ISA

In 1960, IBM S/360 introduced the concept of ISA as a layer of abstraction to the underlining CPU hardware microarchitecture. Programs written on an ISA are guaranteed to run on any CPU that implement the ISA. ISA provides a standardized way to expose the features of a system's hardware that allows manufactures to enhance the implementation without breaking programs. ISA is implemented using microcode engine, wich consists of some storage, microcode ROM which holds the microcode programs, and an execution unit that translate the standard instruction to the ones specific to the hardware implementaiton.

The drawback of microcode engine is it is slower than direct decoding. (Modern microcode engine has approached 99% of the speed.) However, the benefit of abstraction is so signifcant that outweight this slight penalty.