Main Control and ALU Control

The datapath discussed previously was controlled by control signals. The following figure shows the structure of a simple control logic including two major components: (i) main control and (ii) ALU control.

Main Control
The main control sends the control signals to all components of a datapath except the ALU. Its input and output are as follows:

  • Input includes 6-bit opcode field from instruction.
  • Output includes 10 control signals for the datapath.
ALU Control
As its name, the ALU control is to control the ALU. Its input and output are as follows and details will be covered later.

  • Input includes:

    • 6-bit opcode field from instruction and
    • 6-bit function (funct) field from (R-type) instruction.

  • Output includes a 4-bit ALUctl signal for ALU. Some of the ALUctl is on the right.