Package mars.simulator

Class DelayedBranch

java.lang.Object
mars.simulator.DelayedBranch
public class DelayedBranch extends Object
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.
Version:
June 2007
Author:
Pete Sanderson

  • Constructor Details

    • DelayedBranch

      public DelayedBranch()

  • Method Details

    • register

      public static void register​(int targetAddress)
      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.
      Parameters:
      targetAddress - The address to branch to after executing the next instruction