The instructions are certainly not free of cost, but they don't have to implement new function units for every new instruction. Often new instructions are just

The instructions are certainly not free of cost, but they don't have to implement new function units for every new instruction. Often new instructions are just slightly different/more general modes of existing ones. And on the other side legacy instructions will be emulated via microcode instead of having dedicated units. The software equivalent would be compatibility shims and wrapper functions with different signatures sharing a common, highly parameterized internal implementation.