The ideas in combinational circuits (discussed in topics 2-4) and
sequential methods (discussed in topics 5 and 6) when brought together
as one system gives sequential building blocks, usually in the form or
registers and counters.
A register is a set of flip-flops with combinational logic to
implement state transitions that allow information to be stored
and retrieved from them. In the simplest form, a flip-flop
is a one-bit register.
A counter is simply a register with combinational logic to
implement counting, that is it is possible to retrieve the
contents, add or subtract one to the contents, and then store it back
into the register in one