Design steps:

 1. Design the instruction system of the machine: specify the instruction type, instruction quantity, format function of each instruction; 

 2. Preliminary overall design: register setting, bus configuration, calculator design, connection between components, etc.; 

 3. Draw the instruction flow chart: mark the time operation of each instruction each component 

 4. Arrange the operation schedule: divide each operation into micro operations according to the instruction flow chart, list the micro operations that the machine should perform according to the time period; 5. List the micro operation signal expression, simplify realize the circuit.

 Basic composition:

 1. The instruction register is used to store the executing instruction. The instruction is divided into two parts: operation code address code. The operation code is used to indicate the nature of the instruction operation, such as addition subtraction; the address code gives the operand address of the instruction information about the formation of the operand address (in this case, the address forming circuit forms the operand address). There is an instruction called a branch instruction that changes the normal order in which instructions are executed. The address code part of the instruction gives the address of the instruction to be executed. 

 2. Operation code decoder: used to decode the operation code of the instruction, generate the corresponding control level, complete the function of analyzing the instruction. 

 3. Sequential circuit: used to generate time mark signal. In microcomputers, the time mark signal usually has three levels: instruction period, bus period clock period. The micro operation command generation circuit generates various micro operation commands to complete the operation specified by the command. The main basis of these commands is the timestamp the operational nature of the instructions. This circuit is actually the circuit implementation of each micro operation control signal expression, is the most complex part of the combinational logic controller. 

 4. Instruction counter: used to form the address of the next instruction to be executed. Generally, instructions are executed sequentially stored in memory sequentially. Therefore, in general, the address of the next instruction to be executed can be formed by adding 1 to the current address, the micro operation command "1" is used for this purpose. If a branch instruction is executed, the address of the next instruction to be executed is the address to which the branch is to be transferred. The address is in the address code field of the branch instruction is sent directly to the instruction counter.

  The reason why microprogrammed controller is proposed is that the design of combinational logic is inconvenient, inflexible, modified extended.