MIPS-X is a high performance RISC microprocessor that can run with a peak throughput of 16 MIPS. The design began in the spring of 1984, and in the summer of 1987 we were able to execute Pascal programs on a prototype system. This paper describes the design of MIPS-X examining the strengths and weaknesses of our tools, methodology and design environment. We found that consistency within a representation and across representations of the design was an important issue and that having an executable description of the design in the form of a functional simulator was key to the success of our project. We describe a technique we call comparison-mode simulation that made final verification and system integration much easier.