1. The instruction register is used to store the instruction being executed. The instruction is divided into two parts: operation code and address code. The operation code is used to indicate the nature of the operation of the instruction, such as addition, subtraction, etc.; the address code gives the operand address of this instruction or information about the operand address (the operand address is formed by the address forming circuit at this time). One type of instruction is called a transfer instruction, which is used to change the normal execution order of the instruction. The address code part of this instruction gives the address of the instruction to be transferred to be executed.
2. Operation code decoder: used to decode the operation code of the instruction, generate the corresponding control level, and complete the function of analyzing the instruction.
3. Sequential circuit: used to generate time mark signal. In a microcomputer, the time mark signal generally has three levels: instruction cycle, bus cycle and clock cycle. The micro-operation command generating circuit generates various micro-operation commands for completing the operations specified by the instructions. The main basis for these commands is the time stamp and the nature of the operation. This circuit is actually the circuit realization of each micro-operation control signal expression (such as the above A→L expression), and it is the most complicated part of the combinational logic Mitsubishi PLC controller.
4. Instruction counter: used to form the address of the next instruction to be executed. Generally, instructions are executed sequentially, and instructions are stored sequentially in memory. Therefore, under normal circumstances, the address of the next instruction to be executed can be formed by adding 1 to the current address, and the micro-operation command "1" is used for this purpose. If the execution is a branch instruction, the address of the next instruction to be executed is the address to be transferred to. The address is in the address code field of the transfer instruction, and it is sent directly to the instruction counter.