CPU is running NOPs


I needed to verify whether CPU bus interface is working properly, so I wrote a some code, to dump consecutive CPU address reads to serial console.

Current RP2040 bus PIO program is hardcoded to feed NOP (0xEA) to CPU on every Read.

CPU reading NOPs

As you can see, CPU is doing a RESET sequence, then reads RESET vector from 0xFFFC which is two NOPs: 0xEAEA, so the next read is NOP from 0xEAEA, NOP from 0xEAEB, NOP from 0xEAEC, and so on…

CPU reading NOPs

If you want to know what is going on with this NOP-feeding, please watch the video from Ben Eater, who does way better job of explaining this, than I could ever will: https://youtu.be/LnzuMJLZRdU?t=1064