ALU Test Script

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ALU Test Script


My ALU passes the test script... but I don't think that it should!

While I was working on the "no status bits" version of the ALU, as suggested by various posters to these forums, I commented out the zr and ng output pins in the chip definition. I just noticed this morning while running ALU.tst  script that my ALU only had two outputs listed in the "output" box: Out[16] and ng. I had forgotten to uncomment zr! My ALU has a zr pin, but because I'd commented out the output zr pin mine is an internal pin that doesn't connect to anything except the output of a Not - but it passes the script test just fine.

This is not a big problem, I suppose, except that, when it comes to integrating the ALU into a larger project I can imagine receiving a lot of strange errors about missing pins that might have been confusing and hard to track down.

Looking at the script file, it seems like maybe there's no way to check whether a pin is internal or external? Maybe name collisions like this can't be tested for.

- N
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Re: ALU Test Script

I just noticed that you never received a reply.

I would need to see your HDL that passed that you don't think should have to really figure out what is going on. I'm not sure what you actually commented out, I can think of a couple of things you might be referring to.

Feel free to post your code here -- we can delete it once we are done looking it over.
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Re: ALU Test Script

Hey WBahn! That was a while ago, but I think I remember what was going on.

Project 02 has two versions of the test script for the ALU: one with status bits, and one without. It is recommended to make the no-status bits ALU first, as it is less complicated and therefore easier to implement. I followed this advice, and commented out the zr and ng pins in my initial version.

I went on to finish the ALU by adding the status pins, but I forgot to uncomment out the zr pin:

// This file is part of
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/02/ALU.hdl

 * The ALU (Arithmetic Logic Unit).
 * Computes one of the following functions:
 * x+y, x-y, y-x, 0, 1, -1, x, y, -x, -y, !x, !y,
 * x+1, y+1, x-1, y-1, x&y, x|y on two 16-bit inputs, 
 * according to 6 input bits denoted zx,nx,zy,ny,f,no.
 * In addition, the ALU computes two 1-bit outputs:
 * if the ALU output == 0, zr is set to 1; otherwise zr is set to 0;
 * if the ALU output < 0, ng is set to 1; otherwise ng is set to 0.

// Implementation: the ALU logic manipulates the x and y inputs
// and operates on the resulting values, as follows:
// if (zx == 1) set x = 0        // 16-bit constant
// if (nx == 1) set x = !x       // bitwise not
// if (zy == 1) set y = 0        // 16-bit constant
// if (ny == 1) set y = !y       // bitwise not
// if (f == 1)  set out = x + y  // integer 2's complement addition
// if (f == 0)  set out = x & y  // bitwise and
// if (no == 1) set out = !out   // bitwise not
// if (out == 0) set zr = 1
// if (out < 0) set ng = 1

        x[16], y[16],  // 16-bit inputs        
        zx, // zero the x input?
        nx, // negate the x input?
        zy, // zero the y input?
        ny, // negate the y input?
        f,  // compute out = x + y (if 1) or x & y (if 0)
        no; // negate the out output?

        out[16], // 16-bit output
 //       zr, // 1 if (out == 0), 0 otherwise
        ng; // 1 if (out < 0),  0 otherwise

   // Put you code here:

		Not16(in=xOutz, out=notx);
		Not16(in=yOutz, out=noty);



This ALU will pass the full ALU.tst. The simulator treats the 'zr' pin that's attached to the last Not gate as an internal pin that isn't connected to anything else - a case of the system doing what it's supposed to to, but not what I meant it to do! The ALU.tst script checks the value coming off of the pin named 'zr' to make sure it's correct, but doesn't seem to have a way to check that the pin is an output pin.

This did not cause me too much trouble, but I was thinking it would be pretty annoying to have an error like this buried a few layers deep in a complex project where all the individual components test out OK, but connecting them together doesn't!
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Re: ALU Test Script

Your thinking is mostly correct, but there is some subtlety involved which makes it not as bad as you were imagining.

When you test a part, the test script has access to all of the signals within the part in order to make debugging easier. So that is why your test script is passing. The test script actually has no way of knowing what the ports are at all.

But when you USE the part in a larger design, now your larger design can't access those signals and neither can the test script. So if you used your CPU with the zr pin commented out in your Computer design the hardware simulator would throw an error when loading the Computer if it tried to use the zr pin.