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While browsing the System Programming Guide, Part 1

I am reading this
Is any of you already success to change behavior of some instructions, thus OS crash all the time and the BIOS too.

It's for the science team

It's been done.

Of course, that only works if you have a way to load unsigned microcode. AMD added a signature check to their later CPUs, and Intel CPUs have always checked the signature.

There's a hardware exploit that allows bypassing the signature check on some Intel CPUs.

The. title of this thread is rather misleading. A microcode update is not permanent, and is lost on reboot. It can hardly destroy a CPU.

Well, couldn't a microcode update cause a hardware malfunction so catastrophic that the wrong voltages get sent to the CPU, thus destroying it? OK, I admit, that is reaching.

As far as I am aware the voltages to the CPU are not controlled by the microcode.

They might be controlled by the firmware's SMM code, which is affected by microcode.

I'd be more concerned about microcode blowing one of the OTP fuses inside the CPU.

When someone can provide a proof of concept of custom microcode trashing a CPU then I'll believe it's a possibility. If that's what you want to do I would have thought a BIOS modification was a simpler means.