2-2-2-5 w/o user intervention, I'll be pretty impressed, but I won't be shocked if it defaults to 2-3-3-6, or the standard 3-3-3-8.

Do you have a tool that would measure RAM latency on the mac? I'd be interestd in finding out but have got a clue hoe it could be measured without third party software.

 

[ddtlm]

psycho bob:

Both the AMD K8 cores and the G5 have memory bandwidth of 6.4GB/s as they essentially operate in the same way the only difference being the route the data takes. So are we saying that the only real difference between the G5 and K8 is the reduced latency of the latter?

Yes. Often enough, all that matters is how fast you get your data from RAM.

ajkst1:

What is the fastest FSB? Is it the 1.25 Ghz on the 2.5 Ghz G5 or something from AMD? Am I right or is my prof right?

AMD wins easily. (Though in both cases the FSB has more bandwidth than the system RAM, AMD has a much lower latency, which is very important.)

Bigheadache:

I for one hope that an integrated memory controller appears somewhere on the PowerPC roadmap.

Moto is on it, the MPC8641, based on the G4. Should uncork some performance, depending on the task, but who knows if Apple will use it. Note 1MB L2, dual channel DDR2, PCI-express support, not bad at all.

http://www.freescale.com/webapp/sps/site/overview.jsp?code=DRPPCDUALCORE

They even had a nice PDF:

http://www.freescale.com/files/32bit/doc/fact_sheet/MPC8641DFACT.pdf

edit: Hey, the 8641 even has four integrated gigabit ethernet controllers. Someone could make such a sweet minimac wrapped around a 8641D (the dual core version).

 

[wrldwzrd89]

Just b/c its supported doesn't mean it will run at that speed. How do you know if the G5 will default to JEDEC 3200 spec cas3 for stability's sake? On alot of PC motherboards, even if they are capable of 2-2-2-5 latencies, they won't initially run at that until you go into BIOS to set it.

If someone has a friend with Corsair XL or G.Skill RAM in their computer, ask them if you can borrow it and put in in your G5. If it does run at 2-2-2-5 w/o user intervention, I'll be pretty impressed, but I won't be shocked if it defaults to 2-3-3-6, or the standard 3-3-3-8.

I remember reading somewhere (maybe it was on these forums?) that what the G5 actually does is slow the memory down. The G5 can reduce the memory's speed as much as it wants to - even down to 100 MHz (DDR50) or lower, if that's what's needed before the G5 likes the memory speed. It won't do this if you're using standard 3-3-3-8 RAM though.

 

[psycho bob]

I remember reading somewhere (maybe it was on these forums?) that what the G5 actually does is slow the memory down. The G5 can reduce the memory's speed as much as it wants to - even down to 100 MHz (DDR50) or lower, if that's what's needed before the G5 likes the memory speed. It won't do this if you're using standard 3-3-3-8 RAM though.

Never heard of this. The G5 will slow faster DDR modules down to the machines spec, so fitting DDR500 would be slowed to 400. The G5 does have the ability to slew the sytem bus (If I remember correctly, it can certainly slew the CPU speed) depending upon usuage.

 

[wrldwzrd89]

Never heard of this. The G5 will slow faster DDR modules down to the machines spec, so fitting DDR500 would be slowed to 400. The G5 does have the ability to slew the sytem bus (If I remember correctly, it can certainly slew the CPU speed) depending upon usuage.

I'm not terribly surprised that you haven't heard about this - I have to get ready to go to work, so I don't have time to hunt down a link right now. Maybe I'll get around to it later (when my iLife '05/iWork order FINALLY ships )