Intel Inside Macs Paves the Way for Affordable CPU Upgrades
Many of the early IBM PC clones were built around motherboards manufactured in Taiwan. Then came the Compaq Deskpro and its clones, which used standard ISA slots and gave users more control over what they could add to the machine.
Proprietary anything tended to fail miserably in the PC compatible world. Just look at IBM's PS/2 line of desktop computers from the 1980s. They were very solid computers meant for business use. Their cases could be compared to the case of the Power Mac G5 and Mac Pro in terms of construction.
However, IBM insisted on using its own Micro Channel Architecture slots on all but the entry-level models instead of the ISA slots everyone else used. While theoretically better than ISA, MCA didn't catch on due to the expensive cards and IBM's licensing fees, so very few other manufacturer adopted the technology (only NCR, Apricot, Tandy, Research Machines, and Olivetti used MCA at all, and then in only part of their product line). Therefore the PS/2s became a bit of a pain to deal with toward the end of their useful life, as cards were rare and expensive. A used ethernet card, for example, could cost $50 or more in the 90s.
On the Mac side of things, Apple had NuBus, SCSI, ADB, and the Mac serial ports. None of these technologies were compatible with anything on the PC side (with the exception of SCSI, which few PCs supported, and there were cables that allowed use of RS-232 serial devices with the Mac's DIN-8 serial ports), so some people avoided Macs for their lack of compatibility with the "PC standard". (For example, a Mac's 800K floppy wasn't compatible with a PC's 720K format.)
The processors in Macs were also more difficult to upgrade, and sometimes the motherboard was designed specifically to address certain chips (for example, the Mac IIx was designed to work only with a 16 MHz 68030 processor, even though the CPU was removable).
Today this has changed, and now I can take the hard drive from my PC, put it in a Mac, and access the data just as if it was a Mac hard drive. RAM (provided it's the right type) is generally interchangeable (and I say generally, because Macs tend to be more picky about how close the RAM is to its specifications), video cards sometimes work in both Mac and PCs (provided there are drivers, and sometimes only after the firmware has been "flashed"), and now Macs and PCs use the same Intel processors.
PCs have been targeted by overclockers far more than Macs. In the past, users had to move jumpers and play with DIP switches to get their PCs to run the CPU at a higher speed, but these days most PCs provide an option to change the clock speed of the processor in the BIOS, along with the bus speed.
Apple's recent issue with MacBooks overheating, and their subsequent firmware update that reportedly slows the processor down slightly, shows that this is possible for the Intel Macs, too.
If the processor in my PC is way too slow, I can generally exchange it with a faster one. This was possible with the daughter cards used in many of the PCI Power Macs and Mac clones sold in the mid to late 90s, as well as with the ZIF sockets in G3-based and G4-based Power Macs.
The Power Mac G5 essentially eliminated the possibiity of processor upgrades, but the use of Intel processors in the 2006 Macs brings it back. For instance, the Mac mini can be upgraded to a Core 2 Duo processor relatively easily. This is excellent news, as a seemingly un-upgradeable, "cheap" desktop computer is in some ways more expandable than some higher-end systems. It also helps to remove some of the old stigma left from the CRT iMacs that Macs are generally not upgradeable.
In my IBM ThinkPad 600E, the slow 200-300 MHz processor that came can easily be swapped out for a faster CPU. Since the machine runs very cool as it is with a 300 MHz processor, upgrading to a 500-600 MHz chip isn't something that will cause the computer to have problems.
Unfortunately my 12" PowerBook G4 runs too hot to be able to upgrade the processor with something faster, although there may be hope for the MacBooks. My sister's 2 GHz black MacBook runs fairly cool, with the fan only coming on occasionally. If the processor isn't permanently soldered in place, it may be able to be upgraded as faster ones become available.
Given that the Intel processors are used in both PCs and Macs, the price of upgrades for Intel Macs should be very reasonable, increasing the value-for-money ratio for Mac users looking to upgrade to something faster.
That said, as of now the Intel Macs are new enough and fast enough that upgrading them shouldn't matter for most users - at least for the next year or two.
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