Co-design has become an established process for both developing high-performance computing (HPC) architectures (and, more specifically, CPU architectures) as well as HPC applications. The co-design process is frequently based on models. This paper discusses an approach to CPU architecture modelling and its relation to modelling theory. The approach is implemented using the gem5 simulator for Arm-based CPU architectures and applied for the purpose of generating co-design knowledge using two applications that are widely used on HPC systems.