Each silicon disk can make 10s of CPUs, thus people call it a batch CPUs. Since the CPU is very very tiny, you can't expect everything come out perfect, and therefore there will be tolerance on specs. Since it is too hard to do a visual inspection on each and every single transistors(they do sample it), they simply do tests on clock speed, voltage, and stability. Now some chips will be right on the mark, while most isn't, meaning that they need a bit more voltage to get to the expected clock speed and generate a bit more heat. The ones that passes the most extreme tests are named 975. Those that doesn't meet that becomes 965, then 960,950,940,930,920 accordingly. Of course they don't need to test it over and over as test result will indicate the quality of the chip, meaning that all they need to do is one set of tests to detemine what the chip will be. After that, they load the bios into the chip as well as print the name on it.
Now the bios in those premium chips does more than others, which gets sub-divided to different groups (else what do they make money?) So even if your chip is actually 975, you can't simply run it is 975. You can OC the chip to run at greater speed, but you will never have access to those extra functions. There are CPUs that the internal bios is more flexible, which are called unlocked version. Again, unlocked chips are more expensive.
Sometimes, when demands are high, they cut corners. Say each batch yields 50 chips, and they need to ship 4000 920s out of the door tomorrow. Sometimes they will stamp a 975 as 920 to meet demand. This is the reason why so many people likes to OC because you will never know if you actually got a good chip.
The pricing of those chips should be now obvious. While the rare cost premium, others are simply controlled by supply and demand.
I recently see
this at HardOCP. The chip is not new, but you can see now complicated it actually is.
Again, I am not an expert on this, but I do read about it a bit.
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