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20" iMac CD CPU Upgrade

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macrumors 6502
Original poster
Jul 5, 2005
329
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If i upgrade my 20" CD to a C2D will it be just as much of a 64-bit machine as the iMacs that originally came with a C2D? Also would I be able to upgrade the RAM to 3GB instead of the 2Gb limit that I have now?

Thanks in advance for any info.
 
Upgrading the CPU would not give your machine the ability to use 4gb. It's a limitation of the chipset, not the cpu.

It should be functionally equivalent to a white C2D iMac.
 
I didn't think it could take 4GBs, I was asking about 3GBs like the Apple C2Ds.

And yes I know the Al iMacs can take 4Gbs.

Has anyone here personally upgraded the CD to a C2D and can confirm that the machine can then go to 3GBs? I wouldn't want to go to the trouble to upgrade the CPU and still be at 2Gbs.

Thanks
 
NegaTV1--

Like your sig...
and I want one of those 1200 baud modems SOOOO bad!
 
You're limited by the firmware that Apple provides for EFI. You're going to be stuck at 2 GB but the socket at least gives you an upgrade path from Yonah to Merom.
 
Eldorian,

So you tried this or know of someone who tried and was stuck at 2GB? This would kill craking the case for me -- a faster CPU would be great, but not if I can't crank up the RAM to at least 3GB --- aperture has become a RAM hog..
 
Eidorian said:
You're limited by the firmware that Apple provides for EFI. You're going to be stuck at 2 GB but the socket at least gives you an upgrade path from Yonah to Merom.
Click to expand...

I keep getting conflicting reports on this issue. Some people say its the memory controller, some say its the firmware. All I know for sure is that swapping the CPU will not help. People with CD and CS minis who have moved to C2Ds have had no luck upgrading their RAM to 3GB.
 
flopticalcube said:
I keep getting conflicting reports on this issue. Some people say its the memory controller, some say its the firmware. All I know for sure is that swapping the CPU will not help. People with CD and CS minis who have moved to C2Ds have had no luck upgrading their RAM to 3GB.
Click to expand...
The Core Duo and the original Core 2 Duo iMac use the same chipset, 945PM, and the latter has a 3 GB RAM limit compared to the 2 GB of the former.

Over on the Windows only side the 945PM/GM usually tops off around 3.25 GB of available system RAM.
 
Eidorian said:
The Core Duo and the original Core 2 Duo iMac use the same chipset, 945PM, and the latter has a 3 GB RAM limit compared to the 2 GB of the former.

Over on the Windows only side the 945PM/GM usually tops off around 3.25 GB of available system RAM.
Click to expand...

Can you find an example of a Windows CD laptop that had a 3.25GB limit? That would put the nail in the coffin of firmware vs hardware.
 
Well, after poking around I have found that CD MacBooks can support a single 2GB module but no more. So I would guess it is a firmware rather than a hardware thing. iMac CDs could probably support 2X2GB if the firmware allowed. The CD or C2D doesn't seem to come into. Anyone want to hack their EFI? Didn't think so... ;)
 
A few bits of info that might be useful if you're looking into this:

If I remember rightly, the core duo chips were not 64bit, the core 2's are. But 32 bit chips can address 4gb of ram anyway - 64bit can address more.

The 3.25gb thing probably isn't a hardware limit - it's a limit within windows. I have a core2 imac, with 4gb of ram. Osx sees 4gb, windows sees only 3gb. I looked into it, and windows can see upto 4gb, because it's 32bit (despite the hardware being 64bit) but it uses a lot of memory to give access to other things than RAM, like the video card. That takes upto 1gb, which is why I see only 3gb.

The 2gb limit is possibly hardware based though - it's common for the chipset or motherboard to only support a 'reasonable' amount of RAM - i.e. what would be considered a lot when it was made. The reason is that as you support more RAM, you need more wires on the motherboard, more pins on the chipset, and more inside the chipset to support it. That all costs extra, an makes the computer more complex to make and more prone to problems.
 
psonice said:
A few bits of info that might be useful if you're looking into this:

If I remember rightly, the core duo chips were not 64bit, the core 2's are. But 32 bit chips can address 4gb of ram anyway - 64bit can address more.

The 3.25gb thing probably isn't a hardware limit - it's a limit within windows. I have a core2 imac, with 4gb of ram. Osx sees 4gb, windows sees only 3gb. I looked into it, and windows can see upto 4gb, because it's 32bit (despite the hardware being 64bit) but it uses a lot of memory to give access to other things than RAM, like the video card. That takes upto 1gb, which is why I see only 3gb.
Click to expand...

Not true. On Napa-based iMacs, MacBooks and minis the upper limit is fixed at 3.3GBs regardless of Windows or OSX.

The 2gb limit is possibly hardware based though - it's common for the chipset or motherboard to only support a 'reasonable' amount of RAM - i.e. what would be considered a lot when it was made. The reason is that as you support more RAM, you need more wires on the motherboard, more pins on the chipset, and more inside the chipset to support it. That all costs extra, an makes the computer more complex to make and more prone to problems.
Click to expand...

Again, not true. CD MacBooks have been shown to support single 2GB modules so the hardware can support it.
 
flopticalcube said:
Not true. On Napa-based iMacs, MacBooks and minis the upper limit is fixed at 3.3GBs regardless of Windows or OSX.
Click to expand...

Right.. guess there are two issues then. OSX can certainly see more, so if OSX is seeing 3.3gb too then it's definitely hardware. Windows still won't see more than ~3gb though, that's definitely a software limit. Thanks for the correction.

flopticalcube said:
Again, not true. CD MacBooks have been shown to support single 2GB modules so the hardware can support it.
Click to expand...

That's somewhat irrelevant - it doesn't matter to the chipset if it's 1x2gb module or 2x1gb, if it only has enough bits to see 2gb in total at any point between the CPU and the RAM slots, then that's the limit.
 
No sure how we got on to Vista...

So is there a consensus that if I upgrade my CD iMac to a C2D iMac, OSX will still not be able to access any more than 2GBs of RAM?
 
Is there no-one out there brave enough to tinker with Apple's EFI implementation? Perhaps there is a string somewhere in there along the lines of 'HangIfMoreThan2GBRamInstalled=true'. I would love to try but unfortunately do not have the skillz.
 
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