A register is a group of binary storage cells capable of holding binary information. A group of flip-flops constitutes a register, since each flip-flop can work as a binary cell. An
n-bit register, has n flip-flops and is capable of holding n-bits of information. In addition to flip-flops a register can have a combinational part that performs data-processing tasks.

Various types of registers are available in MSI circuits. The simplest possible register is one that contains no external gates, and is constructed of only flip-flops. Figure 8.1 shows such a type of register constructed of four S-R flip-flops, with a common clock pulse input. The clock pulse enables all the flip-flops at the same instant so that the information available at the four inputs can be transferred into the 4-bit register. All the flip-flops in a register should respond to the clock pulse transition. Hence they should be either of the edge-triggered type or the master-slave type. A group of flip-flops sensitive to the pulse duration is commonly called a gated latch. Latches are suitable to temporarily store binary information that is to be transferred to an external destination. They should not be used in the design of sequential circuits that have feedback connections.

Figure 8.1