[dnlrn]

Microcode is not written to the CPU, it gets loaded on every boot. This can happen during the BIOS POST, during the OS bootloader or even while the OS is booting. Therefore, yes its possible to run older microcode (at least on Linux), since you just have to not write the newer version on boot. If the BIOS contains the new microcode, you can flash the previous version of the BIOS.

[throwaway7767]

> Microcode is not written to the CPU, it gets loaded on every boot. This can happen during the BIOS POST, during the OS bootloader or even while the OS is booting. Therefore, yes its possible to run older microcode (at least on Linux), since you just have to not write the newer version on boot. If the BIOS contains the new microcode, you can flash the previous version of the BIOS.

Did you read the last paragraph of my message? Because you're not really disputing anything I said. (to clarify, when I say "You cannot load old microcode anywhere", I define "old" to mean "older than the currently running microcode", I.E. you cannot downgrade it at runtime after it's gotten a new one loaded to RAM.

If you're willing to run outdated system firmware (with associated bugs, security vulnarbilities, etc), you can do it - just like I said in the message you're replying to. But that's not what I'd call a good solution.