For each security advisory, affected processors are identified at the Security Center under that specific advisory.
Microcode requirements can be found on the Security Center for each specific advisory.
Intel always recommends running the latest microcode for your system by using an updated BIOS or by OS loading of microcode updates if supported by the platform. See the Security Center for each advisory with the latest details on what options are available. This is especially true when mitigating potential side-channel cache exploits.