Machine Model
Some types of machine models: 
There are also many implementations of these machine models: # ISA Characteristics
Most RISX architectures, like MIPS, have three classes of instructions: 1. ALU instructions 2. Load and Store instructions 3. Branches and jumps
\[CPI = P_{ALU} \times CPI_{ALU} + P_{BR} \times CPI_{BR} + P_{LD} (P_{HIT} CPI_{HIT} + P_{MISS} CPI_{MISS})\]