It's possible to disable the unused second CPU in Open Firmware with the following commands:

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That should save your electricity bill a few quid and lower the temperature a bit too. :)

What is the keystrokes on a G5 to get into the OF?

alt+lcommand+o+f afair

Will this also work on a MDD powermac? is it something that needs to be done at every boot, or will this stay once you do it?

I imagine itwill work on a G4 too and it sticks until you rest the nvram.

However..... When I check the CPU count in OSX it shows only one CPU but when I check the count with MorphOS it shows two CPU's still.

I suspect this is because it is passing the boot argument to the OSX kernel rather than disabling it for any OS that is installed.

Any MorphOS devs reading this that could tell us if it's possible (now or at a future date) to disable the 2nd CPU in MorphOS?

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I'd rather try to boot a linux liveCD and check if linux sees 1 or 2 cpus. Maybe it's just that MorphOS reports the cpu model and not the current state of it ?

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Good idea, I'll try it later tonight unless someone beats me to it.

If MOS is only reporting the CPU model then disabling the 2nd CPU has not made a difference to the temperature. Though thinking about it logically this makes sense as it isn't being used by MorphOS even when enabled so it won't be drawing much electricity nor generating much heat.

If I disable and then pull one of the cpus will MOS get confused?
Can I run with just one?

Just got my heatsinks/cpus in.
These things are HUGE.
They weigh 4 pounds each!
I'd really like to leave one of them out of the machine.

The machine will not boot without both of them physically installed.

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That is strange, as I have a damaged Dual 2.5GHz G5 PowerMac that only reports one of the two installed G5 CPU's as working, and when I swap them for each other, it still reports the same unit number as working. So, this leads me to believe that the fault is with the motherboard, not either of the G5 CPU modules. It has no problem in booting MacOSX with just the one G5 CPU module being reported as working, so I wonder why the PowerMac won't boot if one of the two G5's is removed from the system?

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Have you tried resetting the nvram?

Hold Cmd-Opt-P-R and power on, keep the keys held down until you hear the boot chime for the second time.

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In the Apple developer tools there was a thing called CHUD tools processor pref pane that let you disable a CPU in a dual machine.
You can also do it through terminal. I don't recall if it survived reboots. If you dual boot, it might be worth taking a look.

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Thanks, I'll look into it and report back.

Hmmm.... Maybe with one removed it will boot then. I just seemed to remember attempting it when my 2.5 G5 killed a CPU from a cooling leak. Give it a try, Im curious now.

I remember someone told that if you remove one CPU the system fans will run 100% all the time. If you try, let us know if this is true or not.

Any news on this topic?

> Any news on this topic?

Seems Intuition's suspicion in posting #5 was correct:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=11055&forum=11&start=23

I tried the OF lines on post #1. My system works fine (I think) but I can't really tell the difference.

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In other words: setting cpus to 1 in OF ist just a hint for the OS. It's up to the OS to actually deactivate one CPU. Thus, wenn booting MOS, both CPUs will bi active then...?

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Has anyone confirmed that this actually lowers power consumption when running MorphOS on a dual G5 PowerMac? I read in another thread that even though the 2nd CPU can be disabled for use by the OS and software, it does not prevent it from using the same amount of electricity, or producing heat.

If it actually can be turned off and saves electricity and heat production, I will definitely use these commands on my dual 2.7GHz G5. I am not sure how to test this, and don't have an amp meter to check the amount of power being used before and after these commands are executed to run MorphOS.

Even without the 2nd G5 CPU disabled using these commands, I wonder if both G5's ramp up to the same operating speed, when any MorphOS software demands more computing power from the one G5 CPU it is accessing? I would guess that the 2nd G5 remains at an idle speed constantly while running MorphOS, but again, I have no way to test and confirm this guess.

> Has anyone confirmed that this actually lowers power consumption when running MorphOS
> on a dual G5 PowerMac?

No.

> I wonder if both G5's ramp up to the same operating speed, when any MorphOS software
> demands more computing power from the one G5 CPU it is accessing? I would guess
> that the 2nd G5 remains at an idle speed constantly while running MorphOS

Both G5 CPUs remain at maximum clock speed constantly while running MorphOS.

I tested it long time ago (along with PB G4) and the power draw was the same as with 2 CPUs. I stored data results somewhere but can't find it atm. I can do the tests again someday if needed.

[ Edited by Korni 29.08.2015 - 18:53 ]

I think i will stick with a single cpu g5

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No need to test again, I trust your results from the first test you did.