Before Spectre and Meltdown, I've never heard of users updating microcode on a chip through firmware or otherwise. I actually thought it was written into the silicon and immutable. Now I'm seeing a lot of mentions about this., things like disabling branch prediction and such.
To what degree is there any precedent for this? Has it been done officially or otherwise (through hobbyist channels). Can one for example, reprogram an Intel chip disabling everything but RISC instructions? And is there any way to verify executing Microcode has not been maliciously modified?