Microprogram consists of microinstructions which are nothing but the strings of 0's and 1's. In a particular instance, we read the contents of one location of microprogram memory, which is nothing but a microinstruction. Each output line ( data line ) of microprogram memory corresponds to one control signal. If the contents of the memory cell is 0, it indicates that the signal is not generated and if the contents of memory cell is 1, it indicates to generate that control signal at that instant of time.
First let me define the different terminologies that are related to microprogrammed control unit.
Control word is defined as a word whose individual bits represent the various control signal. Therefore each of the control steps in the control sequence of an instruction defines a unique combination of 0s and 1s in the CW.
A sequence of control words (CWs) corresponding to the control sequence of a machine instruction constitutes the microprogram for that instruction.
The individual control words in this microprogram are referred to as microinstructions.
The microprograms corresponding to the instruction set of a computer are stored in a aspecial memory which will be referred to as the microprogram memory. The control words related to an instructions are stored in microprogram memory.