Project 2 Un-Pinpointable Errors

The auto-grader is pretty good at detecting an illegal transition and telling
you where it is.  However, under certain circumstances, it can detect that
there was an illegal transition somewhere, but it can't pinpoint where.
In such cases, fixing an inefficiency may reveal an un-pinpointable error.

Here are some of the common errors that fall in this category:

1. First and foremost, RE-READ the Project 2 handout!  If you follow the
    guidelines in the handout, you will have no illegal transitions.

2. if-goto problems
    If-statements represent the control unit making decisions based on some
    value.  Remember that the control unit can only read the current opcode
    bits of instrReg, the memory ready signal, and the aluResult register (i.e.
    test if aluResult is 0) to make decisions on what next state to go to.
    Since instrReg and aluResult are registers, they must be written in the
    cycle BEFORE the control unit tests their value.  Hence the following is
    ILLEGAL:

	test1: printState(state, "test1"); state.aluResult = state.aluOperand +
	    bus;

	    if (state.aluResult == 0) { /* this is illegal because aluResult
					    doesn't actually get the new value
					    until the next cycle */
		goto ...
	    }
    
    On the other hand, the memory read signal is not a register.  Hence
    it can be read in the same cycle that it is written in.  Hence the
    following is LEGAL.

	test3:
	    printState(state, "test3");
	    if (memoryAccess(...)) {
		goto ...
	    }

3. Body of if-goto
    The body of your if-else-statements should ONLY have a goto.  No
	other work should be done inside an if-statement.  E.g. the following
	is ILLEGAL:

	test:
	    printState(state, "test");
	    if ((state.instrReg>>22) == ADD) {
		bus = programCounter;
		memoryAddress = bus;
	    }

4. Condition of if statement
    The control unit can read instrReg, memory's ready signal, and the
    aluResult register.  Be careful that your if statements only test
    these values.

5. memoryAccess problems

    Make sure your memoryAccess function is EXACTLY the same as in the
    handout.

    memoryAccess should NOT be called in the same cycle that loads
    memoryAddress or memoryData.  E.g. the following is ILLEGAL:

	test4:
	    printState(state, test4");
	    bus = state.aluResult;
	    state.memoryData = bus;
	    ready = memoryAccess(...);

6. beginning the FSM
    Remember to call printState at the beginning of each state.