The serial transfer of information from register A to register B is done with shift registers,
as shown in the block diagram of Fig. 6-4(a). The serial output (SO) of register A is connected
to the serial input (Sf of register B. To prevent the loss of information stored in the source
register, the information in registerA is made to circulate by connecting the serial output to its
serial input. The initial content of register B is shifted out tlrough its serial output and is lost
unless it is transferred to a third shift register. The shift control input determines when and
how many times the registers are shifted. This is done with an AND gate that allows ciock
pulses to pass into the CLK terminals only when the shift control is active.