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Hidden Cores On Phenom CPUs Can Be Unlocked
An anonymous reader writes "One of the major ways a semiconductor manufacturer manages to make the most of its chips is through binning. Chips able to cope with high clock speeds with all cores running end up as premium product lines, while others end up as models rated at lower speed grades, or with fewer cores. In the case of AMD's Phenom CPUs, dual and triple core models are quad cores with some disabled, while some newer quad core CPUs are actually six core models with two disabled. To this end both ASUS and MSI have announced that they have modified versions of AMD 890FX- and 890GX-based motherboards to unlock these hidden cores. Much like overclocking, there is no guarantee that you will gain anything by unlocking the hidden cores — everything depends on just why your CPU ended up in a certain product line."
Yeah, maybe. Then again, GP has a point and you're being an asshat.
TFS makes a comparison to overclocking. It points out that there is no guarantee of a benefit - but doesn't point out that there is a risk. In the case of overclocking, the risk is that you will overheat a chip that was rated at a particular clock speed for good reason. Of course you can combat this risk by improving the cooling system. You can combat the risk because you know exactly what the risk is.
Now in the case of "hidden cores", what's the risk? Do you even know? Do you know what kind of flaw would lead them to legitimately disable a core? Is that one core unable to tolerate the same clock speed as the others? Is it functionaly broken such that it will return incorrect results for some operations? How would you tell the difference between that, vs. a chip that was perfectly fine but sold in a degraded state to balance out supply and demand?
You could shell out for a special motherboard just to test your chip, and if no flaw in the normally-disabled chip causes any damage to the rest of the chip (or do you have some basis on which to rule that possibility out?) you at least won't lose anything. Or, could the defect be intermittant such that your tests might miss it?
And if your computer is for hobbying and you enjoy working with a potentially-unstable system, good for you. A lot of people think that's a fine trade-off for what they're going to do with their systems. None of which invalidates GP's question - which is "what exactly might a disabled-by-default core do if you turn it on when it really was disabled for a reason?"
If a core is just flat out non-functional then yes, you are right, a system wouldn't boot. However that it works mostly doesn't mean there isn't a problem. There could be a single instruction that has a flaw, so everything is fine unless that instruction gets executed but when that happens you get a crash or worse, data corruption.
If you think Prime95 is an accurate test, you are kidding yourself. Prime95 tests the FPU mainly, and is good for heat testing. It is not a full CPU test. So maybe the FPU works great, but one of the other units doesn't.
So no, you don't know that nothing is broken. You assume nothing is broken. Maybe that's fine, however then no bitching if you get data corruption or the like because there was a problem that you didn't know about.
The diagnostic doesn't put any sort of uniform stress on anything other than memory. Even wondered why it does a ton of passes on a ton of different modes with a ton of patterns on RAM? That's testing for as many possible RAM failure modes as it can. No attempt is made to test the CPU. You're stressing some parts of the CPU, but you're neglecting the vast majority (e.g. floating point and SIMD).
If anything, it might be a diagnostic for your cooling system. Sure, it helps ensure that nothing is blatantly wrong with the CPU, and it does a better job at testing the CPU than memtest86, but it isn't even remotely a comprehensive test of CPU functionality.
This isn't overclocking we're talking about here. When you overclock, you stress the entire CPU more as a whole. When tests like memtest86 and Prime95 start failing, you know that your CPU is definitely unstable. Then you back off and you hope the untested parts of the CPU will do OK with whatever safety margin you gave it.
When you enable a core, it might have some broken parts, or it might not. Those parts can be flaky, or they can be borked, period. Unless you run software that has a chance of testing those parts, you will never find out. E.g. if the hardware for a specific floating point instruction is borked, memtest86 will be useless, and Prime95 will be useless unless it happens to use that specific instruction. If the transistor in charge of forbidding kernel memory access from user mode is borked, you won't find out until an unstable application takes down your entire system by scribbling all over the kernel.
I am absolutely sure there is no test that will match what Intel and AMD do - because they know exactly how their CPUs work and what to test for. I do know that you can do a whole lot better than memtest86 or Prime95. I haven't checked whether someone actually has attempted to produce a comprehensive architecture test of this sort.
Your mistake is attempting to extrapolate from tools used for testing overclocking (which typically results in overall instability) as a means to test for disabled and possibly subtly broken hardware. Any failures from a defective core are likely to show up only with workloads that exercise the defective bits, and the rest of the CPU will work fine.