What will happen if I remove the CPU while the power is on?
Does anything happen to the computer?
Has anyone tried this?
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You will cause a rift in the space time continuum.
Seriously... (Based on a normal desktop machine - not a server that has hot-swappable CPUs) It will most likely be one of the following:
Based on my past experience, I will go for the second. When the Pentium 3 came in two variations - the card (Slot 1) and the regular kind (Socket 370), I bought myself an upgrade from the 450 MHz to 1 GHz (or 950 MHz, I forget), and the converter was a bit dodgy - the slightest vibration would cause it to move and lose contact.
When this happened, all that would happen was that the screen would freeze and the computer would be unresponsive - but, surprisingly, no long term damage and a simple restart fixed it. I can not guarantee the same would happen to you, and I would highly recommend that you do not try it.
It is now very dangerous to remove CPU from any modern system. Modern CPUs have power draw in excess of 50W and they are operating at <1.5V, that gives some 30 amperes of current. This is already very dangerous. In addition, modern CPU requires active cooling, which is usually latched to the motherboard.
and then, if you have the luxury to remove the CPU together with the heatsink, what will most likely happen is that the high current will then have to flow through a very high contact resistance the moment you try to remove the CPU from e.g. the LGA or socket. this will result in sparking and frying the CPU.
and back then, when CPU uses really low power, unplugging the CPU did no damage except floating all control line (which may or may not wreck the system depends on what the controller lines are doing - most of the time the control line are active low, and therefore taking the CPU out of the question sometimes means activating all output -- i have tried once back then using 80c86 microcontroller.
Depending on how fast you perform the task you can get the CPU removed without even burning it, although you wouldn't be able to put it back while computer still on.
You can also try to remove PCI cards while the computer is on, some cards don't even halt the computer, others may burn chips and your hand, those are the most amazing if you want to live life in the edge.
I would avoid doing this on real hardware. I believe that VirtualBox allows you to dynamically change the number of virtual processors while the virtual machine is running, enabling emulation of hot swapping a CPU (which can be done on real hardware on expensive servers). However, I suspect that it might not allow you to go to zero so you may need to download the source code and add that capability. If you find a way to do this on Amazon EC2, try going to -1 processors and let me know how you get on. A month of runtime, followed by a month of runtime with -1 processors would balance out my bill.
I have turned on a computer with no CPU. It turns on, beeps and waits for a power off. I assume the same is true if you take it out while running, although you could damage things though if it is there to begin with and then you take it out.
For extra kicks, turn on a high consumption program or a few (Crysis, 3DS Max, Adobe Premiere etc...) and start it (or them) at full blast. Then take out the cpu... and the ram ... and the hard drives... and the ... - No. Leave the graphics card in so you get feedback other than explosions and short circuits.
Seriously though, I do not operate on machines that are connected to an outlet even if they are powered down. Laptops (maybe iPads soon, too) that need servicing should have the battery removed. But you knew that already...
One last thought, what happens if you take it out and put it back in a few seconds later? (While running Crysis??)
Not exactly what you ask for but maybe still interesting.
On my Old Old Box, a Pentium MMX machine, I once took the graphics card in and out multiple times while it was running. I was a child at the time and was just "very careful what I touched" - I know this is dangerous and would highly recommend everyone to not try this.
Anyway, I'm not sure why (maybe electrical issues), but the whole machine turned silent while the card was inserted, and ran smoothly again once the card was out - except that the screen output was a bit boring. So at least for graphic cards, it appears it does not do any harm to remove it.
Been there, done that, this is what happened.
I was working on some computers a while back. They were Pentium or newer. I don't remember exactly which. I thought one was off, but it turned out it was just asleep (hibernated) and still plugged in (rookie mistake). I removed the CPU and the power supply fan immediately spun a few cycles and stopped. This was when I noticed the power was plugged in.
The CPU was destroyed, but the motherboard was fine.
Prior to that I did PC phone tech support. One two different occasions I had someone fry their computer by doing something they shouldn't have: Once attaching an hard dive data cable why the power was on, and another time accidentally dropping a screw on the motherboard why the power was on. Both of those computers stopped dead and never came on again (AFAIK).
On another occasion someone removed the hard drive from a running computer in our lab. That computer continued to run, but Windows reported errors anytime you tried to access a file or start a new program. Rebooting it resulted in a hard drive error, which is when we discovered the stolen drive. When the drive was replaced it continued to have flaky performance reading the hard drive and writing to the CD burner until it was replaced.
On the flip side my cousin once spilled 32 ounces of Mountain Dew on his motherboard while the computer was on, and it ended up being fine (after unplugging it, rinsing it and letting it dry really good).
I have other stories of things going both directions. . .
TL;DR It may work sometimes, but you run a really high risk of something touching something it shouldn't (or just getting close enough for the electricity to ark) and you could fry your computer or hurt yourself.