Which technique were introduced because a single job could not keep both the CPU and the I/O devices busy?

The technique were introduced because a single job could not keep both the CPU and the I/O devices busy.

The host repeatedly checks if the controller is busy until it is not. It is in a loop that status register's busy bit becomes clear. This is called Polling and a mechanism for the hardware controller to notify the CPU that it is ready is called Interrupt .