Specify the problem in terms of logic gates. You can spend gates building special-purpose ALUs, you can sink everything into RAM/ROM and have a complex program

Specify the problem in terms of logic gates. You can spend gates building special-purpose ALUs, you can sink everything into RAM/ROM and have a complex program written in an extremely simple RISC instruction set or strike any balance between. Going with gates instead of transistors keeps things tidy and not tied to any particular strategy for physically building the system.