EMARS/mars/simulator/DelayedBranch.java

package mars.simulator;

/*
Copyright (c) 2003-2007,  Pete Sanderson and Kenneth Vollmar

Developed by Pete Sanderson (psanderson@otterbein.edu)
and Kenneth Vollmar (kenvollmar@missouristate.edu)

Permission is hereby granted, free of charge, to any person obtaining 
a copy of this software and associated documentation files (the 
"Software"), to deal in the Software without restriction, including 
without limitation the rights to use, copy, modify, merge, publish, 
distribute, sublicense, and/or sell copies of the Software, and to 
permit persons to whom the Software is furnished to do so, subject 
to the following conditions:

The above copyright notice and this permission notice shall be 
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR 
ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF 
CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION 
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

(MIT license, http://www.opensource.org/licenses/mit-license.html)
 */

/**
 * Represents a (potential) delayed branch.  Note it is necessary only when
 * delayed branching is enabled.  Here's the protocol for using it:
 *
 * (1) When a runtime decision to branch is made (by either a branch or jump
 * instruction's simulate() method in InstructionSet), then if delayed branching
 * is enabled, the register() method is called with the branch target address but
 * the program counter is NOT set to the branch target address.
 *
 * (2) At the end of that instruction cycle, the simulate() method in Simulator
 * will detect the registered branch, and set its trigger.  Don't do anything yet
 * because the next instruction cycle is the delay slot and needs to complete.
 *
 * (3) At the end of the next (delay slot) instruction cycle, the simulate()
 * method in Simulator will detect the triggered branch, set the program
 * counter to its target value and clear the delayed branch.
 *
 * The only interesting situation is when the delay slot itself contains a
 * successful branch!  I tried this with SPIM (e.g. beq followed by b) 
 * and it treats it as if nothing was there and continues the delay slot 
 * into the next cycle.  The eventual branch taken is the original one (as one
 * would hope) but in the meantime the first statement following the sequence
 * of successful branches will constitute the delay slot and will be executed!
 *
 * Since only one pending delayed branch can be taken at a time, everything
 * here is done with statics.  The class itself represents the potential branch.
 * 
 * @author Pete Sanderson
 * @version June 2007
 **/

public class DelayedBranch {
   // Class states.
   private static final int CLEARED    = 0;
	private static final int REGISTERED = 1;
	private static final int TRIGGERED  = 2;

   // Initially nothing is happening.
	
	private static int state = CLEARED;
	private static int branchTargetAddress = 0;
	
	/**
    *  Register the fact that a successful branch is to occur.  This is called in
	 *  the instruction's simulated execution (its simulate() method in InstructionSet).
	 *  If a branch is registered but not triggered, this registration will be ignored 
	 *  (cannot happen if class usage protocol is followed).  If a branch is currently 
	 *  registered and triggered, reset the state back to registered (but not triggered)
	 *  in order to carry over the delay slot for another execution cycle.  This is the
	 *  only public member of the class.
	 *
	 *  @param targetAddress The address to branch to after executing the next instruction
	 */
	public static void register(int targetAddress) {
	  // About as clean as a switch statement can be!
	  switch (state) {
	    case CLEARED    :  branchTargetAddress = targetAddress;
		 case REGISTERED :
		 case TRIGGERED  :  state = REGISTERED;
	  }          
	}

	/**
    *  Trigger a registered branch.  This is called at the end of the MIPS simulator
	 *  instruction execution cycle (simulate method in Simulator), so a registered 
	 *  branch will be triggered right away.  The next execution cycle will be the
	 *  delay slot and at the end of THAT cycle, the trigger will be detected and the
	 *  branch carried out.  This method has package visibility.
	 *
	 *  Precondition: DelayedBranch.isRegistered()
	 *
	 *  Postcondition: DelayedBranch.isTriggered() && !DelayedBranch.isRegistered()
	 *
	 */
	 static void trigger() {
	  // About as clean as a switch statement can be!
	  switch (state) {
		 case REGISTERED :
		 case TRIGGERED  :  state = TRIGGERED;
		 case CLEARED    :
	  }          
	}

	/**
    *  Clear the delayed branch. This must be done immediately after setting the
	 *  program counter to the target address.  This method has package visibility.
	 */
	 static void clear() {
	  state = CLEARED;
	  branchTargetAddress = 0; 
	}

   /**
	 *  Return registration status.  Is false initially, true after register() is called
	 *  but becomes false after trigger() or clear() are called.  This method has package
	 *  visibility.
	 *
	 *  @return true if branch is registered but not triggered, false otherwise.
	 */

    static boolean isRegistered() {
	   return state == REGISTERED;
	}
	
   /**
	 *  Return trigger status.  Is false initially, true after trigger() is called
	 *  but becomes false after clear() is called.  This method has package visibility.
    *
    *  @return true if branch is registered but not triggered, false otherwise.
	 */

   static boolean isTriggered() {
	   return state == TRIGGERED;
	}	


  /**
   *  Return branch target address.  This should be retrieved only to set the program
	*  counter at the end of the delay slot.  This method has package visibility.
	*
	*  Precondition: DelayedBranch.isTriggered()
	*
	*  @return Target address of the delayed branch.
	*/
	 static int getBranchTargetAddress() {
	    return branchTargetAddress;
	}
	
}  // DelayedBranch