Raukodur said:
is this something you are absolutely sure about?
How would you find something like that out?
It is not something I am absolutely sure about for the Intel Core specifically. I don't work for Intel (I
have worked for two companies that manufactured semiconductors and a hardware company very closely involved with a semiconductor manufacturer) and doubt that Intel have made a public statement about exactly how they disable the second core. Also, I doubt that any Intel employee that did know for certain would be permitted to comment.
I do know that it is very common industry practice to disable capacity in all kinds of integrated circuits in this manner. Devices are manufacturered on silicon wafers - repeating the device across the wafers' surface many times.
Although the silicon is extremely pure, it may still contain atomic imperfections. Additionally, each stage of processing and manufacture can introduce new imperfections and contamination. It is common for the wafer (or the devices on the wafer) to be tested between processing steps. Because each processing step is very expensive, it sometimes makes financial sense to scrap an individual wafer rather than finishing the manufacture.
Once processing is complete, each device is electronically tested and characterised. Due to the nature of the manufacturing process, it often isn't as simple as a pass/fail.
For instance, poorly manufacturered capacitors inside the device can 'leak' current, causing higher-than-expected current consumption of the device (it could be a single capacitor that leaks badly, or thousands of capacitors that each leak a little more than they should). Conversely, if the manufacturer is lucky (or gets very good at controlling yield), they could actually end up with a device that has a lower power consumption than they specified (the manufacturer will design-in n allowance for some sub-optimal performance). It isn't just the capactors inside the IC that might be mis-manufactured - other components can be affected, such as a resistor whose resistance is higher or lower than intended.
If the distribution and nature of the failed components happens in a certain way, major functionality might be affected. Memory is the highest-density and a fairly complex structure to lay out in silicon. As well as its use in discrete RAM chips such as you find on an SDRAM memory stick, memory structures appear in flash devices, processors and many other ICs.
Because memory structures are at particular risk from processing flaws (due to its high density), they are often duplicated in expensive devices such as CPUs. This lets a manufacturer choose which bank to disable after manufacture (if both banks are okay, the manufacturer will choose the one with the higher power consumption or choose randomly). Some manufacturer realised early on that by laying out the memory slightly differently they can actually allow the CPU to access and use both banks if they both work and only one bank if one is faulty. This is the main reason why many CPUs are now available with different cache sizes (where one size happens to be double that of the other).
Out-of-tolerance components in the integrated circuit don't just affect the power consumption of the device, they also affect the heat output (heat generated is a function of the power consumption), timing and switching speed of components like transistors. The last two together are the primary reason why CPUs might not work reliably at higher clock speeds, but are able to when the clock speed is reduced.
If you have read this far, you probably have a new insight into Intel's CPU business model. Once you are designing in an extra copy of the memory for a CPU in case you have to disable half of it, it is only a small step to do the same for a whole second core. Intel`s Yonah product range is actually just a single device. Once they have characterised each device on the wafer, they decide what speed, power consumption and memory cache and number of cores each particular device is to have and permanently disconnect the unused parts of the CPU.
If you want to learn more,
Wikipedia is a good place to start.
