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Apple keeps PPCs as the primary chip for their computers. They may also announce dual-core G4 and/or G5 chips, which are both under development by Freescale and IBM, respectively.
However, the current PPC chipset for low-end, G4-based machines, changes dramatically. The new chipset, made by Intel, has a built-in x86-64 processor. Or, there is a separate Intel x86-64 processor, but for low-end machines, a single-chip solution is more likely.
Apple plans to stay with the PPC as their primary architecture. Eventually, high-end, G5-based machines also have a secondary x86-64 processor, perhaps even a dual-core one.
Sample configurations:
Mini Mac: 1.5GHz G4 + 1.5GHz Pentium M derivative
Powerbook: Dual-core 1.7GHZ G4 + 1.7GHz Pentium M derivative
G5 tower: Dual-Core 3.0GHz PPC750MP + 3.0GHz dual-core Pentium D derivative
Why add an x86-64 processor to the mix ? Apple has been working with dynamic recompilation technology. Intel has been working on virtualisation. Alongside with a re-implementation of Windows libraries, perhaps based on an enhanced Wine...
Macs will be able to run Windows software natively.
You will be able to install most Windows applications and run them at native speeds, without having to run Windows. If you need to, you will also be able to run Windows itself at native speeds.
As we know, Tiger can run on x86-64, and dynamic recompilation can make PPC apps run, albeit imperfectly and rather slowly, on x86-64. However, Apple will not switch away from PPC; Mac applications will still be written for PPC. If there are computationally expensive operations that can be parallelised, applications could be written to take advantage of the secondary processor.
So, the secondary processor will be used by the OS, by Windows applications, and by specially-modified applications that run partly on PPC and partly on x86-64.
Developers will be able to port their Windows applications to Macs very quickly; they will keep most of their application intact during the porting process. With Wine, the Win32 frontend will be dynamically translated into Cocoa, and will be tweakable at run-time, by a front-end extender. So, with minor tweaks, an x86 or x86-64 Windows application will become a Mac application. Of course, developers will be encouraged to keep making fully native PPC applications, because PPC remains for the main processors of the Macs.
How can Apple do this ? Well, Intel chips are inexpensive. A basic Intel processor costs about 50$, and a dual-core costs about 200$. For lower-end machines, the processor can be integrated with the chipset, further reducing costs. As for higher-end machines, they've already got two G5 processors - and will instead get a dual-core G5 and a dual-core Intel processor, which won't increase the price from current levels.
Has Apple got the technology right now ? No. They've only got prototypes, and that's why we won't see this before 2006 at the earliest, and then again, only with a few select models.
What is the greatest advantage of doing this ? Total Windows compatibility. No longer will there be a reason not to get a Mac, since it runs all Windows applications at native speeds. You couldn't switch because you depended on specific Windows applications that would never get ported, and you couldn't stand running an emulator ? Problem solved.
The new Mac Mini runs all your software, Mac and Windows. It's not expensive, and it's better than any PC. You can no longer hide behind a lack of support for your favourite applications.
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