Power Mac CPU interchangeability

[Dystopia]

Some time ago Phil from Phil's Computer Lab did a pretty thorough investigation into how the Voodoo2 scales with CPU speed. His project never touched the Mac though. I've been thinking of doing a Mac version of that project.

To that end I'd need a suitable Mac and a variety of CPUs, ideally a range from about 100Mhz to about 1.4Ghz. On the PC side there's the Slot 1 440BX platform that can take nearly that whole range, but I suspect that's an aberration that can't be replicated on the Mac side. What I don't know is how interchangeable are old Mac CPUs. I happen to have a B&W G3 400 (my 6100 is out of consideration as it lacks PCI slots). I know there were a variety of third party upgrade cards, but they seem hard to find, and often absurdly expensive.

Apple first party CPUs are much easier and cheaper to get, but how cross compatible are they? Are they basically restricted to the Mac they were originally meant for, or can I put them in any Mac with a compatible socket? And if the latter where are the boundaries? If I can get away with using only 2-3 Macs for this project that's okay, but if I need a dozen that's a problem. With the G4s I'm given to understand the earliest G4 Power Macs were basically just G3s with a G4 processor card. Can I put one of those G4 cards in my G3? Can I put any G4 card from an AGP Power Mac G4 into say a Quicksilver? Is dual processor a CPU card feature or a motherboard feature i.e. can I put a DP card into a Mac originally sold as SP or vice versa? And what about pre-G3 PPC CPUs? That last one gets confusing quickly. 604 Macs seem harder to come by than 601 or 603 Macs. What can I put in them?

[MacTron]

The G4 modules from MDDs and Xserve are  compatibles betwen them. The G4 modules from Sawtoth to Quicksilver are  compatibles betwen them.
In both cases we have to take into account bus speed, and in less cases power supply to the module.

[Dystopia]

So an MDD CPU won't work in an earlier G4? With the bus speed issue, I assume a higher bus CPU in a slower motherboard will cause the CPU to run slower. Will the reverse overclock the CPU or is it controlled by whichever part has the slower bus? What's the power supply issue, do some of the PSUs not provide enough power for some possible CPU combinations? Also what about the G3? Mine has a 400Mhz CPU, can I put a slower one in?

Heh. I imagine it's not too often someone asks that last question.

[GaryN]

The MDD CPU will not mount in the earlier G4s. The layout and connectors are different.

[reeskm]

So just to clarify:

The 1.0 GHz G4 from an original Xserve (RackMac1,1) will not work in a quicksilver? Even though it has the same bus speed of 133MHz?

It makes sense the slot load CPU (1.33GHz from the RackMac1,2) won't work as the bus is 166 MHz.

[Dystopia]

I have procured a 450Mhz Sawtooth cheaply and would like to put an 800Mhz Quicksilver CPU in it. Are there any gotchas to watch out for? I can see from videos that the Quicksilvers had much more substantial coolers, can I just use the Sawtooth heatsink and attach a fan to it? Are the heatsink mounts different? If I inadequately cool the G4 will it throttle or will it cook itself?

[MacTron]

So just to clarify:

The 1.0 GHz G4 from an original Xserve (RackMac1,1) will not work in a quicksilver? Even though it has the same bus speed of 133MHz?

It makes sense the slot load CPU (1.33GHz from the RackMac1,2) won't work as the bus is 166 MHz.

This is not a Bus speed issue. It's related with the UniNorth chip version, The MDD and the xServe have an UniNorth 2.0 which is incompatible with the 1.x used in the old G4 family .

[reeskm]

The MDD and the xServe have an UniNorth 2.0 which is incompatible with the 1.x used in the old G4 family .

Thanks MacTron! That's the info I was looking for!

So it looks like my best bet is a CPU that is aftermarket for the QS, or the QS2002 CPUs, which should be compatible with my QS2001? I think the answer is yes...