PINE64

How SD Cards Work;  SPI and the SD Card

The above link explains how SPI and the SD card work.  This is not only a good read for understanding in SD cards, but is also a great primer for understanding SPI as well.

That looks like some of the stuff I had to learn 40 years ago in the electrician trade school. JK flipflops, FIFO, Nands, Nors, ect. I was a industrial electrician and had to service a lot of expensive equipment. The upper echelon would freak when I cleaned $10,000 circuit boards in de-ionized water.