Some AMD AM4 motherboard makers are griping that the current UEFI BIOS is close to 16MB. Larger 32MB chips are more expensive. Motherboard makers are loath to use more expensive components.
Some point to CPU microcode which is inaccurate. The microcode is resident on the CPU itself, the BIOS is a set of functions the are executed by the CPU.
The CPUID instruction returns a word that has the CPU vendor, stepping and so-on. This is standard with processors since the original Pentium. So any BIOS can be run on a CPU and if the CPUID shows a model that is not in the list, the BIOS can still run a load an operating system using the basic x64 coding.
What is really happening is whitelisting. By refusing a CPU they are attempting to force a purchase of additional hardware. This is more common with laptops but it has become more common with desktop motherboards too.
The typical UEFI BIOS is now loaded with so much baggage that it is hard to fit it all into 16MB. Getting rid of whitelists saves on code space and improves on overall performance.
Our Lenovo X220 has a UEFI BIOS but it is much smaller mostly as there are not so many Sandy Bridge / Ivy Bridge processors to contend with. The BIOS is a simple menu driven setup.
We know that many motherboards have 2 BIOS chips so that if one fashed chip fails, the other can be used to recover the motherboard. We suspect
The code used in a BIOS is usually a small subset of the entire instruction set. A BIOS has no need for SSE etc to handle priperal initialization etc. All that is needed is to setup the devices and load the operating system.
To date AMD has announced a total of 45+ processors for the AM4 platform. This rises to 55 with the recent GE and Athlon models, and more if the BIOS listings about more Ryzen-2000 series processors are to be believed. Socket AM3 has some 152 processors that work.
AMD had to send some Bristol Ridge processors along with a cooler to many to be able to update the BIOS on a motherboard that would not POST with the new Ryzen 2000 series processors. Now AMD has a new marketing badge with motherboards that already have BIOS support for the new processors.