No, that's not the case, since >30 years. Microcode is only used for implementing some complex instructions (mostly system instructions). Most regular instructions (and the rest of the core) don't use microcode and their expansions into uOps are hardwired. Also the entire execution unit is hardwired.
There are typically some undocumented registers (MSRs on x86) that can control how the core behaves (e.g., kill switches for certain optimizations). These can then be changed by microcode updates.
No, that's not the case, since >30 years. Microcode is only used for implementing some complex instructions (mostly system instructions). Most regular instructions (and the rest of the core) don't use microcode and their expansions into uOps are hardwired. Also the entire execution unit is hardwired.
There are typically some undocumented registers (MSRs on x86) that can control how the core behaves (e.g., kill switches for certain optimizations). These can then be changed by microcode updates.