● Both masters and slaves contain a serial shift register. The master starts a transfer of a byte by writing it to its SPI shift
register. As the register transmits the byte to the slave on the MOSI signal line, the slave transfers the contents of its
shift register back to the master on the MISO signal line .
In this way, the contents of the two shift registers are exchanged. Both a write and a read operation are performed with the slave simultaneously.