/**
   * Calls a function according to the given function number stating that the given number of
   * arguments have been pushed onto the stack
   *
   * <p>If callerIsBuiltIn then the caller is a builtIn function that called this function through
   * callFunctionFromBuiltIn. If address is -1 then a native function should be looked up and
   * called.
   */
  public void callFunction(
      short address, short numberOfArguments, String functionName, boolean callerIsBuiltIn)
      throws ProgramException {
    stackFrames.addElement(new Integer(workingStackSegment.getStartAddress()));
    workingStackSegment.setStartAddress(getSP() + 5);

    if (callerIsBuiltIn) {
      pushValue(MAIN_STACK, VMProgram.BUILTIN_FUNCTION_ADDRESS);
    } else {
      pushValue(MAIN_STACK, program.getPC());
    }
    pushFromRAM(MAIN_STACK, Definitions.LOCAL_POINTER_ADDRESS);
    pushFromRAM(MAIN_STACK, Definitions.ARG_POINTER_ADDRESS);
    pushFromRAM(MAIN_STACK, Definitions.THIS_POINTER_ADDRESS);
    pushFromRAM(MAIN_STACK, Definitions.THAT_POINTER_ADDRESS);
    ram.setValueAt(
        Definitions.ARG_POINTER_ADDRESS, (short) (getSP() - numberOfArguments - 5), false);
    ram.setValueAt(Definitions.LOCAL_POINTER_ADDRESS, getSP(), false);

    // enable in the arg segment only the number of args that were sent to the called function.
    argSegment.setEnabledRange(
        argSegment.getStartAddress(), argSegment.getStartAddress() + numberOfArguments - 1, true);

    if (address == VMProgram.BUILTIN_FUNCTION_ADDRESS) {
      // Perform some actions normally done in the function() method
      localSegment.setEnabledRange(
          localSegment.getStartAddress(),
          localSegment.getStartAddress() - 1,
          true); // no local variables
      callStack.pushFunction(functionName + " (built-in)");
      staticSegment.setEnabledRange(0, -1, true); // empty static segment
      // Read parameters from the stack
      short[] params = new short[numberOfArguments];
      for (int i = 0; i < numberOfArguments; ++i) {
        params[i] = argSegment.getValueAt(i);
      }
      // Call the built-in implementation
      builtInFunctionsRunner.callBuiltInFunction(functionName, params);
    } else if (address >= 0 || address < program.getSize()) {
      program.setPC(address);
      program.setPC(address); // make sure previouspc isn't pc-1
      // which might happen if the calling
      // function called this function in the
      // last line before the "return" and
      // was declared just before this function.
      // In this case encountering the "function"
      // command will issue an error about
      // "missing return"...
    } else {
      error("Illegal call address");
    }
  }
  // Verifies that the given index of the given segment is valid.
  private void checkSegmentIndex(MemorySegment segment, int segmentCode, int index)
      throws ProgramException {
    short loc = (short) (index + segment.getStartAddress());

    if (segmentCode == HVMInstructionSet.THIS_SEGMENT_CODE) {
      if (loc < Definitions.HEAP_START_ADDRESS || loc > Definitions.HEAP_END_ADDRESS)
        error("Out of segment space");
    } else {
      int[] range = segment.getEnabledRange();
      if (loc < range[0] || loc > range[1]) {
        error("Out of segment space");
      }
    }
  }
  /** Returns the value of the function to the top of the stack. */
  public void returnFromFunction() throws ProgramException {

    // make sure that there's somewhere to return to (old local <> 0)
    if (stackSegment.getValueAt(Definitions.LOCAL_POINTER_ADDRESS) == 0)
      throw new ProgramException(
          "Nowhere to return to in "
              + getCallStack().getTopFunction()
              + "."
              + getCurrentInstruction().getIndexInFunction());

