Define Serial In - Serial Out Shift Registers?

An essential four-bit shift register can be constructed using four D flip-flops. The register is the first cleared forcing all four outputs to zero the input data is then applied sequentially to the D input of the first flip-flop on the left (FF0). All through each clock pulse one bit is transmitted from left to right. Suppose a data word to be 1001. The least significant bit (LSB) of the data has to be shifted through the register from FF0 to FF3.

In order to obtain the data out of the register they must be shifted out serially. This can be done non-destructively or destructively for destructive readout the original data is lost and at the end of the read cycle all flip-flops are reset to zero.

To evade the loss of data an arrangement for a non-destructive reading can be done by adding two AND gates an OR gate and an inverter to the system and the construction of this circuit is shown below.

The data is loaded to a register when the control line is HIGH (that is WRITE). The data is able to be shifted out of the register when the control line is LOW (that is READ).