I really appreciate this example, especially the keyboard. I’m playing around with bare metal programming on Raspberry Pi and USB is complicated. Instead, this one has a matrix of switches with diodes[1] for the keyboard which you can just scan the switches with the gpio pins. Oh cool, I see he’s also coding in bare metal as well, so that makes sense.

For storage, you can also interface with an SDcard with SPI directly with gpio as well.[2] Raspberry Pi also has SPI controller(s) as well so you could use the built in slot but I’m not sure how complicated it is vs bit banging it with gpio.

[1] https://pcbheaven.com/wikipages/How_Key_Matrices_Works/

[2] Three video series but on Z80 https://youtu.be/To9ZCaAoc_s and https://youtu.be/o2AE_CkTIAU and https://youtu.be/HLdPMOpk-a4