The basic components of a controller

1. The instruction register is used to store executing instructions. Instructions are divided into two parts: opcode and address code. Opcodes are used to indicate the operational nature of instructions, such as addition, subtraction, etc; The address code provides the operand address of this instruction or relevant information that forms the operand address (in this case, the operand address is formed through an address forming circuit). There is a type of instruction called a transition instruction, which is used to change the normal execution order of instructions. The address code part of this instruction gives the address of the instruction to be executed.
2. Opcode decoder: used to decode the opcode of instructions, generate corresponding control levels, and complete the function of analyzing instructions.
3. Sequential circuit: used to generate time stamp signals. In microcomputers, time stamp signals generally have three levels: instruction cycle, bus cycle, and clock cycle. The micro operation command generation circuit generates various micro operation commands to complete the specified operations of the instructions. The main basis for the generation of these commands is the time stamp and the operational nature of the instructions. This circuit is actually the implementation of various micro operation control signal expressions (such as the A → L expression above), and it is the most complex part of the combinational logic controller.
4. Instruction counter: used to generate the address of the next instruction to be executed. Usually, instructions are executed sequentially, and instructions are stored sequentially in memory. So, in general, 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 a transfer instruction is being executed, the address of the next instruction to be executed is the address to be transferred. The address is located in the address code field of this transfer instruction and is directly sent to the instruction counter.