Page "Serial Peripheral Interface Bus" Paragraph 24

Transmissions normally involve two shift registers of some given word size, such as eight bits, one in the master and one in the slave ; they are connected in a ring.

Data is usually shifted out with the most significant bit first, while shifting a new least significant bit into the same register.

After that register has been shifted out, the master and slave have exchanged register values.

Then each device takes that value and does something with it, such as writing it to memory.

If there is more data to exchange, the shift registers are loaded with new data and the process repeats.