Conclusion
So is the ARM1 microcoded or not? The instruction decoder is clearly made up of microinstructions executed sequentially or with branching. It makes sense to look at this as microcode. But on the other hand, the microcode is fairly simple and forms a small part of the total control circuitry. A large amount of hardcoded logic interprets the microinstruction outputs to generate the control signals. My conclusion is the ARM1 should be called “partially microcoded” or maybe “hybrid microcode / hardwired control”.This article owes a lot to Dave Mugridge’s analysis of the ARM1, especially Inside the ARMv1 — instruction decoding and sequencing. Thanks to the Visual 6502 team for the ARM1 simulator and data used in my analysis.
