I$\text{'}$m working on a project involving a common bus circuit and need some help understanding its operation. Here are the details:

$4-$bit Registers: I$\text{'}$m using $4-$bit bidirectional universal shift registers, but only for parallel inputs and outputs. Each register, as well as the $2$x$1$ MUX, has LEDs to indicate their status.

Base Register: The base register is the only one that can interact with the external environment. It can receive data from outside and send data to the other registers.

$2$x$1$ MUX Control:

When the MUX control signal $(157)$ is $0,$ the base register can send data to the other registers.

When the MUX control signal $(157)$ is $1,$ the internal registers can communicate with each other without the base register's intervention.

Could someone explain in detail how this circuit works, especially how the base register interacts with the other registers and how the $2$x$1$ MUX controls this interaction? Any diagrams or examples would be greatly appreciated.

Thanks in advance for your help!