Question

Although I tagged this homework, it is actually for a course which I am doing on my own for free. Anyway, the course is called ‘From Nand to Tetris’ and I’m hoping someone here has seen or taken the course so I can get some help. I am at the stage where I am building the ALU with the supplied hdl language. My problem is that I can’t get my chip to compile properly. I am getting errors when I try to set the output flags for the ALU. I believe the problem is that I can’t subscript any intermediate variable, since when I just try setting the flags to true or false based on some random variable (say an input flag), I do not get the errors. I know the problem is not with the chips I am trying to use since I am using all builtin chips.

Here is my ALU chip so far:

/**  * The ALU.  Computes a pre-defined set of functions out = f(x,y)  * where x and y are two 16-bit inputs. The function f is selected   * by a set of 6 control bits denoted zx, nx, zy, ny, f, no.  * The ALU operation can be described using the following pseudocode:  *     if zx=1 set x = 0       // 16-bit zero constant  *     if nx=1 set x = !x      // Bit-wise negation  *     if zy=1 set y = 0       // 16-bit zero constant  *     if ny=1 set y = !y      // Bit-wise negation  *     if f=1  set out = x + y // Integer 2's complement addition  *     else    set out = x & y // Bit-wise And  *     if no=1 set out = !out  // Bit-wise negation  *  * In addition to computing out, the ALU computes two 1-bit outputs:  *     if out=0 set zr = 1 else zr = 0 // 16-bit equality comparison  *     if out<0 set ng = 1 else ng = 0 // 2's complement comparison  */  CHIP ALU {  IN  // 16-bit inputs:     x[16], y[16],     // Control bits:     zx, // Zero the x input     nx, // Negate the x input     zy, // Zero the y input     ny, // Negate the y input     f,  // Function code: 1 for add, 0 for and     no; // Negate the out output  OUT // 16-bit output     out[16],      // ALU output flags     zr, // 1 if out=0, 0 otherwise     ng; // 1 if out<0, 0 otherwise  PARTS: // Zero the x input Mux16( a=x, b=false, sel=zx, out=x2 );  // Zero the y input Mux16( a=y, b=false, sel=zy, out=y2 );  // Negate the x input Not16( in=x, out=notx ); Mux16( a=x, b=notx, sel=nx, out=x3 );  // Negate the y input Not16( in=y, out=noty ); Mux16( a=y, b=noty, sel=ny, out=y3 );  // Perform f Add16( a=x3, b=y3, out=addout ); And16( a=x3, b=y3, out=andout ); Mux16( a=andout, b=addout, sel=f, out=preout );  // Negate the output Not16( in=preout, out=notpreout ); Mux16( a=preout, b=notpreout, sel=no, out=out );  // zr flag Or8way( in=out[0..7], out=zr1 );   // PROBLEM SHOWS UP HERE Or8way( in=out[8..15], out=zr2 ); Or( a=zr1, b=zr2, out=zr );  // ng flag Not( in=out[15], out=ng );  } 

So the problem shows up when I am trying to send a subscripted version of ‘out’ to the Or8Way chip. I’ve tried using a different variable than ‘out’, but with the same problem. Then I read that you are not able to subscript intermediate variables. I thought maybe if I sent the intermediate variable to some other chip, and that chip subscripted it, it would solve the problem, but it has the same error. Unfortunately I just can’t think of a way to set the zr and ng flags without subscripting some intermediate variable, so I’m really stuck!

Just so you know, if I replace the problematic lines with the following, it will compile (but not give the right results since I’m just using some random input):

// zr flag Not( in=zx, out=zr );  // ng flag Not( in=zx, out=ng ); 

Anyone have any ideas?

Edit: Here is the appendix of the book for the course which specifies how the hdl works. Specifically look at section 5 which talks about buses and says: ‘An internal pin (like v above) may not be subscripted’.

Edit: Here is the exact error I get: ‘Line 68, Can’t connect gate’s output pin to part’. The error message is sort of confusing though, since that does not seem to be the actual problem. If I just replace ‘Or8way( in=out[0..7], out=zr1 );’ with ‘Or8way( in=false, out=zr1 );’ it will not generate this error, which is what lead me to look up in the appendix and find that the out variable, since it was derived as intermediate, could not be subscripted.

Answer 1

The solution as Pax suggested was to use an intermediate variable as input to another chip, such as Or16Way. Here is the code after I fixed the problem and debugged:

CHIP ALU {  IN  // 16-bit inputs:     x[16], y[16],     // Control bits:     zx, // Zero the x input     nx, // Negate the x input     zy, // Zero the y input     ny, // Negate the y input     f,  // Function code: 1 for add, 0 for and     no; // Negate the out output  OUT // 16-bit output     out[16],      // ALU output flags     zr, // 1 if out=0, 0 otherwise     ng; // 1 if out<0, 0 otherwise  PARTS: // Zero the x input Mux16( a=x, b=false, sel=zx, out=x2 );  // Zero the y input Mux16( a=y, b=false, sel=zy, out=y2 );  // Negate the x input Not16( in=x2, out=notx ); Mux16( a=x2, b=notx, sel=nx, out=x3 );  // Negate the y input Not16( in=y2, out=noty ); Mux16( a=y2, b=noty, sel=ny, out=y3 );  // Perform f Add16( a=x3, b=y3, out=addout ); And16( a=x3, b=y3, out=andout ); Mux16( a=andout, b=addout, sel=f, out=preout );  // Negate the output Not16( in=preout, out=notpreout ); Mux16( a=preout, b=notpreout, sel=no, out=preout2 );  // zr flag Or16Way( in=preout2, out=notzr ); Not( in=notzr, out=zr );  // ng flag And16( a=preout2, b=true, out[15]=ng );  // Get final output And16( a=preout2, b=preout2, out=out ); }