It is observed that we need to store the information of each control signal in control store. The status of a particular control signal is either high or low at a particular instant of time.
It is possible to reserve one bit position for each control signal. If there are n control signals in a CPU, them the length of each control signal is n . Since we have one bit for each control signal, so a large number of resources can be controlled with a single microinstruction. This organization of microinstruction is known as horizontal organization.
If the machine structure allows parallel uses of a number of resources, then horizontal organization has got advantage. Since more number of resources can be accessed parallel, the operating speed is also more in such organization. In this situation, horizontal organization of control store has got advantage.
If more number of resources can be accessed simultaneously, than most of the contents of control store is 0. Since the machine architecture does not provide the parallel access of resources, so simultaneously we cannot generate the control signal. In such situation, we can combine some control signals and group them together. This will reduce the size of control word. If we use compact code to specify only a small number of control functions in each microinstruction, then it is known as vertical organization of microinstruction.
In case of horizontal organization, the size of control word is longer, which is in one extreme point and in case of vertical organization, the size of control word is smaller, which is in other extreme.