    // done in order to clear the method stack's contents
    workingStackSegment.setStartAddress(getSP());

    bus.send(ram, Definitions.LOCAL_POINTER_ADDRESS, ram, Definitions.R13_ADDRESS); // R13 = lcl
    bus.send(
        stackSegment,
        stackSegment.getValueAt(Definitions.LOCAL_POINTER_ADDRESS) - 5,
        ram,
        Definitions.R14_ADDRESS); // R14 = return address
    bus.send(
        stackSegment,
        getSP() - 1,
        stackSegment,
        ram.getValueAt(Definitions.ARG_POINTER_ADDRESS)); // *arg = return value
    setSP((short) (ram.getValueAt(Definitions.ARG_POINTER_ADDRESS) + 1)); // SP = arg + 1
    bus.send(
        stackSegment,
        ram.getValueAt(Definitions.R13_ADDRESS) - 1,
        ram,
        Definitions.THAT_POINTER_ADDRESS); // that = *(R13 - 1)
    bus.send(
        stackSegment,
        ram.getValueAt(Definitions.R13_ADDRESS) - 2,
        ram,
        Definitions.THIS_POINTER_ADDRESS); // this = *(R13 - 2)
    bus.send(
        stackSegment,
        ram.getValueAt(Definitions.R13_ADDRESS) - 3,
        ram,
        Definitions.ARG_POINTER_ADDRESS); // arg = *(R13 - 3)
    bus.send(
        stackSegment,
        ram.getValueAt(Definitions.R13_ADDRESS) - 4,
        ram,
        Definitions.LOCAL_POINTER_ADDRESS); // lcl = *(R13 - 4)

    // removes the top function from the call stack
    callStack.popFunction();

    // check whether there is a "calling frame"
    if (stackFrames.size() > 0) {
      // retrieve stack frame address of old function
      int frameAddress = ((Integer) stackFrames.lastElement()).intValue();
      stackFrames.removeElementAt(stackFrames.size() - 1);
      workingStackSegment.setStartAddress(frameAddress);

      // disable non relevant range of the local segment - enable only the locals
      // of the function that we returned to.
      localSegment.setEnabledRange(
          Math.max(localSegment.getStartAddress(), Definitions.STACK_START_ADDRESS),
          frameAddress - 1,
          true);

      // enable in the arg segment only the number of args that were sent to the function
      // that we returned to.
      argSegment.setEnabledRange(
          argSegment.getStartAddress(), localSegment.getStartAddress() - 6, true);

      // enable this, that according to their retrieved pointers
      thisSegment.setEnabledRange(
          Math.max(thisSegment.getStartAddress(), Definitions.HEAP_START_ADDRESS),
          Definitions.HEAP_END_ADDRESS,
          true);
      thatSegment.setEnabledRange(
          Math.max(thatSegment.getStartAddress(), Definitions.HEAP_START_ADDRESS),
          Definitions.SCREEN_END_ADDRESS,
          true);
    } /* else {
      	error("Nowhere to return to");
      } */
    // Allow return if we previously had "function" even with no call -
    // For the SimpleFunction test

    short returnAddress = ram.getValueAt(Definitions.R14_ADDRESS);
    if (returnAddress == VMProgram.BUILTIN_FUNCTION_ADDRESS) {
      staticSegment.setEnabledRange(0, -1, true); // empty static segment
      builtInFunctionsRunner.returnToBuiltInFunction(popValue(METHOD_STACK));
    } else if (returnAddress >= 0 && returnAddress < program.getSize()) {
      // sets the static segment range
      if (stackFrames.size() > 0) {
        setStaticRange(callStack.getTopFunction());
      } else {
        staticSegment.setStartAddress(Definitions.VAR_START_ADDRESS);
        staticSegment.setEnabledRange(
            Definitions.VAR_START_ADDRESS, Definitions.VAR_END_ADDRESS - 1, true);
      }
      program.setPC((short) (returnAddress - 1)); // set previousPC correctly
      program.setPC(returnAddress); // pc = *sp
    } else {
      error("Illegal return address");
    }
  }