MIPS Processor

I need help understand this file.

How is the ALU control code generated?

--  Execute module (implements the data ALU and Branch Address Adder  --  for the MIPS computer)
LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_ARITH.ALL;
USE IEEE.STD_LOGIC_SIGNED.ALL;
ENTITY  Execute IS
    PORT(    Read_data_1     : IN     STD_LOGIC_VECTOR( 7 DOWNTO 0 );
            Read_data_2     : IN     STD_LOGIC_VECTOR( 7 DOWNTO 0 );
            Sign_extend     : IN     STD_LOGIC_VECTOR( 7 DOWNTO 0 );
            Function_opcode : IN     STD_LOGIC_VECTOR( 5 DOWNTO 0 );
            ALUOp             : IN     STD_LOGIC_VECTOR( 1 DOWNTO 0 );
            ALUSrc             : IN     STD_LOGIC;
            Zero             : OUT    STD_LOGIC;
            ALU_Result         : OUT    STD_LOGIC_VECTOR( 7 DOWNTO 0 );
            Add_Result         : OUT    STD_LOGIC_VECTOR( 7 DOWNTO 0 );
            PC_plus_4         : IN     STD_LOGIC_VECTOR( 7 DOWNTO 0 );
            clock, reset    : IN     STD_LOGIC );
END Execute;
ARCHITECTURE behavior OF Execute IS
SIGNAL Ainput, Binput         : STD_LOGIC_VECTOR( 7 DOWNTO 0 );
SIGNAL ALU_output_mux        : STD_LOGIC_VECTOR( 7 DOWNTO 0 );
SIGNAL Branch_Add             : STD_LOGIC_VECTOR( 7 DOWNTO 0 );
SIGNAL ALU_ctl                : STD_LOGIC_VECTOR( 2 DOWNTO 0 );
BEGIN
    Ainput <= Read_data_1;
                        -- ALU input mux
    Binput <= Read_data_2 
        WHEN ( ALUSrc = '0' ) 
          ELSE  Sign_extend( 7 DOWNTO 0 );
                        -- Generate ALU control bits
    ALU_ctl( 0 ) <= ( Function_opcode( 0 ) OR Function_opcode( 3 ) ) AND ALUOp(1 );
    ALU_ctl( 1 ) <= ( NOT Function_opcode( 2 ) ) OR (NOT ALUOp( 1 ) );
    ALU_ctl( 2 ) <= ( Function_opcode( 1 ) AND ALUOp( 1 )) OR ALUOp( 0 );
                        -- Generate Zero Flag
    Zero <= '1' 
        WHEN ( ALU_output_mux( 7 DOWNTO 0 ) = "00000000"  )
        ELSE '0';    
                        -- Select ALU output        
    ALU_result <= "0000000"  & ALU_output_mux( 7 ) 
        WHEN  ALU_ctl = "111" 
        ELSE      ALU_output_mux( 7 DOWNTO 0 );
                        -- Adder to compute Branch Address
    Branch_Add    <= PC_plus_4( 7 DOWNTO 2 ) +  Sign_extend( 7 DOWNTO 0 ) ;
    Add_result     <= Branch_Add( 7 DOWNTO 0 );
PROCESS ( ALU_ctl, Ainput, Binput )
    BEGIN
                    -- Select ALU operation
     CASE ALU_ctl IS
                        -- ALU performs ALUresult = A_input AND B_input
        WHEN "000"     =>    ALU_output_mux     <= Ainput AND Binput; 
                        -- ALU performs ALUresult = A_input OR B_input
         WHEN "001"     =>    ALU_output_mux     <= Ainput OR Binput;
                        -- ALU performs ALUresult = A_input + B_input
         WHEN "010"     =>    ALU_output_mux     <= Ainput + Binput;
                        -- ALU performs SLT
          WHEN "011"     =>    ALU_output_mux     <= Ainput - Binput;
                        -- ALU performs ?
          WHEN "100"     =>    ALU_output_mux     <= "00000000";
                        -- ALU performs ?
          WHEN "101"     =>    ALU_output_mux     <= "00000000";
                        -- ALU performs ALUresult = A_input -B_input
          WHEN "110"     =>    ALU_output_mux     <= Ainput - Binput;
                        -- ALU performs SLT
           WHEN "111"     =>    ALU_output_mux     <= "00000000" ;
          WHEN OTHERS    =>    ALU_output_mux     <= "00000000" ;
      END CASE;
  END PROCESS;
END behavior;

Looking at this part of the code below. What does it mean when it says Function_opcode( 0 ) OR Function_opcode( 3 )? add $t0, $t1, $t2

the opcode for add is 100000, how is 010 generated?

                        -- Generate ALU control bits    ALU_ctl( 0 ) <= ( Function_opcode( 0 ) OR Function_opcode( 3 ) ) AND ALUOp(1 );
    ALU_ctl( 1 ) <= ( NOT Function_opcode( 2 ) ) OR (NOT ALUOp( 1 ) );
    ALU_ctl( 2 ) <= ( Function_opcode( 1 ) AND ALUOp( 1 )) OR ALUOp( 0 );