ControlWord.java

//  ControlWord.java

//  Class ControlWord
//  -------------------------------------------------------------
/** Creates a control word suitable for controlling the datapath
  * of Patterson & Hennessy Figure 5.24 for one clock cycle.
  */
  public class ControlWord
  {
    //  Bits within the control word
    private static final int NUM_CONTROL_BITS = 10;
    private static final int Jump     = 9;
    private static final int RegDst   = 8;
    private static final int ALUSrc   = 7;
    private static final int MemToReg = 6;
    private static final int RegWrite = 5;
    private static final int MemRead  = 4;
    private static final int MemWrite = 3;
    private static final int Branch   = 2;
    private static final int ALUOp1   = 1;
    private static final int ALUOp0   = 0;

    //  Opcodes
    private static final int r_type   = 0x00;
    private static final int lw       = 0x23;
    private static final int sw       = 0x2B;
    private static final int beq      = 0x04;
    private static final int bne      = 0x05;
    private static final int addi     = 0x08;
    private static final int andi     = 0x0C;
    private static final int j        = 0x02;
    private static final int ori      = 0x0D;
    private static final int slti     = 0x0A;
    private static final int lui      = 0x0F;

    //  This control word and it's mnemonic
    char[] theWord;
    String mnemonic = "   op?";

  //  setBits()
  //  -----------------------------------------------------------------
  /**
    *   Uses a String of 1s and 0s to set the character elements
    *   in the control word to the corresponding values.
    */
      private void setBits(String bits)
      {
        if (bits.length() != NUM_CONTROL_BITS)
        {
          throw new RuntimeException("setbits: " + bits.length() + "!=" +
              NUM_CONTROL_BITS);
        }
        for (int i=0; i<NUM_CONTROL_BITS; i++)
        {
          theWord[i] = bits.charAt(i);
        }
      }


  //  Constructor
  //  -----------------------------------------------------------------
  /**
    *   Constructs a control word for a decoded instruction.
    */
      public ControlWord(int opcode, int rs, int rt, int rd, int shamt,
          int func, int immediate, int address)
      {
        theWord = new char[NUM_CONTROL_BITS];
        for (int i=0; i<NUM_CONTROL_BITS; i++)
        {
          theWord[i] = '?';
        }
        switch (opcode)
        {
          case r_type:
            mnemonic = "r_type";
            setBits("0100100010");
            break;

          case lw:
            mnemonic = "    lw";
            setBits("0011110000");
            break;

          case sw:
            mnemonic = "    sw";
            setBits("0X1X001000");
            break;

          case beq:
            mnemonic = "   beq";
            setBits("0X0X000101");
            break;

          case bne:
            mnemonic = "   bne";
            break;

          case addi:
            mnemonic = "  addi";
            setBits("0110100010");
            break;

          case andi:
            mnemonic = "  andi";
            setBits("0110100010");
            break;

          case j:
            mnemonic = "     j";
            setBits("1XXX0X0XXX");
            break;

          case lui:
            mnemonic = "   lui";
            break;

          case ori:
            mnemonic = "   ori";
            setBits("0110100010");
            break;

          case slti:
            mnemonic = "  slti";
            setBits("0110100010");
            break;


          default:  System.err.println("Unhandled opcode: " + opcode);
        }
      }

  //  toString()
  //  -----------------------------------------------------------------
  /**
    *   Returns a printable representation of the control word.
    */
      public String toString()
      {
        StringBuffer sb = new StringBuffer();
        sb.append(mnemonic + ": ");
        sb.append("Jmp="+theWord[Jump]+" ");
        sb.append("RgDs="+theWord[RegDst]+" ");
        sb.append("ASrc="+theWord[ALUSrc]+" ");
        sb.append("M2Rg="+theWord[MemToReg]+" ");
        sb.append("RgWr="+theWord[RegWrite]+" ");
        sb.append("MRd="+theWord[MemRead]+" ");
        sb.append("MWr="+theWord[MemWrite]+" ");
        sb.append("Br="+theWord[Branch]+" ");
        sb.append("AOp1="+theWord[ALUOp1]+" ");
        sb.append("AOp0="+theWord[ALUOp0]+" ");
        return sb.toString();
      }
  }