B.java

/*
 * B.java
 *
 * Instruction B of the MIPS64 Instruction Set
 * (c) 2007 EduMips64 project - Andrea Milazzo (MancaUSoft)
 *
 * This file is part of the EduMIPS64 project, and is released under the GNU
 * General Public License.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

package org.edumips64.core.is;
import org.edumips64.core.*;
import org.edumips64.utils.*;
/** <pre>
 *         Syntax: B offset
 *         B denote an unconditional branch. The actual instruction is interpreted by the
 *         hardware as BEQ r0, r0, offset.
 *
 *</pre>
  * @author Andrea Milazzo
 */

public class B extends FlowControl_IType {
  final String OPCODE_VALUE = "000100";
  final static int OFFSET_FIELD = 0;

  /** Creates a new instance of B */
  B() {
    super.OPCODE_VALUE = OPCODE_VALUE;
    syntax = "%B";
    name = "B";
  }

  public void ID() throws RAWException, IrregularWriteOperationException, IrregularStringOfBitsException, JumpException, TwosComplementSumException {
    //getting registers rs and rt
    //converting offset into a signed binary value of 64 bits in length
    BitSet64 bs = new BitSet64();
    bs.writeHalf(params.get(OFFSET_FIELD));
    String offset = bs.getBinString();

    String pc_new = "";
    Register pc = cpu.getPC();
    String pc_old = cpu.getPC().getBinString();

    //subtracting 4 to the pc_old temporary variable using bitset64 safe methods
    BitSet64 bs_temp = new BitSet64();
    bs_temp.writeDoubleWord(-4);
    pc_old = InstructionsUtils.twosComplementSum(pc_old, bs_temp.getBinString());

    //updating program counter
    pc_new = InstructionsUtils.twosComplementSum(pc_old, offset);
    pc.setBits(pc_new, 0);

    throw new JumpException();
  }

  public void pack() throws IrregularStringOfBitsException {
    repr.setBits(OPCODE_VALUE, OPCODE_VALUE_INIT);
    repr.setBits(Converter.intToBin(OFFSET_FIELD_LENGTH, params.get(OFFSET_FIELD) / 4), OFFSET_FIELD_INIT);
  }

}