Why does nearly every single "mac news" site keep posting speculation crap about this issue when anyone can see what was done? The kernel is open source. What changed is public knowledge. And yet every article still is still trying to spin a big conspiracy. The more of these stories I see, the more I'm driven away from sites I normally visit due to the writers clearly trying to build hype out of nothing.
Here, let me repost my comments from another site to explain what actually changed.
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I decided to browse through the kernel code in 10.6.2 (now that its out), and here is what I found.
What specifically killed the Atom is in xnu-1486.2.11/osfmk/i386/cpuid.c, around lines 600.
In the past, the kernel would check if the CPU family was 6, and if the model number was higher then 13. This basically means any Intel CPU newer then the Pentium M Dothan (used in the Apple TV) was supported.
The new code now specifically checks for family 13, 14 (Yonah), 15 (Merom), 23 (Penryn), 26 (Nehalem), 30 (Fields), 31 (Dales), and 46 (Nehalem_EX). The Atom is 28, so it doesnt pass the check, and hits code at line 649 that panics with Unsupported CPU.
Interestingly, in xnu-1486.2.11/osfmk/i386/cpuid.h, code was added to define models 28, 30, 31 and 46. Even though the kernel now panics with 28, CPUID_MODEL_ATOM is defined where it wasnt before.
I cant say for certain why the change was made, but to me, it was a rework of a previous flawed check (is CPU model 13 or higher), and replaced it with one that checks for exactly what the kernel wants. There is a decent bit of code in the kernel that is optimized specifically for certain processors, so it makes sense to improve the high level checks.
Oh, and there is a comment in cpuid.c that says suck it atom hackers.
(Certain parts of this post in regards to code comments may or may not be true).
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So in summary, the "Atom Killing" 10.6.2 simply tightened up a logic check that was passing things it shouldn't have. The fact now is that 10.6.2 actually has more Atom specific code in it then 10.6.1, because they now specifically define model 28 as ATOM. Nothing is done with this definition yet, but it is the foundation for kernel checks elsewhere if they wanted to make specific Atom optimizations. There are already plenty of other CPU specific optimizations in the kernel, for things like Nehalem's hyper threading and so on.