[reader50]

According the wikipedia, the U4 (CPC945) memory controller used with dual-core (970MP) PowerMac G5s can handle up to 64 GB.

Apple only rates dualcore G5s to 16 GB, using 2GB DDR2 modules. These were probably the biggest sticks available to test with at the time. Today, newegg has 4GB module pairs at high prices, didn't see any 8GB modules.

So it looks like we can go to at least 32 GB, 64 if 8GB modules become available. Has anyone tried the 4GB sticks, and gone beyond 16 GB? I'd try it if the kits came down further.

[turtle777]

Off Topic: why would you invest that much into such an old machine ?

-t

[imitchellg5]

Not sure that OS X could even address that much RAM with a G5. And certainly most applications can't.

[reader50]

Investing time and effort into maximum upgrades is seldom done because you need it. It's the fun factor instead.

And it's official. I dug up IBM's specs for the CPC945 northbridge. On page 129, the overview of the DDR2 Memory Controller:

• Support for 8 ranks. 8 DIMMs max (4 double-sided pairs).
• 64 GB max capacity using 4 pairs of 8 GB double-sided DIMMs. (Only 62 GB addressable due to 2 GB I/O hole.)

We already have the max number of slots (8) the controller supports. So it will accept up to 8GB modules, for maximum addressable space of 62 GB.

[Lateralus]

Please do this. You will be my hero forever.

[P]

Amusing. And I think that OS X will address all that RAM, but few applications will be able to make use of it.

[macaddict0001]

Is there a ramdisk utility for OSX? That would be one fast SSD. Just out of curiosity, do you know the true ceiling for 970fx models as well?

[reader50]

The 970 / 970FX models use the U3 (CPC925) controller. I didn't find the user manual, but an IBM developer page shows specs in a sidebar.

• the DDR400 dual channel memory subsystem with up to 128-bit RAM, supporting as many as eight 64-bit double-sided DDR DIMMs of up to 1GB each (max system capacity of 8GB), at speeds of up to 200MHz (DDR 400). The IBM CPC925's memory controller supports error correcting code (ECC) for error detection and correction.

This matches the official Apple answer of 8 GB for 8-slot motherboards. Without the user manual though, the only way to be sure would be to plug in a pair of 2 GB DDR modules.

[Big Mac]

I'm pretty sure I've read 2 GB modules are recognized by the U3 controller.

[Todd Madson]

The G5 (well, at least my 2.5 dual) uses PC3200 ram which is of a different form factor than the DDR2/DDR3/DDR4 ram memory.

Good luck finding a 2 GB PC3200 stick that isn't a fortune. I have yet to see a real one in person, all I ever see is 1 gig. I've got 7 in mine presently.

If I ever find a PC3200 2 gig stick I'd pop it in there just for the fun of it but the 2 gig sticks I've seen were all server memory and quite expensive.

[Todd Madson]

I should also mention My G5 came with a manual addendum claiming it could run 16 GB of PC3200 should I be able to find said chips.

[macaddict0001]

Nowhere in the manual for my dual 2.3 7,3 G5 does it mention a max capacity. users guide
Specs are on page 79.
It does however specify 512 Mbit chips for 1 gig modules, and it says on page 49 the maximum number is 16.
I went to Tiger Direct and didn't see any 2GB sticks just now, and since all of the new faster memory no longer uses the 184 pin format its not like you can use that instead.

[reader50]

There's a bunch of DDR 2GB ECC modules. I found one non-ECC module for $110 per stick.

[Todd Madson]

See what I mean? Very expensive. $852.40 (tax not included) for 8 chips so 2 x 8 = 16 gigs.

Kinda spendy for all that old tech but cool just the same. I bet maxing a iMac Corei7 to 16 gigs would be cheaper.

That being said...

My G5 is finally maxed out at 8 gigs now - a friend was moving and found two pc3200 1 gig sticks and he just gave them to me.

I'm not sure this computer will live long enough for me to find big 2 gig sticks affordably.

And the memory would help. One of my music sessions was over 6.3 gigs of ram today....

[Tuoder]

I don't see a non-ultraspecialized application for so much RAM. Anything over 8 would be overkill in such a machine, IMHO.