"OK - I guess the right question is "what has KEPT Apple from adopting Intel (or AMD) processors?" Someone else said it's a cloning issue, but I think Apple still has enough of a hold over their operating system that they would be able to differentiate their systems, even WITH an x86 CPU."
There are several interacting issues at work here.
First, the issue is not using an Intel processor qua processor. If Apple switched to a Pentium 4 but provided no support whatsoever for Windows-type applications, what would be gained for the user?
(Put another way, at the OS level, a user is and should be agnostic to the issue of which processor is used, unless he intends to swap OSes around. This relates to the next point.)
Second, at the time Apple picked the 68000, the advantages of the flat and large address space for what they intended to do were substantial. (At the time IBM picked the 8088 for its PC, the advantages of die cost and manufacturability tipped the balance away from the superior design of the 68000 toward the 8088.)
Once Apple based the Mac on the 68000, with little or no perceived need to support x86 instructions directly, the progression to later 68Ks was clear. The time when Apple _might_ have profitably made a switch to x86 was around 1992-3, when it was pondering the transition to PowerPC with on-the-fly emulation of 68K code. Motorola and the consortium developing the PowerPC (incl. Apple and IBM) worked extensively on this code emulation. And it worked smoothly, as I can attest.
Third, no doubt Apple as been looking very closely at Motorola's lagging performance and decision to emphasize cell phones and other small gadgets for its processors. No doubt Apple has been debating a jump to the Intel ship, while also beating on Motorola and negotiating with IBM.
Fourth, this really illustrates a deeper issue which needs a lot more discussion: the "all the eggs in one basket" with IA-64 when in fact designs and processing _can_ be decoupled.
(To give a hint of where I'm going with this, look at H-P's claims that they could not afford to continue to develop advanced processors based on PA-RISC by themselves, that wafer fabs such as Intel was building have just gotten too expensive. So now we have essentially all computer vendors, except Sun and IBM, awaiting a good performance version of the Itanium. While I am hopeful that Madison, Deefield, Chivano, etc. will give the performance needed, it's painfully clear that the consolidation of manufacturing and design gives _less_ opportunity for evolutionary exploration and learning. Instead of several competing families of architectures, using the best processsing money can buy (at IBM, UMC, TSMC, even Intel), we have a gigantic chip using EPIC architecture...which may turn out to be an expensive detour.)
This last point needs, I admit, a much longer analysis. I'm reminded of a similar effort to consolidate many language proposals into one, grand, canonical Swiss knife of a language: Ada.
As for Apple, the IBM ultralight version of the Power4 looks like a good choice. Apple doesn't need to be "Intel compatible" if in fact they don't plan to be directly compliant.
(And consistent with my thesis above, the trends favor Unix and Linux sorts of OSes, not ".NET" private sandboxes. Or so I am betting.)
--Tim May
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