Processor speed: dual core versus quad core
The number of instructions a processor can execute in a given time period is determined by a number of factors, including the number of CPUs, the clock-speed of those CPUs and the existence and size of on-board cache. Simply put, and all other things being equal, a CPU with a higher clock-speed (described in GHz) will execute more instructions per second than one with a lower clock speed.
Equally, a processor with four cores will execute more instructions than one with two cores, because it can execute twice as many instructions simultaneously. Thus, a 2GHz quad-core processor should be 'faster' than a 3GHz dual-core model.
Core i5 v Core i7 and Hyper Threading
Intel's Core i5 used widely across the Mac range, and the Core i7, used in the 15in MacBook Pro, are both multi-core families of processors.
The Core i5 tends to be dual-core, although Intel does make quad-core i5s, and i7 desktop processors are quad-core, or sometimes six-core. That's one key difference between Core i5 and Core i7. The others are the size of the cache, the integrated graphics (more powerful on Core i7), and support for hyper threading.
Hyper threading, a feature of the Core i7 series, allows the processor to handle twice as many 'streams' as it has cores, by fooling software into thinking it has twice as many cores. So a quad-core processor with hyper threading should be able to execute four times as many sets of instructions in a given time period as a dual-core processor with the same clock speed but without hyper threading.
Hyper threading isn't the only neat trick the current crop of Intel processors can pull. You might have noticed the phrase Turbo Boost on the Mac specs pages on Apple's website. The simplest way to think of Turbo Boost is as a way of safely over-clocking the cores on a processor.
The Turbo Boost controller samples the power consumption and temperature of the cores hundreds of times a second while monitoring the demands made of them by software. If any of the cores are being driven to their theoretical maximum, Turbo Boost can, if enough power is available and the temperature is at a safe level 'over-clock' the core and enable it to work faster. So the four cores in a MacBook Pro's 2.2GHz quad-core i7 can, if needed, be pushed to 3.7GHz subject to power consumption and heat dissipation.
Xeon and the Mac Pro
While the rest of Apple's range uses either the mobile or desktop versions of Intel's processor architecture, the Mac Pro uses the workstation model, Xeon. So what's the difference? Workstation processors are designed for raw power, and are less hindered by the need to keep the temperature down (though power efficiency is still an issue). So, the Xeon E5 used in the Mac Pro is available with up to 12 cores, rather than the four available in the Core i5 in the iMac.
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