ISA

In computing, ISA stands for Instruction Set Architecture. It defines the instruction set of a CPU along withh its encoding. The ISA is all that is needed to develop a compiler and equally to write a simulator for a CPU compatible with this ISA.

FAQ

Q: Does the ISA define the performance?
A: No, not at all! Actually, it is very common to have multiple CPUs which are compatible from an ISA perspective with different performance. One CPU might have a 3 stage pipeline, another one a 5 or 8 stage pipeline, one might have a fast multiplier or divider, another one a slow, bit by bit implementation. Even the bus structure (separate buses for Instruction and Data, or just one bus, or...) is can vary between CPUs compatible with the same ISA.